Record extremes measured over the course of a 24-hour period. These records most commonly apply to snow and rainfall accumulation.

A daily and half-daily index of geomagnetic activity determined from the k indices scaled at two nearly antipodal stations at invariant magnetic latitude 50 degrees (Hartland, England, and Canberra, Australia). The aa values are in units of 1 nT. The index is available back to 1868, and is provided by the Institut du Globe de Paris, France.

In spectroscopy, and in particular the solar Fraunhofer spectrum, a characteristic wavelength of emitted radiation that is partially absorbed by the medium between the source and the observer.

A descriptive word specifically meaning (1) a probability of ~50% for an M- class x-ray flare (see x-ray flare class); (2) disturbed geomagnetic levels such that 16 < Ak index < 30.

A filament displaying motion or changes in shape, location, or absorption characteristics.

A range of heliographic longitudes in either the northern or southern solar hemisphere (seldom both at the same time) containing one or more large and complex active regions formed by the frequent, localized emergence of new magnetic flux. Individual sunspot groups within the complex can have relatively short lifetimes (a week or two); the complex may persist for several solar rotations because additional spot groups form as earlier ones decay.

A prominence above the solar limb moving and changing in appearance over a few minutes of time.

A portion of the solar limb displaying active prominences; typically associated with an active region.

A localized, transient volume of the solar atmosphere in which plages, sunspots, faculae, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are bipolar and may be complex if the region contains two or more bipolar groups.

An active region that exhibits a group or series of spike- like surges that rise no higher than 0.15 solar radii above the limb. (See bright surge on the limb.)

This type of fog requires horizontally moving air, or air that is advecting horizontally from one place to another. When warm and moist air blows over a cold surface, the surface cools the air. Once the air temperature cools enough to equal the dewpoint temperature, condensation is formed and creates a blanket-like thick fog formation. This describes the classic fog that spreads over Golden Gate Bridge in San Francisco.

geomagnetic index of the auroral electrojet, which characterizes the maximum range of excursion (both positive and negative) from quiet levels; measured at a given universal time by using the combined data from a worldwide ring of high-latitude magnetic observatories. AU (A upper) refers to the greatest positive deviation from the quiet time reference and AL (A lower) to the most negative. By definition AE = AU - AL. AO refers to the mean of AU and AL: AO = 1/2 (AU + AL). The AE and companion indexes are provided by the Data Analysis Center for Geomagnetism and Spacemagnetism of Kyoto University, Kyoto, Japan.

The Ak index observed at Fredericksburg, Virginia.

Air quality alerts include air pollution advisories, such as air stagnation, blowing dust, and dense smoke.

A daily index of geomagnetic activity for a specific station or network of stations (represented generically here by k) derived as the average of the eight 3-hourly ak indices.

A transverse wave in magnetized plasma characterized by a change of direction of the magnetic field (rather than a change of intensity).

Record extremes that are the most extreme for any period of time.

First, find your azimuth. Next, the Altitude (or elevation) is the angle between the Earth's surface (horizon) and the sun, or object in the sky. Altitudes range from -90° (straight down below the horizon, or the nadir) to +90° (straight up above the horizon or the Zenith) and 0° straight at the horizon.

Grey to white clouds that form in groups or globular masses and look like rolls in layers or patches. When holding your extended hand to the sky, they are about the size of your thumb. Alto means high while cumulus means heap and signifies convection. These clouds often resemble "sheepback", usually forming after a cold front. But on a warm, humid day, these clouds may develop prior to afternoon thunderstorms.

A uniform grey or blue-grey sheet or layer that covers the entire sky and may produce light precipitin. The sun or moon, although blurry or fuzzy, can be seen through this opaque cloud layer. This cloud often forms ahead of warm fronts leading a storm with light and continuous rain or snow.

The daily Ak index determined from the eight daily am indexes.

The daily Ak index determined from only the Northern Hemisphere stations of the am index network.

High pressure systems are areas, or closed systems, of high pressure and are also known as ridges and anti-cyclones. High pressure systems are associated with clockwise rotating air, in which at the surface the air moves away from the center, and toward the center at high levels. Thus, air is forced to sink in the center of high pressure systems. High pressure systems are associated with dry and clear, fair weather conditions. However, in urban areas with high levels of pollution at the surface, sinking air associated with high pressure can act to trap pollutants, allowing for poor air quality conditions.

Parallel rays of sunlight that penetrate through holes in clouds as columns of sunlit air are divided by darker shaded regions. Perspective effects cause the apparent divergence, and the rays are visible due to reflection of sunlight off of the atmospheric particles. The name originates from their frequent crepuscular occurrence (at dawn and dusk), when the contrast between light and dark are greatest.

Convective cloud meaning accumulated cloud, where nimbus means rain and cumulus means convective. This cloud has a flat cloud bottom with great vertical growth and can extend up to 13 miles. The flat base of the cloud signifies the Lifting Condensation Level (LCL) or the level of the atmosphere of equal temperature and dewpoint temperature. When the cloud top creates an anvil-like structure, the cloud has reached the height of the stable tropopause, where the cloud is forced to no longer grow vertically, but spreads horizontally. This is associated with a thunderstorm cloud and is capable of producing rain, snow, hail, graupel, and lightning and precipitin can usually be seen falling from the cloud base.

A form of cloud to cloud lightning, the most common type of lightning, occurring inside one cumulonimbus cloud due to opposing charges within the cloud. This most frequently occurs when the upper portion of an anvil cloud reaches positive charge, and the middle remains under negative charge. This is often referred to as sheet lightning because it lights up the cloud and surrounding sky with light. Heat lightning is no different from cloud to cloud lightning, it is sometimes referred to as heat lightning when it is too far away for thunder to be heard.

Formally the daily Ak index, determined from the eight daily ap indices. However, for daily operational uses (since several weeks are required to collect the data and calculate the index), the Air Force Weather Agency estimates the value of the Ap index by measuring the geomagnetic field in near-real time at several Western Hemisphere magnetometer stations and statistically weighting the data to represent the Gottingen Ap. The value of this estimated Ap index is reported in SEC daily and weekly summaries of geophysical activity.

That point on the path of a Sun-orbiting object most distant from the center of the Sun. Compare perihelion.

That point on the path of an Earth-orbiting satellite most distant from the center of the Earth. Compare perigee.

A series of magnetic loops, overlying a solar inversion line. Can become visible or enhanced following a nearby coronal mass ejection.

A system of small, arched linear-absorption H-alpha features connecting bright, compact plage of opposite polarity. An AFS is a sign of emerging bipolar magnetic flux and possibly rapid or continued growth in an active region.

A low, horizontal arc cloud formation created by the outflows of thunderstorms. It can act as a small-scale cold front that can encircle the original location of the thunderstorm. Cold outflows from sea breezes or cold fronts can also form this cloud, in the absence of thunderstorms. Two forms include roll clouds and shelf clouds.

The daily Ak index determined from only the Southern Hemisphere stations of the am index network.

The name of this cloud translates to rough or agitated waves. While these clouds give a dark and stormy appearance, but they have been known to dissipate before storm development. These have been spotted in the Plains of the US, forming after convective thunderstorm activity. This is a newly recognized cloud formation, proposed in 2009.

The time period when the sun is between 12 and 18 degrees below the horizon at either sunrise or sunset. The sun does not contribute to the illumination of the sky before this time in the morning, or after this time in the evening. In the beginning of morning astronomical twilight and at the end of astronomical twilight in the evening, sky illumination is very faint, and might be undetectable.

The force exerted by the weight of the atmosphere and gravity. Weather forecasters refer to high pressure and low pressure systems when discussing weather conditions. Weather forecasters refer to high pressure and low pressure systems when discussing weather conditions. Pressure is recorded in many different units: atmospheres (atm), millibars (mb), pascals (Pa), inches of mercury (in), pounds per square inch (PSI), etc. Meteorologists most often use use mb, which is equivalent to hectopascals (hPa), but also use in.

Also known as ”sferics,” transient radio waves produced by naturally occurring electric discharges (e.g., lightning) in the Earth’s atmosphere.

The mean distance between the Earth and Sun equal to 1.496 E11 m.

A faint visual phenomenon associated with geomagnetic activity that is visible mainly in the high-latitude night sky. Aurorae occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. Aurorae are a result of collisions between atmospheric gases and precipitating charged particles (mostly electrons) guided by the geomagnetic field from the magnetotail. Each gas (oxygen, nitrogen molecules, and atoms) emits a particular color depending on the energy of the precipitating particles, and atmospheric composition varies with altitude. Since the faster precipitating particles penetrate deeper, certain auroral colors, originate preferentially from certain heights in the sky. The auroral altitude range is 80 to 1000 km, but typical aurorae are 100 to 250 km above the ground; the color of the typical aurora is yellow-green, from a specific transition of atomic oxygen. Auroral light from lower levels in the atmosphere is dominated by blue and red bands from molecular nitrogen and oxygen. Above 250 km, auroral light is characterized by a red spectral line of atomic oxygen. Aurorae in the Northern Hemisphere are called the aurora borealis or ”northern lights.” Aurorae in the Southern Hemisphere are called aurora australis. The patterns and forms of the aurora include quiescent arcs, rapidly moving rays, curtains, patches, and veils.

A natural occurring display of lights observed in the high latitudes of the polar regions on the globe, but are also often seen as far as 65-72 degrees north and south. The chance for seeing the southern lights increases as you go closer to the South Magnetic Pole. If near the magnetic pole, they can be seen overhead, but from further distances they illuminate the northern horizon with a greenish or yellowish color. It is strongest during the equinoxes, or when the earth is at its greatest tilt. This phenomena occurs when photons are emitted into the ionosphere from ionized nitrogen atoms. They are ionized, or excited, by strong solar wind in the vicinity of Earth's magnetic field lines.

A natural occurring display of lights observed in the high latitudes of the polar regions on the globe, but are also often seen as far as 65-72 degrees north and south. The chance for seeing the northern lights increases as you go closer to the North Magnetic Pole. If near the magnetic pole, they can be seen overhead, but from further distances they illuminate the northern horizon with a greenish or yellowish color. It is strongest during the equinoxes, or when the earth is at its greatest tilt. This phenomena occurs when photons are emitted into the ionosphere from ionized nitrogen atoms. They are ionized, or excited, by strong solar wind in the vicinity of Earth's magnetic field lines.

An elliptical band around each geomagnetic pole ranging from about 75 degrees magnetic latitude at local noon to about 67degrees magnetic latitude at midnight under average conditions. Those locations experience the maximum occurrence of aurorae. The aurora widens to both higher and lower latitudes during the expansion phase of a magnetic substorm.

The equinox that occurs in September. Compare vernal equinox.

The azimuth (az) angle is the compass bearing, relative to true (geographic) north, of a point on the horizon directly beneath the sun. The horizon is defined as an imaginary circle centered on the observer. This is the 2-D, or Earth's surface, part of calculating the sun's position. As seen from above the observer, these compass bearings are measured clockwise in degrees from north. Azimuth angles can range from 0 - 359°. 0° is due geographic north, 90° due east, 180° due south, and 360 due north again.

As viewed from the Earth, the heliographic latitude of the center of the solar disk. The center of the solar disk usually does not coincide with the heliographic equator due to a tilt of the solar axis with respect to the ecliptic. (See B under solar coordinates.)

The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartels to begin in January 1833, and the count has continued by 27-day intervals to the present. The Sun has an average rotation period (as seen from the Earth) of 27.27 days. Therefore, solar longitude slowly drifts with respect to the Bartels rate. Compare Carrington longitude.

This is the only form of precipitation that is determined by windspeed. Under heavy snow conditions in low temperatures, strong winds can blow snow to white-out conditions, which restrict drastically restrict visibility. A blizzard is defined as sustained winds or frequent gusts at or above 35 mph, which blow snow to reduce visibility to a quarter of a mile for at least 3 hours.

Why is the sky blue? This is a great question and has to do with the scattering of light. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. These colors can be seen because components of the atmosphere (gases, dust, soot, ashes, pollen, and salt from the oceans) act to reflect or deflect light in a process called Rayleigh scattering. The various colors of the light have various wavelengths. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue.

A standing shock wave in front of the magnetosphere, arising from the interaction of the supersonic solar wind with the Earth’s magnetic field.

A short-lived brightening of flare or near flare intensity, less than ten millionths of the solar hemisphere in area.

A bright stream of gas seen against the solar disk. BSDs are often flare related and commonly fan out from the flare site. See also bright surge on the limb.

A bright stream of gas emanating from the chromosphere that moves outward more than 0.15 solar radius above the limb. It may decelerate and return to the Sun. Most BSLs assume a linear radial shape but can be inclined and/or fan shaped.

The equivalent blackbody temperature at a specified wavelength of a uniform source filling the resolution element of the telescope.

A transient enhancement of the solar radio emission, usually associated with an active region or flare.

A plot of observed solar active region latitudes vs. time. This diagram, which resembles a butterfly, shows that the average latitude of active region formation drifts from high to low latitudes during a sunspot cycle.

A measure of the North/South orientation of the interplanetery magnetic field measured perpendicular to the ecliptic plane. When Bz is southward, or antiparallel to the Earth's magnetic field, geomagnetic disturbances become much more severe than when Bz is northward.

Calls, or hits, represents each http connection to the API url with your API key. There is a limit per day associated with the service level you agree to. A "Day" is measured by 24 hours based on US Eastern Standard Time.

Calls, or hits, represents each http connection to the API url with your API key. There is a limit per day associated with the service level you agree to. A "Minute" is measured by a 60 second period.

A small horizontal cloud that appears above a cumulus or cumulonimbus cloud and looks like a hood or cap made up of ice crystals. They are formed when strong updrafts occur in a convective tower push a dome-shaped air up above the cloud. The moisture in the dome condenses quickly into an ice fog, and if the ambient air is too dry, the pileus cloud will not form. Pilei can also form over ash clouds and pyro-cumulus clouds. Pileus translates to "with piles".

A system of fixed solar longitudes rotating at a uniform synodic period of 27.2753 days (a sidereal period of 25.38 days). Carrington selected the meridian that passed through the ascending node of the Sun’s equator at 1200 UTC on 1 January 1854 as the original prime meridian. The daily Carrington longitude of the central point of the apparent solar disk is listed (with other solar coordinates) in The Astronomical Almanac published annually by the U.S. Naval Observatory. Compare Bartels’ rotation number.

See U burst.

The projection of Earth’s geographic equator onto the celestial sphere.

An imaginary rotating spherical shell around the Earth and concentric with it.

The most commonly used temperature scale where the freezing point is 0 degrees and the boiling point is 100 degrees.

A solar radio burst in the centimeter wavelength range of 1 to 10 cm.

The rotation of an active region or other feature across the longitude meridian that passes through the apparent center of the solar disk.

The layer of the solar atmosphere above the photosphere and beneath the transition region and the corona. The chromosphere is the source of the strongest lines in the solar spectrum, including the Balmer alpha line of hydrogen and the H and K lines of calcium, and is the source of the red color often seen around the rim of the moon at total solar eclipses.

