Weather Term Definitions


Altimeter - Instrument for measuring altitude.

Altitude - Vertical elevation above mean sea level or any particular location.

Atmosphere - The gaseous or air portion of the physical environment that encircles a planet. In the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. The divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.

Atmospheric Pressure  - The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).

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Barometer - Instrument for measuring atmospheric pressure.

Barometric Pressure - The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).

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Celsius Scale - A temperature scale based on the freezing point of water as 0 and boiling point as 100. Named for Anders Celsius.

Cirrus - One of the three basic cloud forms (the others are cumulus and stratus). It is also one of the three high cloud types. Cirrus are thin, wispy clouds composed of ice crystals and often appear as veil patches or strands. In the mid-latitudes, cloud bases are usually found between 20,000 to 30,000 feet, and it is the highest cloud that forms in the sky, except for the tops, or anvils, of cumulonimbus, which occasionally build to excessive heights.

Civil Twilight - The time between the moment of sunset, when the sun's apparent upper edge is just at the horizon, until the center of the sun is 6 directly below the horizon.

Cumulus - One of the three basic cloud forms (the others are cirrus and stratus). It is also one of the two low cloud types. A cloud that develops in a vertical direction from the base (bottom) up. They have flat bases and dome- or cauliflower-shaped upper surfaces. The base of the cloud is often no more than 3,000 feet above the ground, but the top often varies in height. Small, separate cumulus are associated with fair weather (cumulus humilis). With additional heating from the earth's surface, they can grow vertically throughout the day. The top of such a cloud can easily reach 20,000 or more into the troposphere. Under certain atmospheric conditions, these clouds can develop into larger clouds, known as towering cumulus (cumulus congestus), and may produce a rain shower. Further development may create a cumulonimbus.

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Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

Doppler Radar - Special radar which detects changes in frequency due to movement or air and airborne moisture, thereby able to measure direction and speed of winds and intensity of storms.

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Evaporation - the conversion of water from a liquid into a gas.

Evapotranspiration - The amount of water transferred from the soil to the atmosphere.  Sum of evaporation and transpiration.  Reference Evapotranspiration (ETo) is a term used to estimate the evapotranspiration rate of a reference crop expressed in either inches or millimeters.  ETo varies by location, time, and weather conditions. The main factors that influence ETo include incoming radiation (energy from the sun), outgoing radiation (sensible energy leaving the earth), the amount of moisture in the air, air temperature, and wind speed. ETo can be estimated quite accurately through the use of a "model" (a series of complex mathematical equations).  The "model" that is used commonly is a version of Penman's equation modified by Pruitt/Doorenbos (Proceedings of the International Round Table Conference on "Evapotranspiration", Budapest, Hungary. 1977).

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Fahrenheit Temperature Scale - A temperature scale where water at sea level has a freezing point of +32°F and a boiling point of +212°F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.

Fog - A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on the METAR.

Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.

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Gale - On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).

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.

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Heatstroke - Heatstroke is life-threatening. The victim's temperature control system, which produces sweating to cool the body, stops working. The body temperature can rise so high that brain damage and death may result if the body is not cooled quickly.
Sunstroke - Another term for heatstroke. For additional information please visit the
American Red Cross web site.

Heat Cramps - Heat cramps are muscular pains and spasms due to heavy exertion. They usually involve the abdominal muscles or legs. It is generally thought that the loss of water from heavy sweating causes the cramps.

Heat Exhaustion
- Heat exhaustion typically occurs when people exercise heavily or work in a warm humid place where body fluids are lost through heavy sweating. Blood flow to the skin increases, causing blood flow to decrease to the vital organs. This results in a form of mild shock. If not treated, the victim's condition will worsen. Body temperature will keep rising and the victim may suffer heatstroke.

Heat Index
(Real Feel Temperature) - A number in degrees Fahrenheit that tells how hot it really feels when relative humidity is added to the actual air temperature. Exposure to full sunshine can increase the heat index by 15 degrees F.

Heat Wave - Prolonged period of excessive heat and humidity. The National Weather Service steps up its procedures to alert the public during these periods of excessive heat and humidity.

Humidity - Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity. Also see Relative Humidity.

