Weather Glossary





























The horizontal transfer of any property in the atmosphere by the movement of air. Examples include heat and moisture advection.

This is considered the misture of gases that make up the earth's atmosphere. The principle gases that compose dry air are Nitrogen at 78.084%, Oxygen at 20.946%, Argon at .93%, and Carbon Dioxide .03%

An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.

Acronym for Automated Surace Observing Systen. This system is a collection of automated surface weather instruments that collect data. It performs surface observations in places that either do not have a human observer, or an observer 24-hours a day.


An instrument used to measure atmospheric pressure. Examples include the aneroid barometer and the mercurial barometer.

Barometric Pressure
The pressure exerted by the atmophere at a given point. The measurement can be expressed in millibars(mb) or in inches of mercury(Hg).

A severe weather condition characterized by low tempratures, winds 35mph of greater, blowing snow that can reduce visibilities to 1/4 mile or less for more than 3 hours. A severe blizzard is characterized by temperatures at or below 10 degrees fahrenheit, winds exceeding 45mph, and visibility reduced by snow to near zero.

Blue Norther
Refers to a fast-moving cold front in the southern Great Plains , marked by a dark, blue-black sky, strong north winds and temperatures that may drop 20-30 degrees fahrenheit in a matter of minutes


Atmospheric conditions devoid of wind or any other air motion.

Acronym for Convective Available Potential Energy. The amount of energy available to create convection. Higher values indicate an increasing possibility of severe weather.

The lowest cloud layer that is reported as broken or overcast. If the sky is totally obscured for example by fog, then the ceiling is defined by vertical visibility.

A type of foehn wind. Refers to the warm downslope wind in the Rocky Mountains that may occur after an intense cold spell when the temperatures may rise 20-40 degrees in a matter of minutes.

High clouds usally above 18,000 feet, composed of ice crystals

The historical record of average daily and seasonal weather events. Statistics are generally drawn over several decades. The word is derived from the Greek "klima" meaning inclination, and reflects the importance early scholars attributed to the sun's influence.

Climate Prediction Center
A branch of the National Centers for Environmental Prediction. The center maintains a continuous watch on short-term climate fluctuations and diagnoses and predicts them.

The study of climate. Includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems.

A sudden, heavy rainfall of a showery nature. Related

The merging of two water drops into a single larger drop.

Cold Air Funnel
Funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when the air aloft is very cold. Cold air funnels tend to touch down briefly, but in general are less violent than most other types of tornadoes.

Cold Front
The leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, when a cold front passes the temperature and humidity decrease, the pressure rises, and the wind shifts from southwest to northwest. Precipitation is usually along or ahead of the front in the form of thunderstorms.

The process by which water vapor undergoes a change in state from a gas to a liquid. It's opposite is evaporation.

Motions in a fluid that transport and mix the properties of the fluid. These properties could be heat and/or moisture. Often the term convection is used to describe upward motion of water vapor(moisture) forced to rise by surface heating in turn creating rain or thunderstorms

Wind movement that results in a horizontal net inflow of air into a particular region. Convergent winds at lower levels are associated with upward motion.

Cooling Degree Days
A cooling degree day is a unit used to relate the day's temperature to the energy demands of air conditioning. Cooling degree days are calculated by subtracting 65 from a day's average temperature. For example, if the day's high is 90°F and the day's low is 70°F, the day's average is 80°F. Eighty minus 65 is 15 cooling degree days. Cooling degree days can be used to compare the current summer to past summers. It is a good way to generally keep track of how much demand there has been for energy needed for either heating or cooling buildings. The cooler (warmer) the weather, the larger the number of "heating (cooling) degree days"... and the larger the number of heating (cooling) degree days, the heavier the demand for energy needed to heat (cool) buildings. It can also be used to compare the heat in one part of the country with another. See also Degree Days and Heating Degree Days.

Coriolis Force
A force per unit mass that arises solely from the earth's rotation, acting as a deflecting force. It is dependent on the latitude and speed of the moving object. In the Northern Hemisphere the air is deflected to the right, and in the Southern Hemisphere to the left. The coriolis effect is almost non-existent at the equator.