These clouds look like thin white ice-crystal puffs. Unlike other cirrus clouds, these have supercooler liquid water droplets. If these droplets contact the ice crystals, they will rapidly freeze and transform the cloud into cirrus stratus. Cumulus signifies convection, but the cloud is usually short lived and can produce ice or snow in the form of virga. Each individual cloud puff is termed a "cloudlet" and are as small as a finger when extending a hand to the sky. They are also called Mackerel clouds when the cloudlets are aligned in rows they have the appearance of a herringbone or mackerel.

Thin and generally sheet-like clouds composed of ice-crystals, thin enough to see the sun and moon faintly behind them. When covering the whole sky, they signify abundant moisture in the upper atmosphere.

Thin and wispy clouds blown by strong winds high in the atmosphere, pointing in the direction the of the air movement. They are comprised of fibrous ice crystals. The clouds appear in fair weather conditions and do not produce precipitation that reaches the surface, but small ice crystal can fall from them, creating fall streaks or a form of virga. Cirrus translates to "curl of hair". Another name for these clouds is Mare's tails, due to the light and wispy hook that is often seen.

The time period when the sun is no more than 6 degrees below the horizon at either sunrise or sunset. The horizon should be clearly defined and the brightest stars should be visible under good atmospheric conditions (i.e. no moonlight, or other lights). One still should be able to carry on ordinary outdoor activities.

When there are no opaque (not transparent) clouds. Same as Clear.

See cusp.

The most common type of lightning, occurring inside one cumulonimbus cloud due to opposing charges within the cloud. This most frequently occurs when the upper portion of an anvil cloud reaches positive charge, and the middle remains under negative charge. This is often referred to as sheet lightning because it lights up the cloud and surrounding sky with light. Heat lightning is no different from cloud to cloud lightning, it is sometimes referred to as heat lightning when it is too far away for thunder to be heard.

This is the second most common type of lightning and causes the most damage. A cumulonimbus, or thunderstorm cloud, has charges or energy associated with it. The charges can separate in such that the cloud base is negative and the cloud top is positive, while the ground below remains positive. Then, the negative charges start moving down toward the ground from the base of the cloud, and create a faint "step leader", which is nearly invisible. Once the step leader nears the ground, an electric field is created and pushes the positive charge of the ground up the step leader. This is called the "returning stroke", and is what we call the lightning bolt as it is far more visible than the step leader. So what we see is the discharge that goes up from the ground to the cloud.

When 7/8ths or more of the sky is covered by clouds.

Central Meridian Distance. (See solar coordinates).

See coronal mass ejection.

See central meridian passage.

A zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.

Two points on the Earth’s surface at opposite ends of a geomagnetic field line.

Optical radiation arising from broadband emission from the photosphere.

General term for solar radio noise lasting for hours and sometimes days, in which the intensity varies smoothly with frequency over a wide range in the meter and decimeter wavelengths.

Short for "condensation trails", are artificial clouds made of condensed water vapor created from the exhaust of aircraft engines. The hot exhaust cools and condenses and can form into water droplets if the air is cold enough. The greater the humidity in the atmosphere, the longer these clouds will last.

The bulk transport of plasma (or gas) from one place to another, in response to mechanical forces (for example, viscous interaction with the solar wind) or electromagnetic forces.

By international agreement, the local time at the prime meridian, which passes through Greenwich, England. It was formerly known as Greenwich Mean Time, or sometimes simply Universal Time. There are 24 time zones around the world, labeled alphabetically. The time zone centered at Greenwich has the double designation of A and Z. Especially in the military community, Coordinated Universal Time is often referred to as Z or Zulu Time.

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The outermost layer of the solar atmosphere, characterized by low densities ( < 10E9 cm-3) and high temperatures (> 10E6 K).

An optical device that makes it possible to observe the corona at times other than during an eclipse. A simple lens focuses the Sun onto an occulting disk that prevents the light from the solar disk from proceeding farther along the optical path, effectively providing an artificial eclipse.

An extended region of the corona, exceptionally low in density and associated with unipolar photospheric regions having ”open” magnetic field topology. Coronal holes are largest and most stable at or near the solar poles, and are a source of high-speed solar wind. Coronal holes are visible in several wavelengths. Transequatorial coronal holes are the source of many recurrent geomagnetic disturbances since their lifetimes are months to years. The solar wind emanating from these holes is characteristically high in velocity and low in density.

A typical structure of enhanced corona observed in EUV lines and soft x-rays. Coronal loops represent ”closed” magnetic topology.

An outflow of plasma from or through the solar corona. CMEs are often, but not always, associated with erupting prominences, disappearing solar filaments, and/or flares. CMEs vary widely in structure, density, and velocity. Large and fast CMEs can approach densities of 10E16 g and velocities of 2000 km/s. Earth impacting CMEs can result in significant geomagnetic storms.

Material condensing in the corona and appearing to rain down into the chromosphere as observed in H-alpha at the solar limb. Rarely observed and usually seen following the impulsive phase of a large limb flare.

A large-scale structure in the white-light corona often overlying a principal inversion line in solar photospheric magnetic fields.

A general term for short-time-scale changes in the corona. Includes CMEs.

A nonspherical coordinate system based on a magnetic dipole axis that is offset from the Earth’s center by about 502 km toward a location in the Pacific Ocean (20.4 degrees N 147.3 degrees E). This ”eccentric dipole” axis intersects the surface at 82 degrees N 90 degrees W, and 75 degrees S 119 degrees E.

The broad spectrum of radio noise arriving at the Earth from sources outside the solar system.

An extremely energetic and relativistic charged particle. Galactic Cosmic Rays originate from outside the solar system and the Sun can produce "cosmic rays" during energetic proton events.

Parallel rays of sunlight that penetrate through holes in clouds as columns of sunlit air are divided by darker shaded regions. Perspective effects cause the apparent divergence, and the rays are visible due to reflection of sunlight off of the atmospheric particles. The name originates from their frequent crepuscular occurrence (at dawn and dusk), when the contrast between light and dark are greatest.

In ionospheric radio propagation, that frequency capable of penetration just to the layer of maximum ionization under vertical propagation. Radio waves of lower frequencies are refracted back to the ground; higher frequencies pass through.

See coronal rain.

A sudden deviation in the sunlit geomagnetic field H component (see geomagnetic elements) associated with extra-ordinary solar flare x-ray emission. The effect can be as much as 100 nT and characteristically lasts up to approximately 30 minutes.. The event is also known as a SFE (solar flare effect).

See continuum storm.

Convective cloud meaning accumulated cloud, where nimbus means rain and cumulus means convective. This cloud has a flat cloud bottom with great vertical growth and can extend up to 13 miles. The flat base of the cloud signifies the Lifting Condensation Level (LCL) or the level of the atmosphere of equal temperature and dewpoint temperature. When the cloud top creates an anvil-like structure, the cloud has reached the height of the stable tropopause, where the cloud is forced to no longer grow vertically, but spreads horizontally. This is associated with a thunderstorm cloud and is capable of producing rain, snow, hail, graupel, and lightning and precipitin can usually be seen falling from the cloud base.

Medium sized convective cumulus cloud showing little vertical growth and does not produce precipitation. Mediocris is the transformation between cumulus humilis and cumulus congests.

Puffy white or light grey clouds with a flat base and a sharp outline, often resembling a floating cotton balls. The cloud forms in an unstable environment when warm air from the surface rises until it reaches the Lifting Condensation Level (LCL), where the temperature and dewpoint temperature are equal. This is the flat cloud base and depending on the instability of the environment, the cloud will continue to grow vertically, and can eventually become a cumulonimbus or thunderstorm cloud. Cumulus clouds take two forms, which are associated with good (cumulus humilis) and bad (cumulus congests) weather.

Large convective cumulus cloud with great vertical growth, usually taller than it is wide, due to its strong updrafts. Congestus is Latin for "piled-up" and usually is associated with precipitation and if instability is strong enough, cumulonimbus and thunderstorm clouds will develop. Most often these clouds are indicative for bad weather.

Very small convective cloud, which forms almost immediately when a rising thermal reaches the Lifting Condensation Level (LCL), or where the temperature drops to equal the dewpoint temperature. This cloud usually dissipates after a few minutes because the layer just above it is too stable to allow for vertical growth. Most often these clouds are indicative for pleasant weather.

In the magnetosphere, two regions near magnetic local noon and approximately 15 degrees of latitude equatorward of the north and the south magnetic poles. The cusps mark the division between geomagnetic field lines on the sunward side (which are approximately dipolar but somewhat compressed by the solar wind) and the field lines in the polar cap that are swept back into the magnetotail by the solar wind.

See geomagnetic elements.

A daytime region of the Earth’s ionosphere beginning from approximately 40 km to 90 km altitude. Radiowave absorption in layers in this region can be significantly increased in response to increased ionization associated with solar x-ray flares.

Record extremes measured on a specific calendar day.

Dark gaseous ejections on the Sun visible in Ha. They usually originate from small subflare-like brightenings. Material is usually seen to be ejected, then decelerate at a gravitational rate, and to flow back to the point of origin. DSDs can occur intermittently for days from an active region.

Disparition brusque. See disappearing solar filament.

(1) The angular distance of an astronomical body north (+) or south (-) of the celestial equator. (2) In geomagnetic applications, the angle between true north and the horizontal component of the local geomagnetic field.

A dense fog advisory is issued when fog that reduces visibility to 1/4 mile or less is possible in the advisory area.

The temperature at which the air temperature must be cooled for water vapor to condense, forming water droplets, fog, or clouds.

The temperature at which the air temperature must be cooled for water vapor to condense, forming water droplets, fog, or clouds.

The charging of different areas of a spacecraft or satellite to different potentials in response to sunlight, the charged particle environment, and the design and composition of the materials involved. Discharge may occur through arcing and generally is detrimental.

The differential particle directional flux j(E,w) denotes the number of particles of energy E per unit energy interval, per unit area, per unit time, per unit solid angle of observation, passing through an area perpendicular to the viewing direction. The angle w is the angle between the viewing direction and the local magnetic field. It is approximately obtained from the count rate of a physical detector measuring the flux of particles between energy E and E + dE, geometric factor G, and solid angle of view dW through the relationship j(E,w) = C/(G x dE x dW x dt) where C is the number of detector counts in time dt.

The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approximately 14° per day) than do high latitudes (approximately12° per day).

When the image has been changed such that it longer looks like the original subject matter or incorporates artistic elements, it is considered digital art.

The geomagnetic inclination angle. See geomagnetic elements.

An irregular, imaginary line around the Earth where the geomagnetic inclination angle is measured to be zero. It lies near the geographic equator.

A solar filament that disappears suddenly on a timescale of minutes to hours. The prominence material is often seen to ascend but can fall into the Sun or just fade. DSFs are probable indicators of coronal mass ejections.

The visible surface of the Sun or other heavenly body projected against the sky.

See disappearing solar filament.

A change in the perceived frequency of a radiated signal caused by motion of the source relative to the observer.

The rate at which radiation energy is absorbed in living tissue, expressed in centisieverts per unit time.

A double rainbow is similar to a single rainbow in that it is both an optical and meteorological phenomenon, but the double rainbow portrays the colors in reverse. Thus, the outer band of red is on the inner band on the second rainbow that forms on the outside. It is a mirror image of the original rainbow, as it reflects off of a body of water. Often under extremely moist atmospheric conditions, the body of water that is reflecting the rainbow is the abundant water droplets in the air. In the case of a triple rainbow, the colors are reversed once again to that of the original rainbow.

Light liquid precipitation of small uniform drops falling from stratus clouds.

A term for lightning that develops from a storm that does not produce precipitation, and is the most common cause of natural wildfires. Cumulonimbus and pyrocumulus are capable of producing dry lightning. This form can take any form of lightning: cloud to cloud, cloud to ground, or ground to cloud.

See dark surge on the disk.

See disappearing solar filament.

A measure of variation in the geomagnetic field due to the equatorial ring current. It is computed from the H-components at approximately four near-equatorial stations at hourly intervals. At a given time, the Dst index is the average of variation over all longitudes. The reference level is set so that Dst is statistically zero on internationally designated quiet days. An index of -50 or deeper indicates a storm-level disturbance, and an index of -200 or deeper is associated with middle latitude aurorae. Dst is determined by the World Data Center C2 for Geomagnetism, Kyoto University, Kyoto, Japan.

A strong and long-lived whirlwind that ranges from a half of a meter wide and a few meters tall to more than 10 meters wide and more than 1,000 meters tall. They are similar to tornadoes in that they form around a vertical rotating column o f air. However, dust devils form under fair weather conditions where sunny skies heat the surface, which can produce swirling updrafts of air. These frequently develop in the hot and dry desserts.

The momentum flux (P) of the solar wind. P=density x V E2 where density is in particles/cm E-3 and V in in km/s.

A daytime region of the Earth’s ionosphere roughly between the altitudes of 90 and 160 km. E region characteristics (electron density, height, etc.) depend on the solar zenith angle and solar activity. The ionization in the E layer is caused mainly by x-rays in the range 0.8 to 10.4 nm. (See also sporadic E.)

See corrected geomagnetic coordinates.

The obscuring of one celestial body by another. (1) A Solar Eclipse occurs when the moon comes between the Earth and the Sun. In a total eclipse, the solar disk is completely obscured; in a partial eclipse the solar disk is only partly obscured: (2) A lunar eclipse occurs when the moon enters the shadow cast by the Earth: (3) Spacecraft in the Earth’s shadow are said to be in eclipse.

The great circle made by the intersection of the plane of the Earth’s orbit with the celestial sphere. (Less properly, the apparent path of the Sun around the sky during the year.)

See emerging flux region.

See extremely high frequency.

Extreme ultraviolet Imaging Telescope. Instrument on the SOlar Heliospheric Observer. EIT continuously observes the full disk Sun at 17.1 nm, 19.5 nm, 28.4 nm, and 30.4 nm.

(1) Auroral: A current that flows in the ionosphere in the auroral zone. (2) Equatorial: A thin electric current layer in the ionosphere over the dip equator at about 100 to 115 km altitude. electrostatic discharge (ESD). An abrupt equalization of electric potentials. In space, ESD can occur between objects or portions of a single object (see differential charging); ESD may occur locally within a dielectric or cable. The consequences may include material damage, a spacecraft anomaly, phantom commands, disrupted telemetry, and contaminated data.

See extremely low frequency.

An area on the Sun where new magnetic flux erupts. An EFR is a bipolar magnetic region that first produces a small bipolar plage visible in the chromosphere, which may develop an arch filament system and the initial spots of a sunspot group. An EFR may be isolated from other solar activity or may occur within an active region.

In spectroscopy, a particular wavelength of emitted radiation, more intense than the background continuum.

The integral of the square of the electron density over volume; the units are inverse volume (m-3).

An astronomical almanac listing solar coordinates and the positions of the Sun and other heavenly bodies at regular intervals in time.

See eruptive prominence on limb.

See electrojet.

