Curious what the weather advisory really means? Learn what some of the most commonly used weather phrases mean & more!

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

Advection Fog—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.

All-Time Record—Record extremes that are the most extreme for any period of time.

Altitude—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.

Altocumulus—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.

Altostratus—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.

Anti-cyclone—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.

Anti-sunbeams—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.

Anvil cloud—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.

Anvil Crawler—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.

Arcus—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.

Asperatus—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.

Astronomical Twilight—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.

Atmospheric Pressure—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.

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

Aurora—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 patterns and forms of the aurora include quiescent arcs, rapidly moving rays, curtains, patches, and veils.

Aurora Australis—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.

Aurora Borealis—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.

Autumnal Equinox—The equinox that occurs in September.

Blizzard—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.

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

Cap cloud—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”.

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

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

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

Chromosphere—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.

Cirrocumulus—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.

Cirrostratus—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.

Cirrus—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.

Civil Twilight—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).

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

Cloud to Cloud—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.

Cloud to Ground Lightning—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.

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

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

Continuum Storm (CTM —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.

Convection—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.

Coordinated Universal Time (UTC)—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.

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

Cosmic Ray—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.

Cumulonimbus—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.

Cumulus—Puffy white or light grey clouds with a flat base and a sharp outline, often resembling a floating cotton balls. 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 Congestus—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.

Cumulus Humilis—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.

Cyclone—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.

Daily Record—Record extremes measured on a specific calendar day.

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

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

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

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

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

Double 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.

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

Dry Lightning—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.

Dust devil—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.

Eclipse—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.

Ecliptic—The great circle made by the intersection of the plane of the Earth’s orbit with the celestial sphere.

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

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

Equinox—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.

Evaporation Fog—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.

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

Exosphere—The Earth’s atmosphere above 500-600 km.

Extreme Ultraviolet (EUV)—A portion of the electromagnetic spectrum from approximately 10 to 100 nm.

Extremely High Frequency (EHF)—That portion of the radio frequency spectrum from 30-300 GHz.

Extremely Low Frequency (ELF)—That portion of the radio frequency spectrum from 30 to 3000 Hz.

Fahrenheit Temperature Scale—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.

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

Fair Weather Cloud—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.

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

Fire Cloud—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.

Fire Tornado—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.

Fire Vortex—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.

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

Fire whirl—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.

Flare—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.

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

Flood Watch / Flood Statement—A flood watch is issued when flooding is possible in the watch area.

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

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

Fog—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.

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

Freezing Fog—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.

Freezing Mist—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.

Frozen Fog—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.

Frozen Mist—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.

Frozen rain—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.

Funnel cloud—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.

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

Geomagnetic Activity—Natural variations in the geomagnetic field classified quantitatively into quiet, unsettled, active, and geomagnetic storm levels according to the observed a index.

Geomagnetic field—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).

Glaze—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.

GMT—Greenwich Mean Time. (See Coordinated Universal Time.)

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

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

Gravity waves—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.

Ground to Cloud Lightning—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.

Hail—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.

Halo—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.

Haze—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.

Heat Advisory—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.

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

Heat Lightning—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.

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

High Maximum Temperature Record—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).

High Minimum Temperature Record—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 System—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.

High Wind Advisory—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.

Hill Fog—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.

Humidity—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.

Hurricane Local Statement—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.

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

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

Ice Pellets—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.

Ionosphere—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.

Iridescent clouds—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.

Kelvin Temperature Scale—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.

Length Of Day—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.

Length Of Visible Light—The time of Civil Sunset minus the time of Civil Sunrise.

Low Frequency (LF)—That portion of the radio frequency spectrum from 30 to 300 kHz.

Low Maximum Temperature Record—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).

Low Minimum Temperature Record—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 System—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.

Mackerel sky—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.

Magnetic Cloud—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.

Magnetosphere—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.

Mammatus—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.

Mare’s Tail—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.

Maximum High Temperature Record—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).

Maximum Low Temperature Record—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).

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

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

Maximum Temperature—The highest temperature during a specific period of time.

Mean Sea-level Pressure—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.

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

Microwaves—Generically, any radio frequency of 500 MHz or more.

Minimum High Temperature Record—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).

Minimum Low Temperature Record—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).

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

Minimum Temperature—The lowest temperature during a specific period of time.

Mist—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.

Monthly Record—Record extremes measured within a specific calendar month.

Mostly Cloudy—When the 6/8th to 7/8ths of the sky is covered by with opaque (not transparent) clouds.

Mostly Sunny—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.

Nautical Twilight—The time period when the sun is between 6 and 12 degrees below the horizon at either sunrise or sunset. The horizon is not defined and the outline of objects might be visible without artificial light.

Nimbostratus—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.

Nocilucent clouds—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.

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

Northern Lights—Also known as Northern Polar Lights. 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.

Overcast—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.

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

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

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

Photosphere—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.

Pileus—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”.

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

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

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

Pressure—The force exerted by the weight of the atmosphere and gravity, also known as atmospheric pressure. 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 mb, which is equivalent to hectopascals (hPa), but also use in.

Pressure Change—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.

Pressure Characteristic—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.

Pressure Falling—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.

Pressure Gradient—The amount of pressure change over a given horizontal distance.

Pressure Gradient Force—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.

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

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

Public Information Statement—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.

Pyro-cumulus—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.

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

Radiation Fog—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.

Radio Blackout NOAA Space Weather Scale—A measure of the severity of solar x-ray bursts that cause radio blackouts at Earth. (See Appendix A).

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.

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

Rainbow—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.

Record Event—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).

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

Recurrence—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).

Reflected 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.

Relative Humidity—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.

Ridge—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.

Rime—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.

Roll cloud—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.

San Francisco Fog—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.

Scarf cloud—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”.

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

Sea Smoke—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.

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

Severe Thunderstorm Warning—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.

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

Sheet Lightning—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.

Shelf cloud—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.

Sleet—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.

Snow—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.

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

Solar Activity—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.

Solar Noon—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.

Solar Wind—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.

Solstice—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.

Southern Lights—Also known as the Southern Pole Lights. 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.

Special Weather Statement—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.

Steam Fog—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.

Stratocumulus—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.

Stratosphere—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.

Stratus—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”.

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

Sun dogs—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.

Sun pillars—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.

Sun rays—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.

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

Sunspot—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.

Supercell—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.

Surface Pressure—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.

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

Temperature—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).

Thermosphere—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.

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

Tornado—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.

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

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

Towering Cumulus—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.

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

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

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

Trough—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.

Tule Fog—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.

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

Ultrahigh Frequency (UHF)—That portion of the radio frequency spectrum from 300 MHz – 3 GHz.

Ultraviolet (UV)—That part of the electromagnetic spectrum between 5 – 400nm.

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

Upslope Fog—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.

Vernal Equinox—The equinox that occurs in March.

Very High Frequency (VHF)—That portion of the radio frequency spectrum from 30 – 300 MHz.

Very Low Frequency (VLF)—That portion of the radio frequency spectrum from 3 – 30 kHz.

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

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

Volcanic Ash Advisory—Atmospheric volcanic ash clouds may endanger aviation.

Wall cloud—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.

Water Spout—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.

White-Light Flare—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.

Wind—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 Direction—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.

Wind Shear—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.

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

Windchill—The felt air temperature on exposed skin due to wind.

Windy—20 to 30 mph winds.

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

Yearly Record—Record extremes measured within a specific calendar year.

Zulu Time—See Coordinated Universal Time.

Source: Weather Underground