Cumulonimbus Cloud
A vertically developed cloud, often capped by an anvil shaped cloud. This cloud is otherwise known as a thundercloud. A cumulonimbus cloud can produce tornadoes, hail, lightning, strong winds and heavy rain.

An area of closed pressure circulation with rotating and converging winds. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low pressure system and the term used for tropical cyclone in the Indian Ocean . Other phenomena with cyclonic flow may be referred to as dust devils, tornadoes, tropical and extratropical systems.


Data Buoys
Buoys placed throughout the Gulf of Mexice and along the Atlantic and Pacific coasts of the U.S. that relay information on air and water temperature, air pressure, wind speed and wave conditions via radio signals.

Degree Days
Degree Days are a practical method for determining cumulative temperatures over the course of a season. Originally designed to evaluate energy demand and consumption, degree days are based on how far the average temperature departs from a human comfort level of 65 °F *. Simply put, each degree of temperature above 65 °F is counted as one cooling degree day, and each degree of temperature below 65°F is counted as one heating degree day. For example, a day with an average temperature of 80 °F will have 15 cooling degree days. The number of degree days accumulated in a day are proportional to the amount of heating/cooling you would have to do to a building to reach the human comfort level of 65 °F. The degree days are accumulated each day over the course of a heating/cooling season, and can be compared to a long term (multi-year) average, or normal, to see if that season was warmer or cooler than usual. See also Cooling Degree Days and Heating Degree Days.

In meteorology it is another name for an area of low pressure, a low or trough. It also applies to the initial stage of a developing tropical cyclone.

Condensation in the form of small water drops that form on grass and other obect near the ground when the temperature has fallen to the dewpoint. Dew generally forms during the nighttime hours and evaporates by mid to late morning.

The temperature to which air must be cooled at a constant pressure to become saturated. Example: If the air temperature is 70 degrees and the dewpoint temperature is 70 degrees the air is saturated and dew will form i.e, the relative humidity is 100%

A rate at which wind flow spreads apart along an axis oriented normal to the flow in question.

Wind movement that results in a horizontal net outlow of air from a particular region. Divergence at lower levels is assiciated with a downward movement of air from aloft.

Doppler Radar
Weather radar that measures the direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar.

A severe localized downdraft from a thunderstorm or shower. This outward burst of cool air creates damaging winds at or near the surface.

Abnormal, dry weather for a specific area that is prolonged and causes serious hydrological imbalance.

Dry Line
The boundary between the dry desert air mass of the southwestern U.S. and moist air mass from the Gulf of Mexico . It usually lies north-south across the central and southern High Plains states druing the spring and summer months. When a dry line passes it results in a decrease in humidity, clearing skies, and wind shift from east/southeasterly to west/southwesterly. Its presence influences severe weather development in the Great Plains .


Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.

Easterly Wave
An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

The energy return of a radar signal after it has hit the target. Related

radar echo.

El Nino
The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial Pacific waters move in and displace the colder waters, cutting off the upwelling process.

The sum total of all the external conditions that effect an organism, community, materail, or energy.

The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.

The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

The total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.

Extratropical Cyclone
Any cyclone not of tropical origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes.

The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds.


Fahrenheit Temperature Scale
A temperature scale where water at sea level has a freezing point of +32 degrees F and a boiling point of +212 degrees 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.

This is a subjective description. Considered as pleasant weather conditions with regard to the time of year and the physical conditions.

Feeder Bands
In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart. In thunderstorm development, they are the lines or bands of low level clouds that move or feed into the updraft region of a thunderstorm.

Flanking Line
A line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a supercell.

Flash Flood
A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.

High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Classified as a katabatic wind.

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.

A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist.

Freezing Drizzle
Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces. It is reported as "FZDZ" in an observation and on the METAR.

Freezing Rain
Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces. It is reported as "FZRA" in an observation and on the METAR.

The transition zone or interface between two air masses of different densities, which usually means different temperatures. For example, the area of convergence between warm, moist air and cool, dry air.