One of the two points of intersection of the celestial equator and the ecliptic. The Sun passes through the vernal equinox on about 21 March and through the autumnal equinox on about 22 September.

With regard to solar flare predictions, a probability of >50% that an active region will produce C class x-ray flares. (See x-ray flare class.)

A solar prominence that becomes activated and is seen to ascend away from the Sun; sometimes associated with a coronal mass ejection. (See also disappearing solar filament.)

See electrostatic discharge.

For the Earth, an estimate made from NOAA particle measurements of the instantaneous power dissipated daily in a single auroral zone by auroral particle precipitation. The power ranges from approximately 5 gigawatts during quiet intervals up to more than 100 in very active times. The magnitude of this power input corresponds closely to the level of geomagnetic activity.

See extreme ultraviolet.

The most localized form of fog, usually forming over lakes and rivers, sometime oceans, when the water is warmer than the air above it. Moisture evaporates from the water and saturates the adjacent layer of air and condenses. This air rises, it evaporates into the dryer air aloft, thus, giving the appearance of a low layer of steam above the water.

Horizontal motion of the solar atmosphere near a sunspot, having velocities of a few kilometers per second. In the photosphere, matter streams away from the umbra. In the chromosphere, the direction of flow is toward the umbra.

The forecast temperature is usually less than 1.5 degrees C warmer or cooler than the observed temperature.

The Earth’s atmosphere above 500-600 km.

A portion of the electromagnetic spectrum from approximately 10 to 100 nm.

That portion of the radio frequency spectrum from 30-300 GHz.

That portion of the radio frequency spectrum from 30 to 3000 Hz.

Of the white-light corona (the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered or reflected by solid particles (dust) in interplanetary space. The same phenomenon produces zodiacal light.

The upper region of the ionosphere, above approximately 160 km altitude. F region electron densities are highly variable, depending on the local time, solar activity, season, and geomagnetic activity. The F region contains the Fl and F2 layers. The F2 layer is more dense and peaks at altitudes between 200 and 600 km. The Fl layer, which forms at lower altitudes in the daytime, has a smaller peak in electron density.

See follower spot.

White light plage. Bright region of the photosphere seen in white light, seldom visible except near the solar limb. Corresponds with concentrated magnetic fields that may presage sunspot formation.

Fahrenheit is the standard temperature scale used in the United States. The scale ranges from the freezing point at 32 degrees to the boiling point at 212 degrees, which places the two points exactly 180 degrees apart.

The forecast temperature is usually 2.5 to 3.5 degrees C warmer or cooler than the observed forecast.

Very small convective cloud, which forms almost immediately when a rising thermal reaches the Lifting Condensation Level (LCL), or where the temperature drops to equal the dewpoint temperature. This cloud usually dissipates after a few minutes because the layer just above it is too stable to allow for vertical growth. Most often these clouds are indicative for pleasant weather.

The image is not real or the scene is fictitious and cannot be added to the site.

As rain falls from the cloud, it can evaporate before reaching the surface. This most frequently occurs in dry climates with dry surface conditions.

A linear feature in the H-alpha chromosphere of the Sun, occurring near strong sunspots and plage or in filament channels. Fibrils parallel strong magnetic fields, as if mapping the field direction.

A mass of gas suspended over the chromosphere by magnetic fields and seen as dark ribbons threaded over the solar disk. A filament on the limb of the Sun seen in emission against the dark sky is called a prominence.

A broad pattern of fibrils in the chromosphere, marking a portion of a magnetic polarity inversion line where a filament may soon form or where a filament recently disappeared. Filament channels are frequently observed in soft x-rays images as dark lanes.

Fire alerts include all watches, warnings, and advisories that pertain to fire weather, including red flag warnings, fire weather watches, and dense smoke advisories.

A dense convective cloud that develops above wild land and grass land fires, as well as out of control prescribed fires. Strong heating at the surface allows for warm air to rise from the surface (convection) that would not otherwise occur without the presence of the fire. The majority of the smoke gets trapped below a stable layer in the atmosphere, however, this rising air can be so buoyant that it rises beyond the stable layer, producing a cauliflower-like high-level cloud. When these clouds form over fires, it signifies a raging fire with strong wind gusts at the fire front which help to strengthen the fire.

During a natural or prescribed fire, the flames drastically heat the surface which allows for hot air near the surface to rise. The air is so hot that it rises quickly and creates strong winds by pulling air in to replace the rising air. These violent winds near the fire sometimes take a rotating form, and can quickly strengthen into a vertical column or vortex of rotating air and flames. This signifies a fire is in a dangerous and uncontrollable stage.

During a natural or prescribed fire, the flames drastically heat the surface which allows for hot air near the surface to rise. The air is so hot that it rises quickly and creates strong winds by pulling air in to replace the rising air. These violent winds near the fire sometimes take a rotating form, and can quickly strengthen into a vertical column or vortex of rotating air and flames. This signifies a fire is in a dangerous and uncontrollable stage.

A fire weather advisory is issued when dry conditions in the advisory area result in a situation where forest or brush fires are possible.

During a natural or prescribed fire, the flames drastically heat the surface which allows for hot air near the surface to rise. The air is so hot that it rises quickly and creates strong winds by pulling air in to replace the rising air. These violent winds near the fire sometimes take a rotating form, and can quickly strengthen into a vertical column or vortex of rotating air and flames. This signifies a fire is in a dangerous and uncontrollable stage.

A sudden eruption of energy in the solar atmosphere lasting minutes to hours, from which radiation and particles are emitted. Flares are classified on the basis of area at the time of maximum brightness in H-alpha. Importance 0 (Subflare): <2.0 hemispheric square degrees Importance 1: 2.1-5.1 square degrees Importance 2: 5.2-12.4 square degrees Importance 3: 12.5-24.7 square degrees Importance 4: ?;24.8 square degrees [One square degree is equal to (1.214 x 104 km)2 = 48.5 millionths of the visible solar hemisphere.] A brightness qualifier F, N, or B is generally appended to the importance character to indicate faint, normal, or brilliant (for example, 2B).

A flash flood warning is issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

A flood warning is issued when flooding is occurring or is about to occur.

A flood watch is issued when flooding is possible in the watch area.

Time integrated flux. In SEC use, a specified particle or 0.1-0.8 nm flux accumulation over 24 hours.

Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).

The rate of flow of a physical quantity through a reference surface.

The lowest frequency at which echo traces are observed on an ionogram. It increases with increasing D region absorption.

The maximum ordinary mode radiowave frequency capable of vertical reflection from the sporadic E layer of the ionosphere.

The maximum ordinary mode radiowave frequency capable of vertical reflection from the F2 layer of the ionosphere. (See F region.)

Fog is water droplets suspended in the air at the Earth's surface. Fog is often hazardous when the visibility is reduced to 1/4 mile or less.

Fog alerts include all watches, warnings, and advisories that pertain to fog, including dense fog advisories and freezing fog advisories.

In a magnetically bipolar or multipolar sunspot group, the main spot in that portion of the group east of the principal inversion line is called the follower or f-spot. Leader and follower describe the positions of spots with respect to apparent motion due to solar rotation. (Compare leader spot.)

An abrupt decrease, of at least 10%, of the background galactic cosmic ray intensity as observed by neutron monitors. It is associated with major plasma and magnetic field enhancements in the solar wind at or near the Earth.

The apparent distortion of solar features viewed near the limb of the Sun.

Dark shreds of stratus clouds that break away from the stratus, and sit under the base of precipitating nimbostratus.

The system of dark lines superposed on the continuous solar spectrum formed by the absorption of photons by atoms and molecules in the solar and terrestrial atmospheres.

This occurs when water droplets in the air become "supercooled" meaning they remain in a liquid state before contacting a frozen surface. The surface will be covered in ice upon contact. This is a similar process to freezing rain and sleet. This often forms ice crystals on trees and fences

Mist occurring in an environment below freezing. This is similar to freezing or frozen fog but the visibility is higher because the density is lower.

This occurs when water droplets in the air become "supercooled" meaning they remain in a liquid state before contacting a frozen surface. The surface will be covered in ice upon contact. This is a similar process to freezing rain and sleet. This often forms ice crystals on trees and fences

Mist occurring in an environment below freezing. This is similar to freezing or frozen fog but the visibility is higher because the density is lower.

For freezing or frozen rain to form, the temperature of the cloud base is freezing or below freezing, so it comes out as snow, but there may be a warm layer that the snow falls through and causes the snow to start to melt, but then it falls through another cool layer near the surface and it tries to refreeze.

A funnel shaped cloud forming around a rotating column of air that extends from the bottom of a cumulonimbus or towering cumulus cloud. Most often these form from Supercell thunderstorms. When a funnel cloud extends to reach the ground, it becomes a tornado.

High-energy radiation (energies in excess of 100 keV) observed during large, extremely energetic solar flares.

An ISES special message summarizing by code the current and predicted levels of solar activity and geomagnetic activity.

The outer region of the Earth’s atmosphere lying above the thermosphere and composed mostly of hydrogen.

Natural variations in the geomagnetic field classified quantitatively into quiet, unsettled, active, and geomagnetic storm levels according to the observed a index: quiet 0 - 7, unsettled 8 - 15, active 16 - 29, minor storm 30 - 49, major storm 50 - 99, severe storm 100 - 400.

The components of the geomagnetic field at the surface of the Earth. These elements are usually denoted thus in the literature: X-the geographic northward component; Y -the geographic eastward component; Z-the vertical component, reckoned positive downward; H-the horizontal intensity, of magnitude (X2 + y2 )1/2 F-the total intensity (H2 + Z2 )1/2; I-the inclination (or dip) angle, tan -1 (Z/H); D-the declination angle, measured from the geographic north direction to the H component direction, positive in an eastward direction.; D = tan-l (Y/X) However, in SEC use, the geomagnetic northward and geomagnetic eastward components are called the H and D components. The H axis direction is defined by the mean direction of the horizontal component relative to the geomagnetic north by using the small-angle approximation. Thus the D component = H (the horizontal intensity) multiplied by delta D (the declination angle relative to geomagnetic north, expressed in radians).

The magnetic field in and around the Earth. The intensity of the magnetic field at the Earth’s surface is approximately 32,000 nT at the equator and 62,000 nT at the north pole (the place where a compass needle points vertically downward). The geomagnetic field is dynamic and undergoes continual slow secular changes as well as short-term disturbances (see geomagnetic activity). The geomagnetic field can be approximated by a centered dipole field, with the axis of the dipole inclined to the Earth’s rotational axis by about 11.5 degrees. Geomagnetic dipole north is near geographic coordinate 79 degrees N and 71 degrees W (near Thule, Greenland), and dipole south is near 79 degrees S and 110 degrees E (near Vostok, Antarctica). The observed or dip poles, where the magnetic field is vertical to the Earth’s surface, are near 77 degrees N and 102 degrees W, and 65 degrees S and 139 degrees E. The adopted origin of geomagnetic longitude is the meridian passing through the geomagnetic poles (dipole model) and the geographic south pole. (See also corrected geomagnetic coordinates.)

(1) A worldwide disturbance of the Earth’s magnetic field, distinct from regular diurnal variations. A storm is precisely defined as occurring when the daily Ap index exceeds 29, or (2) NOAA Space Weather Scale (G) for geomagnetic storm disturbances (see Appendix A).

See magnetic local time.

A quasi-DC current induced into long conductors such as electrical transmission lines or pipe lines. This occurs during geomagnetic storms at the Earth due to the movement of the field lines in the vicinity of the conductors.

Term applied to any equatorial satellite with an orbital velocity equal to the rotational velocity of the Earth. The geosynchronous altitude is near 6.6 Earth radii (approximately 36,000 km above the Earth’s surface). To be geostationary as well, the satellite must satisfy the additional restriction that its orbital inclination be exactly zero degrees. The net effect is that a geostationary satellite is virtually motionless with respect to an observer on the ground.

This is not a form of precipitation but occurs when supercooled raindrops come in contact with a solid object and freeze immediately. Glaze causes dangerous travel conditions when forming on roads, and can create such a thick and heavy ice coating it can cause great damage to power lines and trees. Glaze is difference from rime in that it is more ice-cube like in appearance and clings to the object on which it was formed. Rime is more milky and crystalline, resembling sugar, and extends from the object on which it formed.

See ground-level event.

Greenwich Mean Time. (See Coordinated Universal Time.)

The forecast temperature is usually 1.5 to 2.5 degrees C warmer or cooler than the observed forecast.

Global Positioning System: a network of Earth-orbiting satellites used for precise position-finding in surveying and navigation.

The commencement of a geomagnetic storm that has no well-defined onset.

Cellular structure of the photosphere visible at high spatial resolution. Individual granules, which represent the tops of small convection cells, are 200 to 2000 km in diameter and have lifetimes of 8 to 10 minutes.

These clouds look like ocean waves, or ripples in water. After air blows up an object, say a mountain, stability forces it to hold an oscillatory pattern. If the air has enough moisture, it will create a wave structure by condensing into a cloud at the crest of the wave, but evaporates at the wave trough.

A coronal emission line at 530.3 nm from Fe XIV. The green line is one of the strongest visible coronal lines. It identifies moderate temperature regions of the corona; it is enhanced in coronal streamers above inversion lines, and diminished in coronal holes.

See Coordinated Universal Time.

This is much more rare than cloud to cloud or cloud to ground lightning. A cumulonimbus, or thunderstorm cloud, has charges or energy associated with it. The charges can separate in such that the cloud base is negative and the cloud top is positive, while the ground below remains positive. The positive charge from the ground starts moving up toward the base of the cloud from the ground, and creates a faint "step leader", which is nearly invisible. Once the step leader nears the cloud, an electric field is created and pulls the negative charge of the cloud base down the step leader. This is called the "returning stroke", and is what we call the lightning bolt as it is far more visible than the step leader. So what we see is the discharge that goes down from the cloud to the ground.

A sharp increase in ground-level cosmic ray count to at least 10% above background, associated with solar protons of energies greater than 500 MeV. GLEs are relatively rare, occurring only a few times each solar cycle. When they occur, GLEs begin a few minutes after flare maximum and last for a few tens of minutes to hours. Intense particle fluxes at lower energies can be expected to follow this initial burst of relativistic particles. GLEs are detected by neutron monitors, e.g., the monitor at Thule, Greenland.

See geomagnetic elements.

The first atomic transition in the hydrogen Balmer series; wavelength = 656.3 nm. This absorption line of neutral hydrogen falls in the red part of the visible spectrum and is convenient for solar observations. The Ha line is commonly used for patrol observations of solar flares, filaments, prominences, and the fine structure of active regions.

A frozen form of precipitation consisting of individual balls or lumps of ice called hail stones, produced from cumulonimbus or thunderstorm clouds. Instead of getting pulled down by gravity to the ground, strong updrafts within the cloud push the droplet up into the coldest part of the top of the cloud and the droplet freezes. Then gravity pulls it back down so it starts to melt, but then another updraft pushes it back up to the top of the cloud so it can freeze again. This cycle continues until the hail stone is heavier than the force of the updraft, and it finally falls to the ground as a frozen ball or lump of ice. Rings can usually be identified on the hailstone as a result of the hail stone's history of melting near the bottom of the cloud and freezing near the top of the cloud. A bigger hail stone signifies a longer lifespan within the cloud, which also signifies a strong thunderstorm with strong updrafts.

