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 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. definition courtesy of: What Is and Sunlit Design

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.

Celsius

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

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. One still should be able to carry on ordinary outdoor activities.

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.

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.

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. 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.

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 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.

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 of 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.

Fahrenheit

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.

Flood Warning

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

Flood Watch

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

Flurries

Snow flurries are an intermittent light snowfall of short duration with no measurable accumulation.

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.

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.

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. 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

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.

High Pressure

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.

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 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.

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.

Low Pressure

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.

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.

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.

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.

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.

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.

Prevailing Wind

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

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.

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).

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.

Scattered Clouds

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

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.

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.

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.

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.

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.

Sunny

When there are no opaque (not transparent) clouds.

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).

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.

Twilight

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

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.

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.

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.