The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.

Fujita-Pearson Scale
A scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. It is normally used to identify the most intense damage exhibited by a tornado. Developed by T. Theodore Fujita and Allen Pearson.

Funnel Cloud
A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus toward the ground, but not in contact with it.


Geostationary Satellite
An orbiting weather satellite that maintains the same position over the equator during the earth's rotation. Also known as GOES, an acronym for Geostationary Operational Evnvironmental Satellite.

Geostrophic Wind
A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together.

The force of attraction of the earth on an object. The direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth's surface.

Green Flash
A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon.

Greenhouse Effect
The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space.

Greenwich Mean Time (GMT)
The name of the twenty-four hour time scale which is used throughout the scientific and military communities. This is the Prime Meridian of Longitude. The globe is divided into twenty-four time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are numbered from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT.

Ground Clutter
A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. This may hide or confuse the proper return echo signifying actual precipitation.

Gulf Stream
The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the United States , beginning near Cape Hatteras . The term also applies to the oceanic system of currents that dominate the western and northern Atlantic Ocean: the Florida current, which flows through the Florida Straits between the Florida Keys and Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and flows northeasterly off the continental slope into the North Atlantic; and the North Atlantic current, which begins around the Grand Banks off Newfoundland and continues east-northeastwards towards the British Isles.

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 than twenty seconds.

Gust Front
The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary.

A weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearing as a temporary dust whirl or debris cloud.


Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are caled hailstones.

A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an opalescent appearance.

A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.

Heat Exhaustion
The effect of excessive heat, particularly when combined with high humidity on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

Heat Index
T he perceived temperature to the human body based on both air temperature and the amount of moisture (humidity) present in the air. The body dissipates heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as a last resort, by panting, when blood is heated above 98.6°F (37° C). Sweating cools the body through evaporation. However, high relative humidity retards evaporation, robbing the body of its ability to cool itself. When heat gain exceeds the level the body can remove, body temperature begins to rise, and heat related illnesses and disorders may develop. This is not the actual air temperature. As relative humidity increases, the air seems warmer than it actually is because the body is less able to cool itself via evaporation of perspiration. As the heat index rises, so do health risks. When the heat index is 90°-105°F, heat exhaustion is possible. When it is above 105°F, it is probable. Heatstroke is possible when the heat index is above 105°F, and very likely when it is 130°F and above. Physical activity and prolonged exposure to the heat increase the risks. Sometimes called "apparent temperature."

Heat Lightning
Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.

Heat Stroke
Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105 degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.

Heat Wave
A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks.

Heating Degree Day
Indicator of household energy consumption for space heating. It was found that for an average outdoor temperature of 65 degrees Fahrenheit, most buildings require heat to maintain a 70 degree temperature inside. Similarly, for an average outdoor temperature of 65 degrees or more, most buildings require air-conditioning to maintain a 70 degree temperature inside. Example: If the weather has been cool, and the mean temperature is, say, 55 degrees, then there have 10 heating degree days (65 minus 55 equals 10). It is a good way to generally keep track of how much demand there has been for energy needed for either heating or cooling buildings. The cooler (warmer) the weather, the larger the number of "heating (cooling) degree days"... and the larger the number of heating (cooling) degree days, the heavier the demand for energy needed to heat (cool) buildings. Also see Degree Days and Cooling Degree Days.

High Clouds
A term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. The main types of high clouds are cirrus, cirrocumulus, and cirrostratus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

High Pressure System
An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposite of an area of low pressure or a cyclone.

Hook Echo
A radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating favorable conditions for tornadic development. However, hook echoes and tornadoes do not always accompany each other.

The humidex is a measurement used by Canadian meteorologists to reflect the combined effect of heat and humidity. It differs from the heat index used in the United States in using dew point rather than relative humidity. Humidex values are generally higher than the heat index.

The amount of water vapor in the air. It is often confused with relative humidity or dew point.

The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean .

Hurricane Warning
A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are expected in a coastal area or group of islands within a 24 hour period. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement.