A frozen form of precipitation consisting of individual balls or lumps of ice called hail stones, produced from cumulonimbus or thunderstorm clouds. Instead of getting pulled down by gravity to the ground, strong updrafts within the cloud push the droplet up into the coldest part of the top of the cloud and the droplet freezes. Then gravity pulls it back down so it starts to melt, but then another updraft pushes it back up to the top of the cloud so it can freeze again. This cycle continues until the hail stone is heavier than the force of the updraft, and it finally falls to the ground as a frozen ball or lump of ice. Rings can usually be identified on the hailstone as a result of the hail stone's history of melting near the bottom of the cloud and freezing near the top of the cloud. A bigger hail stone signifies a longer lifespan within the cloud, which also signifies a strong thunderstorm with strong updrafts.

An optical phenomenon produced by ice crystals in cirrus clouds. The ice crystals in the upper troposphere refracts and reflects the light, and can sometimes split the light into colors, creating an arc or circle in the sky around the sun or moon.

A faint ring of enhanced emission seen around most or all of the edge of the occulting disk of a coronagraph. Indicative of a spatially large CME on the front side (Earthward) or back side of the Sun. The source region is usually nearer to solar central meridian than the limbs. A partial halo is currently defined as spanning less than 120 degrees of solar latitude at the limb while full halo CMEs encompass 360 degrees. Full halo CMEs from the front side of the Sun almost always result in geomagnetic storms at Earth, especially when accompanied by a solar proton event.

An aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colors.

A heat advisory is issued when the heat index is expected to exceed 105 °F (100 °F in New York City) or if nighttime lows are expected to be greater than 80 °F for two or more nights.

An index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature

A form of cloud to cloud lightning, the most common type of lightning, occurring inside one cumulonimbus cloud due to opposing charges within the cloud. This most frequently occurs when the upper portion of an anvil cloud reaches positive charge, and the middle remains under negative charge. This is often referred to as sheet lightning because it lights up the cloud and surrounding sky with light. Heat lightning is no different from cloud to cloud lightning, it is sometimes referred to as heat lightning when it is too far away for thunder to be heard.

Referring to coordinates on the solar surface referenced to the solar rotational axis.

The boundary surface between the solar wind and the external galactic medium.

The study of wave oscillations in the Sun using acoustic, gravity, and surface gravity waves.

The magnetic cavity surrounding the Sun, carved out of the galaxy by the solar wind.

A feature of the white light corona (seen in eclipse or with a coronagraph) that looks like a ray extending away from the Sun out to about 1 solar radius, having an arch-like base containing a cavity usually occupied by a prominence.

See estimated hemispherical power input.

See high frequency.

That portion of the radio frequency spectrum between 3 and 30 MHz.

With reference to zones of geomagnetic activity - 50 to 80 degrees geomagnetic latitude. The other zones are equatorial, middle latitude, and polar.

The "maximum temperature" is the highest temperature measured during a specified period of time, most commonly per day (e.g. "today's high temperature"). Therefore, a "high maximum temperature record" is the highest high temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

The "minimum temperature" is the lowest temperature measured during a specified period of time, most commonly per day (e.g. "today's low temperature"). Therefore, the "high minimum temperature record" is the highest low temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

High pressure systems are areas, or closed systems, of high pressure and are also known as ridges and anti-cyclones. High pressure systems are associated with clockwise rotating air, in which at the surface the air moves away from the center, and toward the center at high levels. Thus, air is forced to sink in the center of high pressure systems. High pressure systems are associated with dry and clear, fair weather conditions. However, in urban areas with high levels of pollution at the surface, sinking air associated with high pressure can act to trap pollutants, allowing for poor air quality conditions.

A high wind advisory is issued when sustained winds of 31 mph or greater are expected to occur for at least 1 hour. This advisory can also be issued if winds of 46 mph or greater are expected for any length of time.

A feature of the solar wind having velocities exceeding approximately 600 km/s (about double average solar wind values). High-speed streams that originate in coronal holes are less dense than those originating in the average solar wind.

This is the only fog that forms adiabatically. When humid air gradually moves up slope or up a hill, air expends and cools adiabatically, and if the temperature of the air drops to the dewpoint temperature, fog is produced.

Solar flares that occur repetitively in an active region with essentially the same position and with a common pattern of development.

The Solar Hour Angle of the Sun for any local location on the Earth is zero° when the sun is straight overhead, at the zenith, and negative before local solar noon and positive after solar noon. In one 24-hour period, the Solar Hour Angle changes by 360 degrees (i.e. one revolution).

The amount of water vapor in the air, expressed as a percentage. Relative humidity is the ratio of the amount of moisture in the air to the amount that is needed to saturate the air. Thus, it is a function of both moisture content and temperature, as its name states, humidity is "relative" to temperature.

This statement is issued when it is necessary to inform the public of hurricane or tropical storm watches and warnings. These statements contain detailed information of when and what adverse conditions to expect as a result of the tropical system affecting the statement area.

A hurricane statement, or hurricane local statement, provides more detailed information about how the storm will impact your area.

A Hurricane Warning means hurricane conditions are expected in the next 24 hours.

A Hurricane Watch means hurricane conditions are possible in the next 24 hours.

A filament-associated two-ribbon flare, often occurring in spotless regions. The flare is generally slow (30-60 minutes rise time in Ha and x-ray) and follows the disappearance of a previously quiescent filament. The Hyder flare is named for Dr. C. Hyder, who published studies of such flares in 1967.

Same as Sleet; defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces.

See interplanetary magnetic field.

The angle between the local geomagnetic field direction and the horizon. (See geomagnetic elements.)

The integral directional particle flux J(E,w) is literally the mathematical integral, with respect to the energy E, of the differential particle flux j(E,w). It denotes the number of particles of energy equal to or greater than E, per unit area, per unit solid angle, per unit time, passing through an area perpendicular to the viewing direction.

An international consortium of magnetic observatories that exchange data in near-real time by satellite relay.

The magnetic field carried with the solar wind.

The space between planets and other solid bodies in the heliosphere. Popluated by solar and cosmic particles, magnetic fields, and photons.

The most common type of lightning, occurring inside one cumulonimbus cloud due to opposing charges within the cloud. This most frequently occurs when the upper portion of an anvil cloud reaches positive charge, and the middle remains under negative charge. This is often referred to as sheet lightning because it lights up the cloud and surrounding sky with light. Heat lightning is no different from cloud to cloud lightning, it is sometimes referred to as heat lightning when it is too far away for thunder to be heard.

The geomagnetic latitude at which a particular line of force of the geomagnetic field, characterized by L (the altitude of the field line at the equator), intersects the Earth.

The locus of points on the solar surface where the radial magnetic field vanishes. Inversion lines separate regions of opposing polarity and are often superposed by thin, dark filaments. Inside active regions, the areas close to and along inversion lines are preferred places of flare occurrence. Filament channels, plage corridors, arch-filament systems, and fibril patterns surrounding active regions can be used to infer the positions of inversion lines.

A plot or record of the group path height of reflection of ionospherically returned (echoed) radio waves as a function of frequency.

The region of the Earth’s upper atmosphere containing free electrons and ions produced by ionization of the constituents of the atmosphere by solar ultraviolet radiation at short wavelengths < 100nm) and energetic precipitating particles. The ionosphere influences radiowave propagation of frequencies less than about 300 MHz. (See D region, E region, F region.)

A disturbance in the F region of the ionosphere, which occurs in connection with geomagnetic activity. In general, there are two phases of an ionospheric storm, an initial increase in electron density (the positive phase) lasting a few hours, followed by a decrease lasting a few days. At low latitudes only the positive phase is usually seen. Individual storms can vary, and their behavior depends on geomagnetic latitude, season, and local time.

An optical phenomenon produced by ice crystals in cirrus clouds. The ice crystals in the upper troposphere refracts and reflects the light, and can sometimes split the light into colors, creating an arc or circle in the sky around the sun or moon.

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A 3-hourly quasi-logarithmic local index of geomagnetic activity relative to an assumed quiet-day curve for the recording site. Range is from 0 (quiet) to 9 (severely disturbed). The K index measures the deviation of the most disturbed component (see geomagnetic elements).

Of the white-light corona (the corona seen by the eye at a total solar eclipse) that portion which is caused by sunlight scattered by electrons in the hot outer atmosphere of the Sun.

A temperature scale in which 0 degrees is absolute zero, or the point at which all molecular motion ceases. The maximum boundary is the triple point, or the temperature at which the three phases of water co-exist. The Kelvin is 273.16th of this scale. Kelvin is rarely used to describe temperatures to the general public, but is usually used in scientific calculations for understanding weather patterns amd forecasting.

Occurs upon the interface between two fluids. For example when air travels over a lake, it creates ripples in the water. In the atmosphere, when one layer of air sits above a still cloud layer, it will create ripples or waves in the clouds.

Occurs upon the interface between two fluids. For example when air travels over a lake, it creates ripples in the water. In the atmosphere, when one layer of air sits above a still cloud layer, it will create ripples or waves in the clouds.

A 3-hourly planetary index of geomagnetic activity calculated by the Institut de Physique du Globe de Paris, France, from the K indexes observed at a large, symmetrically located network of stations. The Km indexes are used to determine the am indexes.

A 3-hourly planetary index of geomagnetic activity calculated by the Institut fur Geophysik der Gottingen Universitat, Germany, from the K indexes observed at 13 stations primarily in the Northern Hemisphere. The Kp indexes, which date from 1932, are used to determine the ap indexes.

Heliographic longitude of a solar feature. (See solar coordinates.)

Lagragian orbit number 1. A location on the Earth/ Sun line where gravational forces can be balanced to maintain a stable orbit. Approximately 1.5 million km upstream of the Earth. Solar wind monitors located there allow a 20-60 minute (depending on solar wind velocity) warning of geomagnetic disturbances at Earth.

Lake and marine alerts include all watches, warnings, and advisories that pertain to weather over lakes or oceans, such as rip current statements, gale watches and warnings, and small craft advisories.

Large Angle Spectrometric COronagraph experiment on SOHO capable of imaging CMEs from 1.1-32 solar radii.

With reference to the effect of energetic particles on spacecraft microcircuits, a serious type of single event upset in which the microcircuit is either permanently stuck or cannot be reset without being turned off and on.

See long duration event.

In a magnetically bipolar or multipolar sunspot group, the main spot in that portion of the group west of the principal inversion line; also called the preceding or p-spot. Leader and follower describe the positions of spots with respect to apparent motion due to solar rotation.

The time of Actual Sunset minus the time of Actual Sunrise. The change in length of daylight between today and tomorrow is also listed when available.

The time of Civil Sunset minus the time of Civil Sunrise.

A lens-shaped cloud that normally develops on the downwind side of a mountain or mountain range. This occurs when stable, moist air flows over a mountain, creating a series of oscillating waves. If the temperature at the crest of the wave equals the dewpoint temperature, condensation occurs in a lens formation. As the air falls town the trough of the wave, where the temperature and dewpoint temperature are not equal, then evaporation occurs. Thus, a "wave cloud", or a series of lenticular clouds, is capable of forming. These are often mistaken for UFO's because of the saucer-like shape. They can separate into altocumulus standing lenticular (ACSL), Stratocumulus standing lenticular (SCSL), and cirrocumulus standing lenticular (CCSL).

Among satellite operators, a common abbreviation for Low Earth Orbit.

See low frequency.

Observed in white light, a bright tongue or streaks penetrating or crossing sunspot umbrae. Light bridges typically develop slowly and have lifetimes of several days. The appearance of a light bridge is frequently a sign of impending active region division or dissolution. The more brilliant forms occur with overlying bright plage and often occur during the most active phase of the sunspot group.

A plot of intensity in a particular wavelength or band of wavelengths against time, especially with reference to a solar flare. For example, the time history of the x-ray output of a flare.

The edge of the solar disk.

For certain solar spectral lines, a lessening of the intensity of the line from the center of the solar disk to the limb, caused by the existence of a temperature gradient in the Sun and the line-of-sight through the solar atmosphere. Limb darkening also occurs in some radio wavelengths.

A flare at the edge (limb) of the solar disk; the elevated portions of the flare are seen with particular clarity against the dark sky background.

Active live blogs will automatically load new content as it arrives.

Inactive live blogs mean that the event has passed or the author is no longer covering it live. New content will not be loaded automatically.

In the magnetotail, the two regions (north and south) separated by the neutral sheet.

With reference to x-ray events, those events that are not impulsive in appearance. The exact time threshold separating impulsive from long-duration events is not well defined, but operationally, any event requiring 30 minutes or more to decay to one-half peak flux is regarded as an LDE. It has been shown that the likeliihood of a coronal mass ejection increases with the duration of an x-ray event, and becomes virtually certain for durations of 6 hours or more.

That component of the solar magnetic field vector parallel to the direction of view, radial from the solar surface at disk center.

A system of prominences in the form of loops associated with major flares, bridging the magnetic inversion line. The lifetime of an LPS is a few hours. Loop prominences observed in H-alpha are distinctly brighter than other prominences, and material typically flows downward along both legs from condensation ”knots” near the top of the loop. Can be observed on the solar disk with good viewing conditions.

That portion of the radio frequency spectrum from 30 to 300 kHz.

The "maximum temperature" is the highest temperature measured during a specified period of time, most commonly per day (e.g. "today's high temperature"). Therefore, the "low maximum temperature record" is the lowest high temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

The "minimum temperature" is the lowest temperature measured during a specified period of time, most commonly per day (e.g. "today's low temperature"). Therefore, the "low minimum temperature record" is the lowest low temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

Low pressure systems are areas, or closed systems, of low pressure and are also known as troughs and cyclones. Low pressure systems are associated with counter-clockwise rotating air, in which at the surface the air moves toward the center, and away from the center at high levels. Thus, air is forced to rise in the center of low pressure systems. Low pressure systems are associated with active weather as this rising air allows for convection under the right atmospheric conditions.

The lowest frequency that allows reliable long-range HF radio communication by ionospheric refraction.

The ratio of the maximum frequency reflected once from an ionospheric layer over a 3000 km range to the critical frequency of the layer.

These clouds look like thin white ice-crystal puffs. Unlike other cirrus clouds, these have supercooler liquid water droplets. If these droplets contact the ice crystals, they will rapidly freeze and transform the cloud into cirrus stratus. Cumulus signifies convection, but the cloud is usually short lived and can produce ice or snow in the form of virga. Each individual cloud puff is termed a "cloudlet" and are as small as a finger when extending a hand to the sky. They are also called Mackerel clouds when the cloudlets are aligned in rows they have the appearance of a herringbone or mackerel.

