Glossary of Terms

(A B C D E F G H I J K L M N O P Q R S T U V W X Y Z)

(Beaufort Scale Comfort Index Heat Stress Index)

 

Term Definition
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 dew point 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.
AIR DENSITY The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0 degrees Celsius or 39.2 degrees Fahrenheit.
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.
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. 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.
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. 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.
Azimuth 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.
BAROMETRIC PRESSURE The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above the point in question. The measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). Also known as atmospheric pressure.
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.
Blue sky 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.
CALIBRATION ERROR The inaccuracy that the manufacturer permits when the unit is calibrated in the factory.
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".
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. They are also called Mackerel clouds when the cloudlets are aligned in rows they have the appearance of a herringbone or mackerel.
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). One still should be able to carry on ordinary outdoor activities.
CLOUD BASE For a given cloud or cloud layer. The lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles. Virtual Weather Station estimates the cloud base on temperature and humidity readings, using the following equation: Cloud Base (ft) = 250(Temperature - Dew Point)
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.
COOLING DEGREE DAY A cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees a given temperature It is used to estimate the energy requirements, and is an indication of fuel consumption for air conditioning or refrigeration. Refer to degree day or heating degree day.
Contrails 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.
Crepuscular 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.
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.
Cumuls Mediocris Medium sized convective cumulus cloud showing little vertical growth and does not produce precipitation. Mediocris is the transformation between cumulus humilis and cumulus congests.
Cumulus 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.
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.
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 to which a sample of air must be cooled, while the mixing ratio and barometric pressure remain constant, in order to attain saturation by water vapor. When this temperature is below O°C, it is sometimes called the frost point.
Dew Point Temperature The temperature at which the air temperature must be cooled for water vapor to condense, forming water droplets, fog, or clouds.
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-Bulb Temperature The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the temperature measured by a regular thermometer exposed to the airstream. Unlike wet bulb temperature, dry bulb temperature does not indicate the amount of moisture in the air. In construction, it is an important consideration when designing a building for a certain climate.
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.
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.
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.
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).
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.
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.
Good Forecast Quality The forecast temperature is usually 1.5 to 2.5 degrees C warmer or cooler than the observed forecast.
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.
GUST A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.
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. 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.
Hail Stone 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.
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 The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.
HEATING DEGREE DAY One heating degree day is given for each degree that the daily mean temperature is below 65 degrees a given temperature. It is used as an indication of fuel consumption. Refer to degree day or cooling degree day.
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.
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.
Hour Angle of the Sun 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).
HUMIDITY The amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity, and specific humidity.
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.
Intracloud 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.
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.
Kelvin-Helmholtz Instability 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.
Kelvin-Helmholtz Waves 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.
LATITUDE The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.
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.
Lenticular 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).
LONGITUDE The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude. See Greenwich Mean 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.
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. 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.
Maximum Temperature The highest temperature during a specific period of time.
Mean Anomaly of the Sun 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 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.
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.
MOON PHASE

The moon phase is caused by sun rays reflecting off the moon's surface while it moves around the earth. The sun illuminates half of the moon at any time while the moon orbits around the earth. The variation in the angle made by the earth-moon line with respect to the earth-sun line causes changing phase of the moon. The moon completes one revolution around the earth in 27.322 days with respect to the background stars. This is called the SIDERIAL period of the moon. During this same time the earth moves about 27 degrees along its orbit around the sun. As a result, the moon takes about two extra days to complete the cycle with respect to the sun-earth line. This longer cycle of the moon that takes about 29.57 days is called SYNDONIC period of the moon. The longer cycle is considered as Lunar month.