Hurricane Watch
A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are a potential threat to a coastal area or group of islands within a 24 to 36 hour period. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.

Hydrologic Cycle
Often called the water cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.

Any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are examples of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation. Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spary. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.

An instrument that measures the water vapor content of the atmosphere.

This situation occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.


Water in a solid state. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail.

Inches of Mercury
The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars. First devised in 1644 by Evangelista Torricelli, an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. Water vapor, ozone, and carbon dioxide are efficient at absorbing or transmitting infrared radiation.

Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLar radiATION.

It is the condition of the atmosphere when spontaneous convection and severe weather can occur. Air parcels, when displaced vertically, will accelerate upward.

Instrument Flight Rules (IFR)
Refers to the general weather conditions a pilot can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist in take off and landing.

Instrument Shelter
A boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitaion, and condensation, while at the same time time providing adequate ventilation.

Intertropical Convergence Zone (ITCZ)
An area where the Northern and Southern Hemispheric trade winds converge. It is a broad area of low pressure where both the coriolis force and the low-level pressure gradient are weak, occasionally allowing tropical disturbances to form. The ITCZ fluctuates, moving northward over the south Atlantic during the Northern Hemipshere summer.

It refers to an increase in an atmospheric property with height. For example...A temperature inversion is when the temperature increases with altitude, which is a departure from the usual decrease of temperature with height.

An atmospheric zone of ionized gases that extends between 50 and 400 miles above the surface of the Earth. It is located between the mesosphere and the exosphere.

The line of equal change in atmospheric pressure during a certain time period. It marks the change in pressure tendedncy.

The line drawn on a weather map connecting points of equal barometric pressure.

The line drawn on a weather map connecting points of equal dew point.

THe line drawn through geographic points recording equl amounts of rainfall during a given time or for a give storm.

A line connecting equal points of value.

A line connecting equal wind speeds.

The line of equal or constant temperature.

A region of accelerated wind speed along the axis of a jet stream.

A area of strong winds that are concentrated in a realatively narrow band in the upper troposphere 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 northward toward the poles.


K Index
The measure of thunderstorm potential based on the vertical temperature lapse rate, the mosture content of the lower atmosphere and the vertical extent of the moist layer.

Katabatic Wind
A wind that is created by air flowing downhill. When the air is warm, it may be called a foehn wind, and regionally may be known as a Chinook, or Santa Ana . When this air is cool, it is called a drainage wind, mountain breeze or glacier wind.

A nautical unit of wind speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. 1 knot = 1.151 statute miles per hour.


Land Breeze
A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the termpeature difference when the sea sruface is warmer than the adjacent land. Predominate during the night, it reaches its maximum around dawn.

Lapse Rate
The change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height. This is a sign of instability.

The location north or south in reference to the equator, which is designated at zero degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.

The side of an object, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore protected from the direct force of the wind.

Lifted Index
A measure of atmospheric instability that is obtained by computing the temperature that the air near the ground would have if it were lifted to a higher level and compared to the actual temperature at that altitude. Positive values indicate more stable air and negative values indicate instability.

A rapid, visible discharge of electricity hotter than the surface of the sun. Lightning is caused by the build up of electrical potential between cloud and ground, between clouds, or between clouds and the surrounding air.

The location east or west in reference to the Prime Meridian, which is designated at 0 degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes. Time zones are correlated to longitude.

Low Clouds
A term used to describe clouds with bases below 6,000 feet. Types of low clouds include stratus, stratocumulus, cumulus and cumulonimbus.

Low Level Jet
Strong winds that are concertrated in relatively narrow bands in the lower part of the atmosphere. It is often amplified at night.

Low Pressure System
An area of a relative pressure minimum that has converging winds and rotates in the same direction as the Earth...counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as a cyclone.

Lunar Eclipse
A 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.


A large downburst iwht an outflow diameter of 2.5miles or larger and damaging winds.

The meteorological scale covering an area ranging from the size of a continent to the entire globe.

Mare's Tail
The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches of strands, often resembling a horse's tail.