A relatively smooth excursion of the H (horizontal) component (see geomagnetic elements) of the geomagnetic field away from a

In general, any identifiable parcel of solar wind. More specifically, a region of about 0.25 AU in radial dimension in which the magnetic field strength is high and the direction of one component of the magnetic field changes appreciably by means of a rotation nearly parallel to a plane. Magnetic clouds are one manifestation of coronal mass ejections in the interplanetary medium.

On Earth, analogous to geographic local time. MLT at a given location is determined by the angle subtended at the geomagnetic axis between the geomagnetic midnight meridian and the meridian that passes through the location. 15 degrees = 1 h. The geomagnetic meridian containing the sub-solar point defines geomagnetic local noon, and the opposite meridian defines geomagnetic midnight. (See geomagnetic field.)

See Mount Wilson magnetic classification.

A plot showing the amplitude of one or more vector components of a magnetic field versus space or time. Solar magnetograms are a graphic representation of solar magnetic field strengths and polarity.

The study of the dynamics of an electrically conducting fluid in the presence of a magnetic field.

The boundary surface between the solar wind and the magnetosphere, where the pressure of the Earth's magnetic field effectively equals the dynamic pressure of the solar wind.

An electric current sheet that more or less coincides with the magnetopause.

The region between the bow shock and the magnetopause, characterized by very turbulent plasma. For the Earth, along the Sun-Earth axis, the magnetosheath is about 2 Earth radii thick.

The magnetic cavity surrounding a magnetized body, carved out of the passing solar wind by virtue of the magnetic field, which prevents, or at least impedes, the direct entry of the solar wind plasma into the cavity.

The extension of the magnetosphere in the antisunward direction as a result of interaction with the solar wind. In the inner magnetotail, the field lines maintain a roughly dipolar configuration. At greater distances, the field lines are stretched into northern and southern lobes, separated by a plasmasheet. There is observational evidence for traces of the Earth’s magnetotail as far as 1000 Earth radii downstream.

These globular clouds usually form underneath the base of a cumulonimbus cloud and are associated with strong storms. These globular lobes clump together to form small patches that last only a few minutes, but can also cover the entire sky lasting up to a few hours. The name translates to mamma and means "mammary cloud" as they resemble the shape of a female breast.

The MSL charts show mean sea-level pressure (blue contours, 4 mb interval), 1000 to 500 mb thickness (yellow contours, 60 m interval), and accumulated precipitation (color fill, see colorbar for intervals). The NAM shows the next 6 hours; the GFS shows the previous 6 hours if the forecast time is even (e.g., 6, 12, 18, 24 hour forecasts), and the previous 3 hours if the forecast time is odd (e.g., 3, 9, 15, 21 hour forecasts; and the RUC shows the previous 1 hour for the forecast times 1, 2, 4, 5, 7, and 8, and the previous 3 hours for forecast times 3, 6, and 9.

Thin and wispy clouds blown by strong winds high in the atmosphere, pointing in the direction the of the air movement. They are comprised of fibrous ice crystals. The clouds appear in fair weather conditions and do not produce precipitation that reaches the surface, but small ice crystal can fall from them, creating fall streaks or a form of virga. Cirrus translates to "curl of hair". Another name for these clouds is Mare's tails, due to the light and wispy hook that is often seen.

An approximately 70-year period, centered near 1670, during which practically no sunspots were observed.

The "high temperature" is the highest temperature measured during a specified period of time, most commonly per day (e.g. "today's high temperature"). Therefore, the "maximum high temperature record" is the highest high temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

The "low temperature" is the lowest temperature measured during a specified period of time, most commonly per day (e.g. "today's low temperature"). Therefore, the "maximum low temperature record" is the highest low temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

The most rain measured in a specified period of time (e.g. daily, monthly, yearly).

The most snow measured in a specified period of time (e.g. daily, monthly, per storm).

The highest temperature during a specific period of time.

The highest frequency that allows reliable HF radio communication over a given ground range by ionospheric refraction. Frequencies higher than the MUF penetrate the ionosphere and become useful for extraterrestrial communications.

See mound prominence.

The movement of the Earth around the Sun is an ellipse. However, if the movement of the Earth around the Sun were a circle, it would be easy to calculate its position. Since, the Earth moves around the sun about one degree per day, (in fact, it's 1/365.25 of the circle), we say the Mean Anomaly of the Sun is the position of the Earth along this circular path. The True Anomaly of the Sun is the position along its real elliptical path.

Mean sea-level pressure is a pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level. The calculation corrects for the altitude difference from the pressure reading at the station elevation, to what it would otherwise be at sea-level. Land elevation affects the pressure reading at the surface. For example, a station on a hill may read a lower pressure than it would read if the station was at the same point and there was no hill. High pressure and low pressure systems are based on mean sea-level pressure to keep them comparable at any geographic location. Thus, surface pressure is different that mean sea-level pressure as it has not yet been corrected for the difference in altitude from sea-level.

The average of a series of temperatures taken over a specific period of time, such as an evening, a day or month.

That portion of the radio frequency spectrum from 0.3 to 3 MHz.

The region of the Earth’s atmosphere between the upper limit of the stratosphere (approximately 30 km altitude) and the lower limit of the thermosphere (approximately 80 km altitude).

See magnetohydrodynamics.

See pulsation.

radiowave signal sometimes associated with optical and/or x-ray flares. Microwave bursts are generally broadband, often extending into the millimeter and decimeter domains. (See also U-burst.)

Generically, any radio frequency of 500 MHz or more.

With reference to zones of geomagnetic activity, 20 degrees to 50 degrees geomagnetic latitude. Other zones are equatorial, polar, and high latitude.

The "high temperature" is the highest temperature measured during a specified period of time, most commonly per day (e.g. "today's high temperature"). Therefore, the "minimum high temperature record" is the lowest high temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

The "low temperature" is the lowest temperature measured during a specified period of time, most commonly per day (e.g. "today's low temperature"). Therefore, the "minimum low temperature record" is the lowest low temperature on record, measured in a specified period of time (e.g. daily, monthly, all-time).

The least rain measured in a specified period of time (e.g. daily, monthly, yearly).

The lowest temperature during a specific period of time.

Small droplets suspended in air near the surface. Droplets are large enough to feel when the air is moving 1 meter per second and is usually associated with stratus clouds. The difference between mist and fog is visibility. If visibility is less than 1 km, then it is fog, otherwise it is called mist.

Record extremes measured within a specific calendar month.

A wave disturbance (also known as a flare blast wave) generated by large flares, which is seen to propagate horizontally across the disk of the Sun at a typical velocity of about 1000 km/s. Its presence is more visible in wings of the H-alpha line.

When the 6/8th to 7/8ths of the sky is covered by with opaque (not transparent) clouds. Same as Considerable Cloudiness.

When the 1/8th to 2/8ths of the sky is covered by with opaque (not transparent) clouds Same as Mostly Clear, except only applicable during daylight hours.

H-alpha structure at the solar limb that is the elevated top of numerous small surges and/or a dense, low-lying prominence.

Classification of the magnetic character of sunspots according to rules set forth by the Mount Wilson Observatory in California. alpha (a). A unipolar sunspot group. beta (B). A sunspot group having both positive and negative magnetic polarities (bipolar), with a simple and distinct division between the polarities. gamma (G). A complex active region in which the positive and negative polarities are so irregularly distributed as to prevent classification as a bipolar group. beta-gamma (BG). A sunspot group that is bipolar but which is sufficiently complex that no single delta (D). A qualifier to magnetic class (see below) indicating that umbrae separated by less than 2 degrees within one penumbra have opposite polarity. beta-delta (BD). A sunspot group of general beta magnetic classification but containing one (or more) delta spot(s). beta-gamma-delta (BGD). A sunspot group of beta-gamma magnetic classification but containing gamma-delta (GD). A sunspot group of gamma magnetic classification but containing one or more delta spot(s). multipath. Describing a degraded condition of radio propagation in which the radio wave splits and arrives at the receiver via different paths. Because each path will generally have different lengths, arrival times, and phases, the signal received will suffer fading.

The time period when the sun is between 6 and 12 degrees below the horizon at either sunrise or sunset. The horizon is well defined and the outline of objects might be visible without artificial light. Ordinary outdoor activities are not possible at this time without extra illumination.

The line that separates solar magnetic fields of opposite polarity, typically determined from solar magnetograms recording the longitudinal magnetic component.

A ground-based detector that counts secondary neutrons generated by processes originating with the impact of atmospheric molecules and atoms by very energetic particles (galactic or solar cosmic rays).

A dark grey low-layered cloud, 'wet' or dirty looking, and is capable of precipitating rain or snow, or a combination thereof. Precipitation associated with this cloud is continuous and light to moderate, and this cloud often brings low visibility.

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A shorthand classification scheme developed to convey to the general public the complex and often confusing levels of disturbances in the solar-terrestrial environment. Three typical events are represented as: Geomagnetic Storms (G), Solar Radiation Storms (S), and Radio Blackouts (R). A numerical qualifier is added to the event type to indicate the severity of the disturbance. These qualifiers are defined as follows: 1 Minor 2 Moderate 3 Strong 4 Severe 5 Extreme

Located in the mesosphere, these are as the highest clouds in the Earth's atmosphere. These clouds are usually too faint to be seen, but most often appear on summer nights between the latitude of 50 and 70 degrees. They are made up of crystals and water ice and originate from bigger and higher clouds called polar mesospheric clouds.

A transient enhancement of solar radio emission, particularly near 250 MHz, consisting of an elevated background emission. These storms may last hours to days.

Describing a degraded condition of radio propagation caused by horizontal gradients in ionospheric electron density. The radio wave is refracted away from its normal great-circle path, which is the shortest distance between two points on Earth. Strong horizontal gradients are associated with the equatorward boundary of the auroral oval (especially in the night sector) and the sunrise terminator.

A natural occurring display of lights observed in the high latitudes of the polar regions on the globe, but are also often seen as far as 65-72 degrees north and south. The chance for seeing the northern lights increases as you go closer to the North Magnetic Pole. If near the magnetic pole, they can be seen overhead, but from further distances they illuminate the northern horizon with a greenish or yellowish color. It is strongest during the equinoxes, or when the earth is at its greatest tilt. This phenomena occurs when photons are emitted into the ionosphere from ionized nitrogen atoms. They are ionized, or excited, by strong solar wind in the vicinity of Earth's magnetic field lines.

A natural occurring display of lights observed in the high latitudes of the polar regions on the globe, but are also often seen as far as 65-72 degrees north and south. The chance for seeing the northern lights increases as you go closer to the North Magnetic Pole. If near the magnetic pole, they can be seen overhead, but from further distances they illuminate the northern horizon with a greenish or yellowish color. It is strongest during the equinoxes, or when the earth is at its greatest tilt. This phenomena occurs when photons are emitted into the ionosphere from ionized nitrogen atoms. They are ionized, or excited, by strong solar wind in the vicinity of Earth's magnetic field lines.

The image does not focus on weather or was not shot outdoors.

The image is obscene or not appropriate for a weather and outdoor photography site.

nanotesla = 10E-9 Tesla

Obliquity is the angle between a planet's equatorial plane and its orbital plane.

Why is the sunset sometimes beautiful colors of red, orange, and pink? The answer is Rayleigh scattering, which is the scattering of light on various atmospheric components such as gases, dust, soot, ashes, pollen, and salt from the oceans. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue. However, when the sun begins to set, the light has to travel farther before we see it and is able to reflect and scatter more colors. The sun color appears more orange-red because the shorter wavelengths (blues and greens) are scattered, and the longer wavelengths are absorbed (reds, oranges, and pinks).

Why is the sunset sometimes beautiful colors of red, orange, and pink? The answer is Rayleigh scattering, which is the scattering of light on various atmospheric components such as gases, dust, soot, ashes, pollen, and salt from the oceans. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue. However, when the sun begins to set, the light has to travel farther before we see it and is able to reflect and scatter more colors. The sun color appears more orange-red because the shorter wavelengths (blues and greens) are scattered, and the longer wavelengths are absorbed (reds, oranges, and pinks).

When the sky is completely covered by an obscuring phenomenon. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.

See solar coordinates.

See leader spot.

See particle flux unit.

1 p cmE-2 sE-1 srE-1

Between 3/8 and 5/8 of the sky is covered by clouds.

Between 3/8 and 5/8 of the sky is covered by clouds. The term "Partly Sunny" is used only during daylight hours.

See polar cap absorption.

The sunspot area that may surround the darker umbra or umbrae. In its mature form it consists of linear bright and dark elements radial from the sunspot umbra.

That point on the orbit of an Earth-orbiting satellite nearest to the Earth. Compare apogee.

That point on the orbit of a Sun-orbiting body nearest to the Sun. Compare aphelion.

Continuation of existing conditions. When a physical parameter varies slowly, the best prediction is often persistence.

An unintended spacecraft command caused by the natural environment. (See single event upset or electrostatic discharge.)

The lowest visible layer of the solar atmosphere; corresponds to the solar surface viewed in white light. Sunspots and faculae are observed in the photosphere.

A small horizontal cloud that appears above a cumulus or cumulonimbus cloud and looks like a hood or cap made up of ice crystals. They are formed when strong updrafts occur in a convective tower push a dome-shaped air up above the cloud. The moisture in the dome condenses quickly into an ice fog, and if the ambient air is too dry, the pileus cloud will not form. Pilei can also form over ash clouds and pyro-cumulus clouds. Pileus translates to "with piles".

Why is the sunset sometimes beautiful colors of red, orange, and pink? The answer is Rayleigh scattering, which is the scattering of light on various atmospheric components such as gases, dust, soot, ashes, pollen, and salt from the oceans. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue. However, when the sun begins to set, the light has to travel farther before we see it and is able to reflect and scatter more colors. The sun color appears more orange-red because the shorter wavelengths (blues and greens) are scattered, and the longer wavelengths are absorbed (reds, oranges, and pinks).

Why is the sunset sometimes beautiful colors of red, orange, and pink? The answer is Rayleigh scattering, which is the scattering of light on various atmospheric components such as gases, dust, soot, ashes, pollen, and salt from the oceans. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue. However, when the sun begins to set, the light has to travel farther before we see it and is able to reflect and scatter more colors. The sun color appears more orange-red because the shorter wavelengths (blues and greens) are scattered, and the longer wavelengths are absorbed (reds, oranges, and pinks).

In a plasma, the angle between the velocity vector of a charged particle and the direction of the ambient magnetic field.

On the Sun, an extended H-alpha emission feature of an active region that is seen from the time of emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background. Magnetic fields are more intense in plage, and temperatures are higher than in surrounding, quiescent regions.

A low-intensity division in plage coinciding with a polarity inversion line and marked by narrow filament segments and/or fibrils spanning the corridor.

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A gas that is ionized sufficiently to be a good electrical conductor and be affected by magnetic fields.

The characteristic frequency of free plasma oscillations, determined by the balance between electron kinetic energy and ion Coulomb attraction.

The outer boundary of the plasmasphere.