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Ice Fog - Fog that is composed of minute ice particles. It occurs in very low temperatures under clear, calm conditions in the polar latitudes and may produce a halo around the sun or moon.

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Jet Stream - An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polar-ward. It is marked by a concentration of isotherms and strong vertical shear.

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Knot - A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.

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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° North and South latitude.

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.

Lunar Eclipse - An eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. The moon does not have any light of its own, instead, it reflects the sun's light. During a lunar eclipse, the moon is in the earth's shadow. It will often look dim and sometimes copper or orange in color.

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Mean Sea Level - The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL.

Mean Temperature - The average of temperature readings taken over a specified amount of time. Often the average of the maximum and minimum temperatures.

METAR - Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.

Microburst - A small, concentrated downburst affecting an area less than 4 kilometers (about 2.5 miles) across. Most micro bursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long. Case Mid-level cooling. Local cooling of the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead to destabilization of the entire atmosphere if all other factors are equal. Mid-level cooling can occur, for example, with the approach of a mid-level cold pool.

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Nautical Mile - A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles.

NOAA - National Oceanic and Atmospheric Administration

NWS - National Weather Service

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Overcast - The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.

Ozone(O3) - A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.

Ozone Layer - An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.

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Precipitation - Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.

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Quantitive Precipitation Forecast (QPF) - A forecast of rainfall, snowfall or liquid equivalent of snowfall.

Quasi-Stationary Front - A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.

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Relative Humidity - A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point. Also see Humidity.

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Solar Radiation - The electromagnetic radiation emitted by the Sun.  The entire energy range of electromagnetic radiation is specified by frequency, wavelength, or photon energy. The Solar Spectrum is the distribution of the radiation by wavelength.  The low end of the spectrum is infrared radiation (heat), and passes through the colors of visual light from red through violet, through ultraviolet radiation, x-rays, and gamma rays.

Solar Irradiance is the amount of solar energy that arrives at a specific area of a surface during a specific time interval (radiant flux density).  A typical unit is W/m. 

The Solar Constant is the amount of solar power flux that passes through the mean Earth orbit. Although not strictly constant due to variations in the suns output, the currently accepted value is 1367 W/m2. Note that Earth-based instruments record lower values of solar power flux because of atmospheric attenuation. 

Solar Energy is the ability of the Sun's energy to do work, such as raise the temperature of water or excite electrons in a photovoltaic cell.  Typical units of measure are kilowatt-hours per square meter (kWh/m), mega joules per square meter (MJ/m), langleys (L) (equal to cal/cm), or British thermal units per square foot (Btu/ft).

Solar radiation data provide information on how much of the sun's energy strikes a surface at a location on earth during a particular time period. The data give values of energy per unit of area.  By showing naturally occurring changes in the amount of solar radiation over the course of days, months, and years, these data determine the amount of solar radiation for a location.

Storms - Click here for in-depth explanation.

Stratus - One of the three basic cloud forms (the others are cirrus and cumulus. It is also one of the two low cloud types. It is a sheetlike cloud that does not exhibit individual elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen in low, uniform layers and rarely extends higher than 5,000 feet above the earth's surface. A veil of stratus may give the sky a hazy appearance. Fog may form from a stratus cloud that touches the ground. Although it can produce drizzle or snow, it rarely produces heavy precipitation. Clouds producing heavy precipitation may exist above a layer of stratus.

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.

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.

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Temperature - Measurement of heat and/or cold.

Thunder - Click here for in-depth explanation.

Tornado - A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

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Ultra Violet (UV) Index - The UV Index describes the levels of exposure to UV rays. The Index predicts UV levels on a 0-10+ scale: 

 UV Index Value 

 Exposure Category 







hat, sunscreen (spf 15+)



hat, sunscreen (spf 15+)
keep to shady areas



hat, sunscreen (spf 15+)
keep to shady areas or stay indoors


Very High

stay indoors as much as possible
take precautions when outdoors

The US UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities.
The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country. A radiative transfer model is then used to determine the amount of UV radiation reaching the ground from 290 to 400 nm in wavelength, using the time of day (solar noon), day of year, and latitude.
As an example, assume the following UV levels for each wavelength are predicted for a given location (these are totally made up numbers, and not even the ratios represent reality):