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 ninteen year period, determined from hourly height readings measured from a fixed predetermined reference level. It is used as a basis for determining elevations, as the referece for all altitudes in upper air measurements, and as the level above which altitude is measure by a pressure altimeter for aviation. Often referred to as MSL

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.

Mercurial Barometer
An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

An area of rotation of storm size that may often be found on the southwest part of a supercell. Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar.

The scale of meteorological phenomena that range in size from several kilometers to around 100 kilometers. This includes MCCs, MCSs, and squall lines. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale.

Mesoscale Convective Complex (MCC)
A large mesoscale convective system (MCS) which is about the size of the state of Ohio or Iowa and lasts at least 6 hours. Generally forming during the afternoon and evening, the complex normally reaches its peak intensity at night when heavy rainfall and flooding become the primary threat. Severe weather may occur at anytime.

Mesoscale Convective System (MCS)
A large organized convective weather system comprised of a number of individual thunderstorms. It normally persists of several hours and may be rounded or linear in shape. This term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (MCC).

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.

The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. A scientist who studies the atmosphere and atmospheric phenomena.

A severe localized wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes.

The smallest scale of meteorological phenomena that range in size from a few centimeters to a few kilometers. Larger phenomena are classified as mesoscale. It also refers to small scale meteorological phenomena with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain.

Middle Clouds
A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

Millibar (MB)
The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.

Mixed Precipitation
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.

Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

The seasonal shift of winds created by the great annual temperature variation that occurs over large areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India . The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States .

Multicell Storm
A thunderstorm made up of two or more single-cell storms.

Multiple Vortex Tornado
A tornado which has two or more condensation funnels or debris clouds, often rotating around a common center.


National Center for Atmospheric Research (NCAR)
A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities. For further information, contact NCAR, located in Boulder , Colorado .

National Centers for Environmental Prediction (NCEP)
As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Aviation Weather Center , the Climate Prediction Center , the Storm Prediction Center , and the Tropical Prediction Center . Formerly known as NMC. For further information, contact the NCEP, with central offices located in Silver Spring , Maryland .

National Climatic Data Center (NCDC)
The agency that archives climatic data from the National Oceanic and Atmospheric Administration as well as other climatological organizations. For further information, contact the NCDC, located in Asheville , North Carolina .

National Hurricane Center (NHC)
A branch of the Tropical Prediction Center , it is the office of the National Weather Service that is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico , and the Eastern Pacific. For further information, contact the NHC, located in Miami , Florida .

National Meteorological Center (NMC)
Now incorporated into the National Centers for Environmental Prediction, it was the division of the National Weather Service that produced, processed, handled, and distributed meteorological and oceanographic information to users throughout the Northern Hemisphere, specifically U.S. governmental organizations.

National Oceanic and Atmospheric Administration (NOAA)
A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources. For further information, contact NOAA, located in Silver Spring , Maryland .

National Severe Storms Forecast Center (NSSFC)
As of October 1995, the responsibilities of this Center were divided into two branches, the Storm Prediction Center and the Aviation Weather Center .

National Severe Storms Laboratory (NSSL)
A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms. For further information, contact the NSSL, headquartered in Norman , Oklahoma .

National Weather Association (NWA)
An organization whose membership promotes excellence in operational meteorology and related activities, recognizing the professional as well as the volunteer. For further information, contact the NWA.

National Weather Service (NWS)
A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. For further information, contact the NWS.

This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.

A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises 78.09%.

A cyclonic storm occurring off the east coast of North America . These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.

The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc.

Numerical Forecasting
The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather.


In meterology, the evaluation of one or more meteorological elements, like temperature, pressure or wind that describe the state of the atmosphere at a given time. A trained observer is one who records the evaluations of the meteorological records.

The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemisty and physics of sea water, and marine biology.

Omega Block
A warm high aloft which has become displaced and is on the polarward side of the jetstream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts.

Also referred to as an outflow boundary. It is the outward flow of air from a system such as a thunderstorm. It is the result of cold downdrafts and its passage includes a wind shift and most often a temperature drop. Outflow boundaries sometimes help produce thunderstorms as they move into regions of instability.