In the magnetosphere, a region of relatively cool (low energy) and dense plasma that may be considered an outer extension of the ionosphere with which it is coupled. Like the ionosphere, the plasmasphere tends to corotate with the Earth.

In the magnetosphere, the core of the magnetotail in which the plasma is hotter and denser than in the tail lobes north and south of it. The plasmasheet is thought to be separated from the tail lobes by the sheet of the ”last closed field lines” and it typically lies beyond geosynchronous orbit.

An anomalous condition of the polar ionosphere where HF and VHF (3-300 MHz) radiowaves are absorbed, and LF and VLF (3-300 kHz) radiowaves are are reflected at lower altitudes than normal. PCAs generally originate with major solar flares, beginning within a few hours of the event and maximizing within a day or two of onset. As measured by a riometer, the PCA event threshold is 2 dB of absorption at 30MIlz for daytime and 0.5 dB at night. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. However, the transpolar radio paths may be disturbed for days, up to weeks, following the end of a proton event.

A nearly continuous ring of filaments occasionally encircling either polar region of the Sun (latitudes higher than 50’).

Fine, ray-like structures of the solar corona and transition region, best observed in the solar polar regions during solar minimum.

In the Earth’s upper atmosphere, a weak, structureless, near-isotropic flux of electrons precipitating into the polar caps.

The forecast temperature is usually more than 3.5 degrees C warmer or cooler than the observed forecast.

A feature in the photosphere, 1 to 3 arc seconds in extent, usually not much darker than the dark spaces between photospheric granules. It is distinguished from a sunspot by its short lifetime, 10 to 100 minutes.

A loop prominence system often seen after a major two-ribbon flare on the visible disk. Lifetimes are several hours.

The force exerted by the weight of the atmosphere and gravity. Different units: atmospheres (atm), millibars (mb), pascals (Pa), inches of mercury (in), pounds per square inch (PSI), etc. Meteorologists most often use mb.

The amount of pressure change at any one location, either increasing or decreasing, during a specific period of time. This is usually observed in three-hour intervals and can be recorded as pressure rising, pressure falling, pressure steady, or pressure unsteady. Also known as pressure characteristic or pressure tendency.

The amount of pressure change at any one location, either increasing or decreasing, during a specific period of time. This is usually observed in three-hour intervals and can be recorded as pressure rising, pressure falling, pressure steady, or pressure unsteady. Also known as pressure tendency or pressure change.

A decrease in pressure during a specific interval and any one location. Pressure falling rapidly refers to a decrease in pressure at a rate of 0.06 inches of mercury or more per hour. This signifies a weather system with lower pressure is approaching.

The amount of pressure change over a given horizontal distance.

A three-dimensional force that accelerates air parcels, in a form of air movement or wind, from areas of high pressure to areas of low pressure.

An increase in pressure during a specific interval and any one location. Pressure rising rapidly refers to an increase in pressure at a rate of 0.06 inches of mercury or more per hour. This signifies a weather system with higher pressure is approaching.

There is little or no change in pressure at any one location in any three-hour observation interval. This signifies a relatively unchanged or stagnant weather pattern.

The amount of pressure change at any one location, either increasing or decreasing, during a specific period of time. This is usually observed in three-hour intervals and can be recorded as pressure rising, pressure falling, pressure steady, or pressure unsteady. Also known as pressure characteristic or pressure change.

A pressure that fluctuates by 0.03 inches of mercury or more from the mean pressure during the period of observation, which is usually a three-hour interval.

An alert issued by a Regional Warning Center to give rapid notification of significant solar or geophysical activity in progress or just concluded.

The direction from which the wind blows most frequently in any location.

A term identifying cloud-like features in the solar atmosphere. The features appear as bright structures above the solar limb and as dark filaments when seen projected against the solar disk. They are most clearly and most often observed in H-alpha.

The measurement of proton flux reaching and sustaining ~ 10 p.f.u. (1 p.f.u. = 1 particle cm-2 s-l sr-1) for at least 15 min. at energies > 10 MeV by the primary SEC geosynchronous satellite. The start time of the event is defined as the earliest time at which event thresholds have been reached. The end time is the last time 10 p.f.u. was observed. This definition allows multiple injections from flares and interplantary shocks to be encompassed by a single event.

Any flare producing significant counts of protons with energies exceeding 10 MeV in the vicinity of the Earth.

A narrative statement that can be used for non-hazardous events (e.g. sun dogs, halos, rainbows, aurora borealis), public educational information and activities, and information regarding service changes, service interruptions, or special information clarifying radar imagery from an unusual source (e.g. smoke, birds, bugs) that may be mistaken for precipitation.

A rapid fluctuation of the geomagnetic field having periods from a fraction of a second to tens of minutes and lasting from minutes to hours. There are two main patterns: Pc (a continuous, almost sinusoidal pattern), and Pi (an irregular pattern). Pulsations occur at magnetically quiet as well as disturbed times.

A dense convective cloud that develops above wild land and grass land fires, as well as out of control prescribed fires. Strong heating at the surface allows for warm air to rise from the surface (convection) that would not otherwise occur without the presence of the fire. The majority of the smoke gets trapped below a stable layer in the atmosphere, however, this rising air can be so buoyant that it rises beyond the stable layer, producing a cauliflower-like high-level cloud. When these clouds form over fires, it signifies a raging fire with strong wind gusts at the fire front which help to strengthen the fire.

A I5-minute index of geomagnetic activity for high-latitude (auroral) stations. After quiet diurnal variations are removed, Q is the largest deviation scaled from the undisturbed level for the two horizontal components. (This differs from the K index, which is scaled from the largest relative deviation.) The I5-minute periods are centered on the hour and at 15, 30, and 45 minutes past each hour. The range of Q is from 0 to 11; the upper limit, in nanotesla, for each index value is given after the dash. Q = 0-10, 1-20, 2-40, 3-80, 4-140, 5-240, 6-400, 7-660, 8-1000, 9-1500, 10-2200, 11-Unlimited

See quiet day curve.

A long, sheet-like prominence nearly vertical to the solar surface. Except in an occasional activated phase, shows little large-scale motion, develops very slowly, and has a lifetime of several solar rotations. Quiescent prominences form within the remnants of decayed active regions, in quiet areas of the Sun between active regions, or at high solar latitudes where active regions seldom form. (See filament.)

A descriptive word specifically meaning (1) a probability of less than 50% for a C-class flare (see x-ray flare class) in a sunspot region and (2) geomagnetic activity levels such that Ak < 8.

Especially in connection with the components of the geomagnetic field (see geomagnetic elements), the trace expected in the absence of activity. The K index and Q index are measured from deviations relative to a QDC. Riometer and neutron monitor deviations are also measured relative to a QDC.

See sunspot number.

Radar returns from electron density irregularities in auroral regions. The strength of radar auroral returns is aspect dependent.

Regions of the magnetosphere roughly 1.2 to 6 Earth radii above the equator in which charged particles are stably trapped by closed geomagnetic field lines. There are two belts. The inner belt is part of the plasmasphere and corotates with the Earth; its maximum proton density lies near 5000 km. Inner belt protons are mostly high energy (10-50 MeV range) and originate from the decay of secondary neutrons created during collisions between cosmic rays and upper atmospheric particles. The outer belt extends on to the magnetopause on the sunward side. The altitude of maximum proton density is near 16,000-20,000 km. Outer belt protons are lower energy (about 200 eV to 1 MeV) and come from the solar wind. The outer belt is also characterized by highly variable fluxes of energetic electrons. The radiation belts are often called the ”Van Allen radiation belts” because they were discovered in 1968 by a research group at the University of Iowa led by Professor J. A. Van Allen.

Fog created by radiational cooling of the ground and the air just above the ground. Usually occurs on clear and humid nights, when the ground cools quickly. Due to the high humidity, the temperature only needs to drop slightly to reach the dew point temperature, and condenses. Little to no winds allow for a deeper fog layer. Tule fog gets its name from radiational fog developing in the Tule Valley of California.

A measure of the severity of solar x-ray bursts that cause radio blackouts at Earth. (See Appendix A).

See radio emission.

Emission of the Sun in radio wavelengths from centimeters to dekameters, under both quiet and disturbed conditions. Some patterns, known variously as noise storms, bursts, and sweeps, are identified as described below. These types of emission are subjectively rated on an importance scale of 1 to 3, 3 representing the most intense.

Precipitation that falls to earth in drops more than 0.5 mm in diameter.

A rainbow is both an optical and meteorological phenomenon. If the atmosphere has sufficient moisture, or vapor droplets in the air, sunlight shinning on it will refract when it enters the droplet, then reflect off the back of the drop, and refracts again as it leaves the drop. This splits up the light into many different angles, from shortest wavelengths on the inside (blues) to longest wavelengths on the outside (reds). The result is a band of light in an arc shape that includes all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet.

Any station uploading in Rapid Fire™ mode sends observations to Wunderground as often as every 2.5 seconds. When any web site visitor opens a Rapid Fire™ weather page, they can watch weather conditions change as they happen in real-time.

A fluted or ripple-like instability that can develop on a fluid or plasma boundary surface and propagate along it. This instability is often invoked to explain phenomena in the ionosphere and magnetosphere.

A process by which differently directed field lines link up, allowing topological changes of the magnetic field to occur, determining patterns of plasma flow, and resulting in conversion of magnetic energy to kinetic and thermal energy of the plasma. Reconnection is invoked to explain the energization and acceleration of the plasmas that are observed in solar flares, magnetic substorms, and elsewhere in the solar system.

A weather observation that exceeds all previous observations within its category, and is officially marked as an extreme event (e.g. a maximum rainfall record, or an all-time high temperature record).

A non-routine statement to report record-setting meteorological and hydrological events that equal or exceed existing records.

Used especially to express a tendency of some solar and geophysical parameters to repeat a trend and sometimes the actual value of the parameter itself every 27 days (the approximate rotation period of the Sun).

An intense coronal emission line at 637.4 nm from Fe X. It identifies relatively cooler regions of the corona.

Why is the sunset sometimes beautiful colors of red, orange, and pink? The answer is Rayleigh scattering, which is the scattering of light on various atmospheric components such as gases, dust, soot, ashes, pollen, and salt from the oceans. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue. However, when the sun begins to set, the light has to travel farther before we see it and is able to reflect and scatter more colors. The sun color appears more orange-red because the shorter wavelengths (blues and greens) are scattered, and the longer wavelengths are absorbed (reds, oranges, and pinks).

Why is the sunset sometimes beautiful colors of red, orange, and pink? The answer is Rayleigh scattering, which is the scattering of light on various atmospheric components such as gases, dust, soot, ashes, pollen, and salt from the oceans. By looking at rainbows, we can see all the colors that make up visible light: red, orange, yellow, green, blue, indigo, and violet. The shortest wavelengths, or highest frequencies (blues), are absorbed more often than the longest wavelengths, or lowest frequencies (reds). The colors that are absorbed are what we see, thus, the sky is most often blue. However, when the sun begins to set, the light has to travel farther before we see it and is able to reflect and scatter more colors. The sun color appears more orange-red because the shorter wavelengths (blues and greens) are scattered, and the longer wavelengths are absorbed (reds, oranges, and pinks).

A double rainbow is similar to a single rainbow in that it is both an optical and meteorological phenomenon, but the double rainbow portrays the colors in reverse. Thus, the outer band of red is on the inner band on the second rainbow that forms on the outside. It is a mirror image of the original rainbow, as it reflects off of a body of water. Often under extremely moist atmospheric conditions, the body of water that is reflecting the rainbow is the abundant water droplets in the air. In the case of a triple rainbow, the colors are reversed once again to that of the original rainbow.

A double rainbow is similar to a single rainbow in that it is both an optical and meteorological phenomenon, but the double rainbow portrays the colors in reverse. Thus, the outer band of red is on the inner band on the second rainbow that forms on the outside. It is a mirror image of the original rainbow, as it reflects off of a body of water. Often under extremely moist atmospheric conditions, the body of water that is reflecting the rainbow is the abundant water droplets in the air. In the case of a triple rainbow, the colors are reversed once again to that of the original rainbow.

A number assigned by SEC to a plage region or sunspot group if one of the following conditions exists: (1) the region is a group of at least sunspot classification C; (2) two or more separate optical reports confirm the presence of smaller spots; (3) the region produces a solar flare; (4) the region is clearly evident in H-alpha and exceeds 5 heliographic degrees in either latitude or longitude. (See also active region.)

A functional relationship between two or more correlated variables that is often empirically determined from data and is used especially to predict values of one variable when values of the others are given.

The amount of water vapor in the air, expressed as a percentage. Relative humidity is the ratio of the amount of moisture in the air to the amount that is needed to saturate the air. Thus, it is a function of both moisture content and temperature, as its name states, humidity is "relative" to temperature.

The international standard relative sunspot number.

High pressure systems are areas, or closed systems, of high pressure and are also known as ridges and anti-cyclones. High pressure systems are associated with clockwise rotating air, in which at the surface the air moves away from the center, and toward the center at high levels. Thus, air is forced to sink in the center of high pressure systems. High pressure systems are associated with dry and clear, fair weather conditions. However, in urban areas with high levels of pollution at the surface, sinking air associated with high pressure can act to trap pollutants, allowing for poor air quality conditions.

The angular distance measured eastward along the celestial equator from the vernal equinox. It is expressed in hours, minutes, and seconds (the circumference of the celestial equator is defined as 24 hours).

The Celestial Sphere is a sphere where we project objects in the sky. We project stars, the moon, and sun, on to this imaginary sphere. The Right Ascension of the Sun is the position of the sun on our Celestial Sphere.

This is not a form of precipitation. Ice deposits in the form of icy feathers, pointing into the wind. This occurs when super cooled cloud or fog droplets come in contact with an object and freeze immediately. Glaze is difference from rime in that it is more ice-cube like in appearance and clings to the object on which it was formed. Rime is more milky and crystalline, resembling sugar, and extends from the object on which it formed.

In the magnetosphere, a region of current that flows from east to west in a disk-shaped region near the geomagnetic equator in the outer of the Van Allen radiation belts. The current is produced by the gradient and curvature drift of the trapped charged particles. The ring current is greatly augmented during magnetic storms because of the hot plasma injected from the magnetotail. This increase in the ring current causes a worldwide depression of the horizontal geomagnetic field during a magnetic storm.

(Relative Ionospheric Opacity meter). A specially designed ground-level radio receiver for continuous monitoring of cosmic noise. The absorption of cosmic noise in the polar regions is very sensitive to the solar low-energy cosmic ray flux. Absorption events are known as PCAs (polar cap absorption) and are primarily associated with major solar flares.

A low, horizontal, tube-shaped and rare type of arcus clouds. This cloud differs from shelf clouds because they are completely detached from any other cloud and appears to be rolling. The most frequent and famous roll cloud is the Morning Glory cloud in Queensland, Australia.

A type of sunspot penumbra characteized by granular (rather than filamentary) structure, brighter intensity than the umbra, and narrow extent, and possibly only partially surrounding the umbra. Penumbrae are typically rudimentary during the sunspot formative and decay phases.