This information is then weighted according to how human skin responds to each wavelength; it is more important to protect people from wavelengths that harm skin than from wavelengths that do not damage people's skin. The weighting function is called the McKinlay-Diffey Erythema action spectrum. For illustration purposes only (these numbers are not correct), assume 290nm radiation causes three times as much damage as 350nm radiation and five times as much damage as 400 nm radiation. Then, if in some unit 290nm UV radiation did 15 units of damage, 350nm radiation would do 5 units and 400 nm radiation would do 3 units. At each wavelength, multiply the actual incoming radiation level by the weighting:

















These weighted irradiances are summed up, or integrated, over the 290 to 400 nm range resulting in a value representing the total effect a given day's UV radiation will have on skin. For our example, the total is 400.
These estimates are then adjusted for the effects of elevation and clouds. UV at the surface increases about 6% per kilometer above sea level. Clear skies allow 100% of the incoming UV radiation from the sun to reach the surface, whereas scattered clouds transmit 89%, broken clouds transmit 73%, and overcast conditions transmit 31%. If we assume that the example location is at 1 kilometer in elevation, and that there will be broken clouds, then the calculation is:  400 x 1.06 x 0.73 = 309.5
Once adjusted for elevation and clouds, this value is then scaled (divided) by a conversion factor of 25 and rounded to the nearest whole number. This results in a number that usually ranges from 0 (where there is no sun light) to the mid teens. This value is the UV Index. Thus, the UV Index for the example city would be:  309.5 / 25 = 12.4, rounded to 12
Currently, the computation of the UV Index does not include the effects of variable surface reflection (e.g., sand, water, or snow), atmospheric pollutants or haze.  Source:  USEPA

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Vapor Pressure - The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.  Vapor Trail - A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail. Visibility - A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.

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Wet Bulb Temperature - The air temperature drops after it rains. In the summertime a thunderstorm can break the oppressive heat of the day. As it precipitates, evaporation occurs and the air temperature drops. The wet bulb temperature is the temperature at which no more evaporation will occur, and thus no further decrease in the temperature. The air will continue to cool until the air can evaporate no more moisture. The temperature, when the cooling continues until the evaporation stops and the air becomes saturated, is the wet bulb temperature.

Wind - 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.

Wind Chill
Wind Chill is the term used to describe the rate of heat loss on the human body resulting from the combined effect of low temperature and wind.  When wind blows across the skin, it removes the insulating layer of warm air adjacent to the skin. When all factors are the same, the faster the wind blows, the greater the heat loss, which results in a colder feeling.  As winds increase, heat is carried away from the body at a faster rate, driving down both the skin temperature  and eventually the internal body temperature.
While exposure to low wind chills can be life threatening to both humans and animals alike, the only effect that wind chill has on inanimate objects, such as vehicles, is that it shortens the time that it takes the object to cool to the actual air temperature (it cannot cool the object down below that temperature).

Wind Gust - A sudden higher change of wind speed outside the normal speed at the time. (Example: Wind currently at 5 miles per hour suddenly gusts up to 12 miles per hour and either settles down to zero or a lower average speed).

Wind Run - The measurement of the "amount" of wind passing a given point during a given period of time, expressed in either "miles of wind" or "kilometers of wind".

Wind Vane - Sensor for wind direction measurement.

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X-Rays - The portion of the electromagnetic spectrum that has a very short wave length. It has a wave length longer than gamma rays, yet shorter than visible light. X-rays can penetrate various thicknesses of all solids, and when absorbed by a gas, can result in ionization.

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Yellow Snow - Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.

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Zodiac - The position of the sun during the course of the year as it appears to move though successive constellations. Also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except Venus and Pluto.

Zulu Time - One of several names for the twenty-four hour time which is used throughout the scientific and military communities. Related terms: Universal Time Coordinate (UTC) and Greenwich Mean Time (GMT).

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Some information courtesy of and
The Weather Channel as well as other various weather information centers.

Special thanks to Lighthouse Weather for the layout of this document.