Mostly Cloudy When the 6/8th to 7/8ths of the sky is covered by with opaque (not transparent) clouds. Same as Considerable Cloudiness.
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. Ordinary outdoor activities are not possible at this time without extra illumination.
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.
Northern 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.
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.
Obliquity Obliquity is the angle between a planet's equatorial plane and its orbital plane.
Orange sky 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).
Orange sunset 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).
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.
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".
Pink sky 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).
Pink sunset 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).
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 ALTITUDE Atmospheric or barometric pressure expressed in terms of altitude which corresponds tothat pressure in the standard atmosphere.
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.
Pressure Rising 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.
Pressure Steady 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.
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 characteristic or pressure change.
Pressure Unsteady 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.
Prevailing Wind The direction from which the wind blows most frequently in any location.
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.
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.
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.
RATE OF CHANGE The derivative or change in a parameters value with respect to time. Virtual Weather Station calculates the rate of change by calculating the derivative of a parameter, and then filtering it over one hour. Thus, the rate of change equation factors all of the measurements taken in the last hour, and may not exactly match the change in one hour.
Red sky 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).
Red sunset 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).
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.
Reflection 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 A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage.
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.
Right Ascension of the Sun 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.
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 LEVEL PRESSURE The atmospheric pressure at mean sea level either directly measured by stations at sea level or empirically determined from the station pressure and temperature by stations not at sea level. Used as a common reference for analyses of surface pressure patterns.
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.
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
  • Recent history of tree, telephone pole, or other structural damage
Severe Thunderstorm Watch 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
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 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.
Southern 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.
Southern 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 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.
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.
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 Declination 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.
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.
SUNRISE The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level. See sunset for comparison.
SUNSET The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level. See sunrise for comparison.
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).
thelonious None needed. Sample.
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 funnel cloud has been spotted or a rotating thunderstorm has been indicated by Doppler 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 warning means tropical storm conditions are possible in the next 24 hours.
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.
Undulated Asperatus/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.
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.
VAPOR PRESSURE The pressure exerted by water vapor molecules in a given volume of air
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.
VIRTUAL TEMPERATURE Virtual temperature is a fictitious temperature that takes into account moisture in the air. The formal definition of virtual temperature is the temperature that dry air would have if its pressure and specific volume were equal to those of a given sample of moist air. Virtual temperature allows meteorologists to use the equation of state for dry air even though moisture is present.
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.
Water Year The term U.S. Geological Survey "water year" in reports that deal with surface-water supply is defined as the 12-month period October 1, for any given year through September 30, of the following year. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 2010 is called the "2010" water year.
Wet-Bulb Temperature The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air. Unlike dry bulb temperature, wet bulb temperature is an indication of the amount of moisture in the air.
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.
WIND CHILL INDEX The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.
WIND DIRECTION The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, etc.).
WIND RUN
The distance or length of flow of the air past a point during a given interval of time.
Windy 20 to 30 mph winds
WIND SPEED The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.
Winter Weather Statement A winter weather advisory is issued when significant accumulations of snow, sleet, or freezing rain may affect the advisory area.

 

Beaufort Scale

Description
Features
Air Speed
calm
smoke rises vertically; water smooth
0-1
light air
smoke shows wind direction; water ruffled
1-3
light breeze
leaves rustle; wind felt on face
4-7
gentle breeze
loose paper blows around
8-12
moderate breeze
branches sway
13-18
fresh breeze
small trees sway, leaves blown off
19-24
strong breeze
whistling in telephone wires; sea spray from waves
25-31
near gale
large trees sway
32-38
gale
twigs break from trees
39-46
strong gale
branches break from trees
47-54
storm
trees uprooted; weak buildings collapse
55-63
violent storm
widespread damage
64-73
hurricane
widespread structural damage
above 74

 

Comfort Index

Scale Condition
Extreme Cold Wind chill below 0 °F
Uncomfortably Cold Wind chill 0 °F - 30 °F
Cool Wind Chill 30 °F - 60 °F
Comfortable Temperature 60 °F - 80 °F
Warm Temperature 80 °F - 90 °F
Uncomfortably Hot Temperature 90 °F - 100 °F
Extreme Hot Temperature above 100 °F

 

Heat Stress Index

Scale Condition
--- Below 80 °F
Caution Above 80 °F Under 90 °F
Extreme Caution Above 90 °F Under 105 °F
Danger Above 105 °F Below 130 °F
Extreme Danger Above 130 °F