The slowly varying (weeks or longer) fluctuation observed in solar radio emission at microwave frequencies (wavelengths from 3-100 cm).

Radio frequencies between 1.55 and 5.20 GHz. For satellite communication. The term usually refers to frequencies used for Earth-space communication near 2.2 GHz.

Solar flux unit. 10 E -22 W 10E-2 m 10E-1 Hz = 10,000 Jansky.

This type of fog requires horizontally moving air, or air that is advecting horizontally from one place to another. When warm and moist air blows over a cold surface, the surface cools the air. Once the air temperature cools enough to equal the dewpoint temperature, condensation is formed and creates a blanket-like thick fog formation. This describes the classic fog that spreads over Golden Gate Bridge in San Francisco.

The usually undesirable response of spacecraft systems to variations in the space environment. High energy particles cause detector noise and/or physical damage to solar cells, electronics, and memory devices (single event upsets or ”bitflips”). Large and varying low-to-medium energy particle fluxes can result in a charge buildup between spacecraft components especially during the eclipse season and during spacecraft maneuvers. Atmospheric drag on spacecraft below approximately 1,000 km can increase during geomagnetic storms, resulting in cross-track and in-track orbit errors and orientation problems. Various communication interference problems result during solar radio bursts from flares when the Sun is within the field of view of the ground tracking dish. Ionospheric irregularities during geomagnetic storms can cause radio telemetry scintillation and fading.

See sudden commencement.

A small horizontal cloud that appears above a cumulus or cumulonimbus cloud and looks like a hood or cap made up of ice crystals. They are formed when strong updrafts occur in a convective tower push a dome-shaped air up above the cloud. The moisture in the dome condenses quickly into an ice fog, and if the ambient air is too dry, the pileus cloud will not form. Pilei can also form over ash clouds and pyro-cumulus clouds. Pileus translates to "with piles".

Sky condition when between 1/10 and 5/10 are covered.

Describing a degraded condition of radio propagation characterized by a rapid variation in amplitude and/or phase of a radio signal (usually on a satellite communication link) caused by abrupt variations in electron density anywhere along the signal path. It is positively correlated with spread F and to a lesser degree, sproadic E. Scintillation effects are the most severe at low latitudes, but can also be a problem at high latitudes, especially in the auroral oval and over the polar caps.

The most localized form of fog, usually forming over lakes and rivers, sometime oceans, when the water is warmer than the air above it. Moisture evaporates from the water and saturates the adjacent layer of air and condenses. This air rises, it evaporates into the dryer air aloft, thus, giving the appearance of a low layer of steam above the water.

Record extremes measured for a specific season (e.g. coldest Winter or warmest Summer).

In the solar wind, the area of demarcation between sectors. which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward the Sun (a negative sector), or away from the Sun (a positive sector). The sector boundary separating fields of opposite polarity is normally narrow, passing the Earth within minutes to hours as opposed to the week or so needed for passage of a typical sector. The solar wind velocities in the boundary region are typically among the lowest observed.

See single event upset.

A severe thunderstorm warning is issued when a storm with any of these severe weather criteria is approaching the warning area:

• Hail 3/4 of an inch or greater
• Winds greater than 58 mph

Severe thunderstorms can and do produce tornadoes.

A severe thunderstorm watch is issued when there is a possibility that thunderstorms in and near the watch box area may produce the following severe weather conditions:

• Hail 3/4 of an inch or greater
• Winds greater than 58 mph

Provides follow-up information on severe weather conditions (severe thunderstorms or tornadoes) which have occurred or are occurring.

Solar flare effect. (See crochet.)

A form of cloud to cloud lightning, the most common type of lightning, occurring inside one cumulonimbus cloud due to opposing charges within the cloud. This most frequently occurs when the upper portion of an anvil cloud reaches positive charge, and the middle remains under negative charge. This is often referred to as sheet lightning because it lights up the cloud and surrounding sky with light. Heat lightning is no different from cloud to cloud lightning, it is sometimes referred to as heat lightning when it is too far away for thunder to be heard.

A low, horizontal, wedge-shaped arcus cloud that is unlike the roll cloud in that is is attached to the base of another cloud, usually associated with a thunderstorm. This is often mistaken for a wall cloud. In general, a shelf cloud forms along the leading edge of a storm and a wall cloud forms on the back side of the storm. Wall clouds are inflow features associated with warm air and lean in towards the storm, while shelf clouds are outflow features associated with cool air and jut out away from the storms, often as a gust front.

See super high frequency.

A discontinuity in pressure, density, and particle velocity, propagating through a compressible fluid or plasma.

An abrupt decrease of HF radio signal strength, lasting from minutes to hours, caused by increased dayside ionization from some solar flares. A SWF is one effect under the broad category of sudden ionospheric disturbances (SIDs).

See sudden impulse.

See sudden ionospheric distrubance.

Referring to a coordinate system fixed with respect to the distant stars.

With reference to the effects of energetic particles on spacecraft microcircuits - an unexpected change in the logic state of a single digital bit. SEUs can be either ”soft” (the microcircuit is not damaged and can be rewritten to either state), or a latch up, which cannot easily be reset.

Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces.

An average of 13 monthly RI numbers, centered on the month of concern. The 1st and 13th months are given a weight of 0.5.

Precipitation in the form of ice crystals, mainly of intricately branched, hexagonal form and often agglomerated into snowflakes, formed directly from the freezing [deposition] of the water vapor in the air.

The vertical height of frozen precipitation on the ground. Data presented from up to three of the closest reporting stations.

Transient perturbations of the solar atmosphere as measured by enhanced x-ray emission (see x-ray flare class), typically associated with flares. Five standard terms are used to describe the activity observed or expected: Very low x-ray events less than C-class. Low - C-class x-ray events. Moderate - isolated (one to 4) M-class x-ray events. High - several (5 or more) M5 or greater x-ray events.

The total radiant energy received vertically from the Sun, per unit area per unit of time, at a position just outside the Earth’s atmosphere when the Earth is at its average distance from the Sun. Radiation at all wavelengths from all parts of the solar disk is included. Its value is approximately 2.00 cal cm-2 min-1 = 1.37 kW m-2 and varies slightly (by approximately 0.1 %) from day to day primarily in response to the apparent size of sunspots blocking emission from the photosphere.

Specifications for a location on the solar surface. The location of a specfic feature on the Sun (for example, a sunspot) is complicated by the fact that there is a tilt of 7.25 degrees between the ecliptic plane and the solar equatorial plane as well as a true wobble of the solar rotational axis. (Only twice a year are the solar north pole and the celestial north pole aligned.) Consequently, to specify a location on the solar surface, three coordinates (P, B, L) are necessary to define a grid. Daily values for the coordinates in Coordinated Universal Time (UTC) are listed in The Astronomical Almanac published annually by the U.S. Naval Observatory. The terms used to refer to the coordinates are defined as follows: P-angle: The position angle between the geocentric north pole and the solar rotational north pole measured eastward from geocentric north. The range in P is plus or minus 26.31°. Bo: Heliographic latitude of the central point of the solar disk; also called the B-angle. The range of B is plus or minus 7.23°, correcting for the tilt of the ecliptic with respect to the solar equatorial plane. Example: If (P, B) = (-26.21°, -6.54°), the heliographic latitude of the central point on the solar disk is -6.54° (the north rotational pole is not visible), and the angle between the projection onto the disk of the geocentric north pole and the solar north rotational pole is 26.21° to the west. L: Heliographic longitude of the central point of the solar disk. The longitude value is determined with reference to a system of fixed longitudes rotating on the Sun at a rate of 13.2 degrees/day (the mean rate of rotation observed from central meridian transits of sunspots). The standard meridian on the Sun is defined to be the meridian that passed through the ascending node of the Sun’s equator on 1 January 1854 at 1200 UTC and is calculated for the present day by assuming a uniform sidereal period of rotation of 25.38 days. Once P, B, and L are known, the latitude, central meridian distance, and longitude of a specific solar feature can be determined as follows: Latitude. The angular distance from the solar equator, measured north or south along the meridian. Central meridian distance (CMD). The angular distance in solar longitude measured from the central meridian. This position is relative to the view from Earth and will change as the Sun rotates. Therefore, this coordinate should not be confused with heliographic positions that are fixed with respect to the solar surface. Longitude. The angular distance from a standard meridian (0 degrees heliographic longitude), measured from east to west (0 to 360 degrees) along the Sun’s equator. It is computed by combining CMD with the longitude of the central meridian at the time of the observation, interpolating between ephermeris values (for 0000 UT) by using the synodic rate of solar rotation (27.2753 days, 13.2 degrees/day).

See sunspot cycle.

See crochet.

The month(s) during a sunspot cycle when the smoothed sunspot number reaches a maximum.

The month(s) during a sunspot cycle when the smoothed sunspot number reaches a minimum.

Solar Time is based on the motion of the sun around the Earth. The apparent sun's motion, and position in the sky, can vary due to a few things such as: the elliptical orbits of the Earth and Sun, the inclination of the axis of the Earth's rotation, the perturbations of the moon and other planets, and of course, your latitude and longitude of observation. Solar Noon is when the sun is at the highest in the sky, and is defined when the Hour Angle is 0°. Solar Noon is also the midpoint between Sunrise and Sunset.

A measure of the severity of solar proton events as depicted in the NOAA Space Weather Scales.

See radio emission.

(1) synodic: 13.39 degrees -2.7 degrees sinE2 J per day (J = solar latitude). (2) sidereal: 14.38 degrees -2. 7 degrees sinE2 J per day. The difference between sidereal and synodic rates is the Earth orbital motion of 0.985 degrees/day.

The boundary between large-scale unipolar magnetic regions on the Sun’s surface, as determined from inversion lines mapped using filaments and filament channels, or large-scale magnetograms. The supposed solar signature of an interplanetary sector boundary.

The outward flow of solar particles and magnetic fields from the Sun. Typically at 1 AU, solar wind velocities are near 375 km/s and proton and electron densities are near 5 cm-3. The total intensity of the interplanetary magnetic field is nominally 5 nT.

Full disk soft x-ray (0.6-6 nm) imager flown on many of the GOES geosynchronous weather satellites.

A point on the ecliptic where the Sun reaches its greatest absolute declination. There are two of these points, halfway between the equinoxes; they mark the beginning of summer and winter.

A region of the Earth centered near 25S and 50W (near the Atlantic coast of Brazil) of low geomagnetic field intensity owing to the fact that the geomagnetic field axis is offset from the center of the Earth. One consequence of the SAA is that trapped particles in the plasmasphere drift closer to the Earth’s surface and can more easily be lost into the atmosphere. The result is that the F region (see ionosphere) is highly variable in this region, and satellites in low Earth orbits suffer greater radiation doses when they pass through the SAA. There is a corresponding location of maximum geomagnetic field intensity in Southeast Asia.

A natural occurring display of lights observed in the high latitudes of the polar regions on the globe, but are also often seen as far as 65-72 degrees north and south. The chance for seeing the southern lights increases as you go closer to the South Magnetic Pole. If near the magnetic pole, they can be seen overhead, but from further distances they illuminate the northern horizon with a greenish or yellowish color. It is strongest during the equinoxes, or when the earth is at its greatest tilt. This phenomena occurs when photons are emitted into the ionosphere from ionized nitrogen atoms. They are ionized, or excited, by strong solar wind in the vicinity of Earth's magnetic field lines.

A natural occurring display of lights observed in the high latitudes of the polar regions on the globe, but are also often seen as far as 65-72 degrees north and south. The chance for seeing the southern lights increases as you go closer to the South Magnetic Pole. If near the magnetic pole, they can be seen overhead, but from further distances they illuminate the northern horizon with a greenish or yellowish color. It is strongest during the equinoxes, or when the earth is at its greatest tilt. This phenomena occurs when photons are emitted into the ionosphere from ionized nitrogen atoms. They are ionized, or excited, by strong solar wind in the vicinity of Earth's magnetic field lines.

A term that emcompasses all the charging effects on a spacecraft due to the environment in space. Occasionally this term is used in a more limited sense to mean surface charging.

A special weather statement is issued for hazards that have not yet reached warning or advisory criteria, or for hazards that do not have a specific advisory of their own.

Rapidly changing, predominantly vertical, spike-like structures in the solar chromosphere observed above the limb. Spicules appear to be ejected from the low chromosphere at velocities of 20 to 30 km/s, reaching a height of about 9000 km and then falling back or fading. The total lifetime is 5 to 10 minutes.

Transient, localized patches of relatively high electron density in the E region of the ionosphere, which significantly affect radiowave propagation. Sporadic E can occur during daytime or nighttime, and varies markedly with latitude. Es can be associated with thunderstorms, meteor showers, solar, and geomagnetic activity.

Luminous material ejected from a solar flare with sufficient velocity to escape the Sun (675 km/s). Sprays are usually seen in H-alpha with complex and rapidly changing form. There is little evidence that sprays are focused by magnetic fields.

A condition of the F region of the ionosphere caused by patches of ionization that scatter or duct radio signals, characterized on ionograms by a wide range of heights of reflected pulses. In equatorial latitudes spread F is most commonly observed at night and may be negatively correlated with geomagnetic activity. At high latitudes spread F occurs throughout the daytime and is positively correlated with magnetic activity. The latitude of minimum occurrence of spread F is near 30 degrees magnetic latitude.

See spray.

The diurnal variation of the geomagnetic field. The Sq variation is explained in terms of solar tidal motions of the ionosphere and thermally driven ionospheric winds.

See solar sector boundary.

See sudden commencement.

The most localized form of fog, usually forming over lakes and rivers, sometime oceans, when the water is warmer than the air above it. Moisture evaporates from the water and saturates the adjacent layer of air and condenses. This air rises, it evaporates into the dryer air aloft, thus, giving the appearance of a low layer of steam above the water.

Record extremes for the period of one storm. The storm period could vary from record to record.

A low layer of rolling grey patchy clouds that sometimes joins together, creating a continuous cloud. This often forms on the back side of the cold front, producing light rain or drizzle.

That region of the Earth’s atmosphere between the troposphere and the mesosphere. It begins at an altitude of temperature minimum at approximately 13 km and defines a layer of increasing temperature up to about 50 km.

A uniform low and layered cloud, often covering the entire sky and sometimes causes drizzle or mist, but usually does not bring rain. Status can be created when thick fog "lifts".

Pieces or shreds of stratus clouds that are not capable of producing precipitation.

A code word designating a major disturbance of the winter, polar, middle atmosphere from the tropopause to the ionosphere, lasting for several days at a time and characterized by a warming of the stratospheric temperature by some tens of degrees. There is no evidence that stratwarms are caused by solar events, or that they affect the lower atmosphere. The primary effect is upon HF propagation.

See flare.

A geomagnetic perturbation lasting 1 to 2 hours, which tends to occur during local post-midnight nighttime. The magnitude of the substorm is largest in the auroral zone, potentially reaching several thousand nanotesla. A substorm corresponds to an injection of charged particles from the magnetotail into the auroral oval.