When the sky is completely covered by clouds.

This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. Weather generally associated with this event include cloudiness, cool tempertures and steady precipitation.

Ozone Layer
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultaviolet radiation. It is located between the troposphere and the stratosphere between 9.5 and 12.5 miles above the Earth's surface. Ozone at the surface is not healthy for humans to breathe.


Palmer Drought Index
A long-term meteorological drought severity index produced by the NOAA/USDA Joint Agricultural Weather Facility. The index depicts prolonged times, in months or years, of abnormal dryness or wetness. It responds slowly. Changing little from week to week, it reflects long-term moisture runoff, recharge and deep percolation, as well as evapotranspiration.

Partly Cloudy
The state of the weather whene clouds are conspicuously present, but do not completely cover the sky at a given time. Sometimes interchanged with mostly sunny.

Pilot Report
A report of in-flight weather by an aircrat pilot or crew member. Often referred to as a PIREP.

Polar Front
A semi-continuous, semi-permanent boundary between polar airmasses and tropical air masses. An integral part of an early meteoroligical theory known as the Polar Front Theory.

Polar-Orbiting Satellite
A satellite whose orbit passes over both of the Earth's poles gathering cloud and temperature data.

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 force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.

Pressure Gradient
The amount of pressure change that occurs over a fixed distance at a fixed altitude.

Prevailing Wind
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

A type of doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere. Used to monitor winds with height in determining severe weather threat.

An instrument used to measure the water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.


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


Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back. Related Terms: Doppler Radar and NEXRAD

The process by which energy is propagated through any medium by virtue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form. Sound waves are another.

Radiation Fog
Fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning. Related

ground fog.

An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and winds aloft. Observations are made when the radiosonde is aloft and emits radio signals as it ascends. May be referred to as a RAOB, an acronym for RAdiosonde OBservation.

A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.

A measure of the process by which a surface can turn back a portion of incident radiation into the medium through which the radiation approached. It also refers to the degree by which precipitation is able to reflect a radar beam. Related albedo.

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.

In meteorology, it is the movement of a weather system in a direction opposite to the direction of the basic flow in which it is embedded.

The rapid freezing of supercooled water droplets as they touch an exposed object, forming a white opaque granular deposit of ice. It is one of the results of an ice storm, and when formed on aircraft it is called rime icing. Related glaze

Roll Cloud
A relatively rare, low-level, horizontal, tube-shaped cloud. Although they are associated with a thunderstorm, they are completely detached from the base of the cumulonimbus cloud.

Rossby Waves
The movement of ridges and troughs in the upper wind patterns, primarily the jet stream, circling the earth. Named for Carl-Gustaf Rossby, a U.S. Weather Bureau (NWS) employee, who first theorized about the existence of the jet stream in 1939.


Saffir-Simpson Damage-Potential Scale
Developed in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then Director of the National Hurricane Center , it is a measure of hurricane intensity on a scale of 1 to 5. The scale categorizes potential damage based on barometric pressure, wind speeds, and surge. Related Saffir Simpson Scale

A measure of the quantity of dissolved salts in sea water. The total amount of dissolved solids in sea water in parts per thousand by weight.

Santa Ana Winds
The hot, dry winds, generally from the east, that funnel through the Santa Ana river valley south of the San Gabriel and San Bernadino Mountains in southern California , including the Los Angeles basin. Classified as katabatic, it occurs most often during the winter and it is an example of a foehn wind.

Any object that orbits a celestial body, such as a moon. However, the term is often used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. Some of the information that is gathered by weather satellites, such as GOES9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world.

Saturation Point
The point when the water vapor n the atmosphere is at its maximum level for the existing temperature.

The process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. This is a primary reason for colors, such as blue skies, rainbows, and orange sunsets.

Low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. They are often along and behind cold fronts and gust fronts, being associated with cool moist air, such as an outflow from a thunderstorm. When observed from a distance, they are sometimes mistaken for tornadoes.