An abrupt increase or decrease in the northward component (see geomagnetic elements) of the geomagnetic field, which marks the beginning of a geomagnetic storm. SCs occur almost simultaneously worldwide but with locally varying magnitudes.

A sudden perturbation, positive or negative, of several nanotesla in the northward component (see geomagnetic elements) of the low-latitude geomagnetic field, not associated with a following geomagnetic storm. (An SI becomes an SC if a storm follows.)

Any of several radio propagation anomalies due to ionospheric changes resulting from solar or geophysical events. Anomalies include short wave fades, enhancements of atmospherics, phase shifts, cosmic noise absorptions, and signal enhancements.

The Declination of the sun is how many degrees North (positive) or South (negative) of the equator that the sun is when viewed from the center of the earth. The range of the declination of the sun ranges from approximately +23.5° (North) in June to -23.5° (South) in December.

An optical phenomenon produced by ice crystals in cirrus clouds. The ice crystals in the upper troposphere refracts and reflects the light, and can sometimes split the light into colors, creating an arc or circle in the sky around the sun or moon.

An optical phenomenon produced by ice crystals in cirrus clouds. The ice crystals in the upper troposphere refracts and reflects the light, and can sometimes split the light into colors, creating an arc or circle in the sky around the sun or moon.

Parallel rays of sunlight that penetrate through holes in clouds as columns of sunlit air are divided by darker shaded regions. Perspective effects cause the apparent divergence, and the rays are visible due to reflection of sunlight off of the atmospheric particles. The name originates from their frequent crepuscular occurrence (at dawn and dusk), when the contrast between light and dark are greatest.

When there are no opaque (not transparent) clouds. Same as Clear.

An area seen as a dark spot, in contrast with its surroundings, on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere. Larger and darker sunspots sometimes are surrounded (completely or partially) by penumbrae. The dark centers are umbrae. The smallest, immature spots are sometimes called pores.

As devised by McIntosh, a 3-letter designation of the white-light characteristics of a sunspot group. The general form of the designation is Zpc. One letter is chosen from each of the following three categories. Z (the modified Zurich class of the group): A-A small single sunspot or very small group of spots with the same magnetic polarity, without penumbra. B-Bipolar sunspot group with no penumbra. C-An elongated bipolar sunspot group. One sunspot must have penumbra, and penumbra does not exceed 5 ° in longitudinal extent. D-An elongated bipolar sunspot group with penumbra on both ends of the group; longitudinal extent of penumbra is more than 5°, but does not exceed 10°. E-An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 10° but not 15°. F-An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 15°. H-A unipolar sunspot group with penumbra. Class H sunspot groups become compact Class D or larger when the penumbra exceeds 5 degrees in longitudinal extent. p (the penumbra type of the largest spot in the group): x-no penumbra r-rudimentary s-small (<2.5 degrees north-south diameter), symmetric a-small, asymmetric h-large (>2.5 degrees north-south diameter), symmetric k-large, asymmetric c (the compactness of the group): x-a single spot o-open i-intermediate c-compact

The approximately 11 year quasi-periodic variation in the sunspot number. The polarity pattern of the magnetic field reverses with each cycle. Other solar phenomena, such as the 10.7 cm solar radio emission, exhibit similar cyclical behavior.

A daily index of sunspot activity (R), defined as R = k(10g + s) where s = number of individual spots, g = number of sunspot groups, and k is an observatory factor (equal to 1 for the Zurich Observatory and adjusted for all other observatories to obtain approximately the same R number). The standard number, RI, once derived at Zurich (see Wolf number), is now being derived at Brussels and is denoted by RI. Often, the term ”sunspot number” is used in reference to the widely distributed smoothed sunspot number.

That portion of the radio frequency spectrum from 3 to 30 GHz.

A thunderstorm cloud or cumulonimbus that is strengthened by a strong continuously-rotating updraft, otherwise known as a mesocyclone. This is one of the four thunderstorm classifications: supercell, squall line, multi-cell, single-cell. Supercells are isolated from other storms and are capable of producing severe weather and tornadoes.

A system of large-scale velocity cells that does not vary significantly over the quiet solar surface or with phase of the solar cycle. The cells are presumably convective in origin with weak upward motions in the center, downward motions at the borders, and horizontal motions of typically 0.3 to 0.4 km/s. Magnetic flux is more intense along the borders of the cells.

The surface pressure is the pressure reading on a barometer, but has not been adjusted to mean sea-level pressure. Land elevation affects the pressure reading at the surface. For example, a station on a hill may read a lower pressure than it would read if the station was at the same point and there was no hill. High pressure and low pressure systems are based on mean sea-level pressure to keep them comparable at any geographic location. Thus, surface pressure is different than mean sea-level pressure as it has not yet been corrected for the difference in altitude from sea-level.

A jet of material from active regions that reaches coronal heights and then either fades or returns into the chromosphere along the trajectory of ascent. Surges typically last 10 to 20 minutes and tend to recur at a rate of approximately 1 per hour. Surges are linear and collimated in form, as if highly directed by magnetic fields.

The wind speed obtained by averaging the observed values over a 1-minute period.

See short wave fade.

Referring to a coordinate system fixed on the Earth.

A map of the whole Sun in absolute heliographic coordinates, displaying an integrated view of solar features observed during a Carrington rotation.

See total electron content.

The NOAA instrument used to estimate the hemispherical power input. (See estimated hemispherical power input.)

Temperature is a measure of the warmth or coldness of an object or substance with reference to a standard value. It can be measured in Kelvin (K), Fahrenheit (F), or Celsius (C).

A solar flare accompanied by a 10cm radio burst of intensity greater than 100% of the pre-burst value.

None needed. Sample.

That region of the Earth’s atmosphere where the neutral temperature increases with height. It begins above the mesosphere at about 80-85 km and extends to the exosphere.

A local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder.

Ability to change time.

A rotating column of air that stretches from the base of a cumulonimbus or towering cumulus cloud. This is the most intense atmospheric phenomena and takes the shape of a funnel cloud. A tornado develops from a funnel cloud with strong and violent rotation that extends entirely to the surface. Upon impact with the ground, strong winds associated with a tornado can kick up dust and debris, and cause great damage. Tornados can be mistaken for other rotating vortices such as: waterspouts, fire whirls (fire vortices), and dust devils.

A tornado warning is issued when a tornado has been spotted or indicated by radar.

A tornado watch is issued when thunderstorms in and near the watch box area may produce tornadoes.

The number of electrons along a ray path between a transmitter and a receiver. Units are electrons per square meter. This number is significant in determining ionospheric effects such as refraction, dispersion, and group delay on radio waves, and can be used to estimate critical frequencies. The TEC is strongly affected by solar and geomagnetic activity.

A large convective cumulus cloud with great vertical growth, usually taller than it is wide, due to its strong updrafts. Congestus is Latin for "piled-up" and usually is associated with precipitation and if instability is strong enough, cumulonimbus and thunderstorm clouds will develop. Most often these clouds are indicative for bad weather.

That region of the solar atmosphere lying between the chromosphere and the corona where the temperature rises from 10 E04 K to 10 E06 K. The transition region is only a few thousand kilometers thick.

Component of magnetic field vector perpendicular to the direction of view and parallel to the solar surface at disk center.

A tropical storm statement, or tropical storm local statement, provides more detailed information about how the storm will impact your area.

A Tropical Storm warning means tropical storm conditions are expected in the next 24 hours.

A Tropical Storm watch means tropical storm conditions are possible in the next 24 hours.

The lowest layer of the Earth’s atmosphere, extending from the ground to the stratosphere at approximately 13 km of altitude.

Low pressure systems are areas, or closed systems, of low pressure and are also known as troughs and cyclones. Low pressure systems are associated with counter-clockwise rotating air, in which at the surface the air moves toward the center, and away from the center at high levels. Thus, air is forced to rise in the center of low pressure systems. Low pressure systems are associated with active weather as this rising air allows for convection under the right atmospheric conditions.

Fog created by radiational cooling of the ground and the air just above the ground. Usually occurs on clear and humid nights, when the ground cools quickly. Due to the high humidity, the temperature only needs to drop slightly to reach the dew point temperature, and condenses. Little to no winds allow for a deeper fog layer. Tule fog gets its name from radiational fog developing in the Tule Valley of California.

This is the time before sunrise and after sunset where it is still light outside, but the sun is not in the sky.

A flare that has developed as a pair of bright strands (ribbons) on both sides of an inversion line of the solar magnetic field.

A noise storm composed of many short, narrow-band bursts in the meter wavelength range (300-50 MHz), of extremely variable intensity. The storm may last from several hours to several days.

Narrow-band bursts that sweep rapidly (seconds) from decimeter to dekameter wavelengths (500-0.5 MHz). They often occur in groups and are an occasional feature of complex solar active regions.

A smooth continuum of broad-band bursts primarily in the meter range (300-30 MHz). These bursts occur with some major flare events; they begin 10 to 20 minutes after flare maximum and can last for hours.

Narrow-band emission that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward dekameter wavelengths (10 MHz). Type II emissions occur in loose association with major flares and are indicative of a shock wave moving through the solar atmosphere.

Short-duration (a few minutes) continuum noise in the dekameter range usually associated with Type III bursts.

A radio noise burst associated with some flares. It has a U-shaped appearance in an intensity vs. frequency plot. The minimum intensity falls roughly between 500 and 2000 MHz. A U-burst is sometimes called a Castelli U.

See ultrahigh frequency.

That portion of the radio frequency spectrum from 300 MHz - 3 GHz.

That part of the electromagnetic spectrum between 5 - 400nm.

The dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.

See unipolar magnetic region.

The name of this cloud translates to rough or agitated waves. While these clouds give a dark and stormy appearance, but they have been known to dissipate before storm development. These have been spotted in the Plains of the US, forming after convective thunderstorm activity. This is a newly recognized cloud formation, proposed in 2009.

A large-scale photospheric region where the magnetic elements are predominantly of one polarity (for example, the solar polar regions).

A shortened form of the more correct Coordinated Universal Time (UTC).

With regard to geomagnetic activity, a descriptive word between quiet and active such that the Ak index is between 8 and 15.

See single event upsets.

This is the only fog that forms adiabatically. When humid air gradually moves up slope or up a hill, air expends and cools adiabatically, and if the temperature of the air drops to the dewpoint temperature, fog is produced.

See Coordinated Universal Time.

See ultraviolet.

See radiation belts.

The equinox that occurs in March. Compare autumnal equinox.

That portion of the radio frequency spectrum from 30 - 300 MHz. very low

That portion of the radio frequency spectrum from 3 - 30 kHz.

As rain falls from the cloud, it can evaporate before reaching the surface. This most frequently occurs in dry climates with dry surface conditions.

The greatest distance toward the horizon at which prominent objects can be identified with the naked eye.

Atmospheric volcanic ash clouds may endanger aviation.

Inflow features with (often warm) air moving towards them. This is often mistaken for a shelf cloud. In general, a shelf cloud forms along the leading edge of a storm and a wall cloud forms on the back side of the storm. Wall clouds are inflow features associated with warm air and lean in towards the storm, while shelf clouds are outflow features associated with cool air and jut out away from the storms, often as a gust front.

An intense vortex that forms over water, usually lakes. Often originates as a tornado that moves over a body of water. The can be destructive as they are associated with strong winds and can propagate up to 20 mph.

A handle is a screen name to be used all across wunderground.com, in our site messages, photo section and blog sections.

See white light.

A major flare in which small parts become visible in white light. This rare continuum emission is caused by energetic particle beams bombarding the lower solar atmosphere. Such flares are usually strong x-ray, radio, and particle emitters

Will you be making money off selling the use of weather, through subscription, app sales, or advertising.

Will you be making money off selling the use of weather, through subscription, app sales, or advertising.

Is the weather going to be embedded in a microchip that will be used within a mobile device or an in-car application.

Air in motion relative to the surface of the earth. Wind develops from pressure differences in the air. An area of high pressure and low pressure oppose each other and set up a pressure gradient force that moves from high to low pressure. This force creates a wind as air is pushed in the according direction. The stronger the pressure difference, the stronger the force, and the stronger the resultant wind. Wind is described as the prevailing direction of which the wind is blowing as a speed in units of miles per hour or knots.

Wind alerts include all advisories related to wind, such as high wind watches and warnings.

The direction that the wind is blowing from. It can be expressed in cardinal directions or from 0 to 360 degrees. North is 360 or 0 degrees, and South is 180 degrees.

A change of variation in wind speed and or direction in the horizontal or vertical. This term usually refers to vertical wind shear, or the change in wind with height, but can also represent horizontal wind shear, or the chance in wind with distance.

The rate at which air is moving horizontally past a point. It may be a 2-minute average speed, or an instantaneous speed.

The felt air temperature on exposed skin due to wind.

20 to 30 mph winds.

Portion of a spectroscopic absorption (or emission) line between the core of the line and the continuum adjacent to the line.

Winter weather alerts include various winter season phenomena such as winter storm warnings, blizzard warnings, and frost and freeze advisories.

A winter weather advisory is issued when significant accumulations of snow, sleet, or freezing rain may affect the advisory area.

An historic term for sunspot number. In 1849, R. Wolf of Zurich originated the general procedure for computing the sunspot number. The record of sunspot numbers that he began has continured to this day.

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Call letters of the radio station over which National Insititute of Standards and Tehnology broadcasts time-standard signals at 2.5, 5, 10, 15, and 20 MHz. Solar-terrestrial conditions and forecasts are broadcast at 18 minutes past the hour.

Designates those radio frequencies between 5.2 and 10.9 GIlz.

Radiation of extremely short wavelength (generally less than 1 nm).

A daily average background x-ray flux in the 0.1 to 0.8 nm range. It is a midday minimum given in terms of x-ray flare class.

A temporary enhancement of the x-ray emission of the Sun. The time-intensity profile of soft x-ray bursts is similar to that of the H-alpha profile of an associated flare.

Rank of a flare based on its x-ray energy output. Flares are classified by the SEC according to the order of magnitude of the peak burst intensity (I) measured at the Earth by satellites in the 0.1 to 0.8 nm band as follows: Peak, 0.1 to 0.8 nm flux (W m E-2) B I < 10E-6 C 10E-6 <= I < 10E-5 M 10E-5 <= I < 10E-4 X I I>= 10E-4 A multiplicative factor is appended to the end of the class (e.g. M8 = 8 x 10E-5 W mE-2)

The end time is defined as the time the flux has decayed to 1/2 the peak flux of the event.

Record extremes measured within a specific calendar year.

A coronal emission line at 569.4 nm from Ca XV. It identifies the hottest regions of the corona.

See geomagnetic elements.

The splitting of some solar spectral emission lines due to the presence of a strong magnetic field. Briefly, the lines split into three or more components of characteristic polarization. The components are circular if the local magnetic field is parallel to the line of sight, and linear if the field is perpendicular to the line of sight. The amount of splitting is proportional to the strength of the field.

See Coordinated Universal Time.

See sunspot classification.

See sunspot number.