Sea Breeze
A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

Sea Breeze Front
A coastal phenomena, it is restricted to large bodies of water and their immediate coast lines. This is usually the landward extent of the sea breeze. Due to the imbalance of heating between land and water, a region of maximum upward motion or convergence occurs by mid-afternoon in the summer some 10 to 15 miles inland.

Sea Fog
A type of advection fog which forms in warm moist air cooled to saturation as the air moves across cold water. Related Arctic Sea Smoke

Sea Level
The height or level of the sea surface at any time. It is used as a reference for elevations above and below. Related mean sea level

Sea Level Pressure
The atmospheric pressure at mean sea level, usually determined from the observed station pressure.

Severe Thunderstorm
A thunderstorm with winds measuring 50 knots (58 mph) or greater, 3/4 inch hail or larger, or tornadoes. Severe thunderstorms may also produce torrential rain and frequent lightning. Related supercell

Severe Weather
Generally, any destructive weather event, but usually applies to localized storms, such as blizzards, intense thunderstorms, or tornadoes.

It is the rate of change over a short duration. In wind shear, it can refer to the frequent change in wind speed within a short distance. It can occur vertically or horizontally. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.

Short Wave
A progressive wave of smaller amplitude, wave length, and duration than a long wave. It moves in the same direction as the basic current in which it is embedded and may induce upward vertical motion ahead of it. They are more numerous than long waves and often disappear with height in the amtosphere.

Skew T-Log P Diagram
A thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. It is used to evaluate and forecast air parcel properties. Some values that can be determined are the Convective Condensation Level (CCL), the Lifting Condensation Level (LCL), and the Level of Free Convection (LFC).

Sky Cover
The amount of the celestial dome that is hidden by clouds and/or obscurations.

Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface.

Small Craft Advisory
An advisory issued for marine interests, especially for operators of small boats or other vessels. Conditions include wind speeds between 20 knots (23 mph) and 34 knots (39 mph).

Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes.

snow flakes
An ice crystal or an aggregate of ice crystals which fall from clouds.

Snow Flurry/Flurries
Light showers of snow, generally very brief without any measurable accumulation.

Snow Flurry/Flurries
Light showers of snow, generally very brief without any measurable accumulation.

Snow Grains
Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle.

Snow Grains
Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle.

Snow Pellets
Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail.

Snow Pellets
Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail.

An ice crystal or an aggregate of ice crystals which fall from clouds.

Solar Eclipse
An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, casting some of the earth's surface in its shadow. The moon's disk shaped outline appears to cover the sun's brighter surface, or photosphere. That part of the earth that is directly in the moon's shadow will see a total eclipse of the sun, while the areas around it will see a partial eclipse.

Solar Radiation
"Current Solar Radiation" is technically know as Global Solar Radiation, a measure of the intensity of the sun's radiation reacing a horizontal surface. The irradiance includes both the direct component from the sun and the reflected component from the rest of the sky. The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, express in Watts/sq. m (W/m2). This is useful in determining current cloud cover conditions (more clouds = lower W/m2) as well as for use in trend analysis of the Earth's atmosphere.

The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. The Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude which lies directly beneath a solstice. In the Northern Hemisphere, the winter solstice falls on or about December 21 and the summer solstice on or about June 21.

Southern Oscillation
A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events. It represents the distribution of temperature and pressure over an oceanic area.

The season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. Customarily, this refers to the months of March, April, and May in the Northern Hemisphere, and the months of September, October, and November in the Southern Hemisphere. Astronomically, this is the period between the vernal equinox and the summer solstice.

A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. The intensity and duration is longer than that of a gust.

Squall Line
A narrow band of line of active thunderstorms that is not associated with a cold front. It may form from an outflow boundary or the leading edge of a mesohigh.

Station Pressure
The atmospheric pressure with respect to the station elevation.

Stationary Front
A front which is nearly stationary or moves very little since the last synoptic position. May be known as quasi-stationary front.

Storm Prediction Center (SPC)
A branch of the National Centers for Environmental Prediction, the Center monitors and forecasts severe and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena across the United States. Formerly known as the Severe Local Storms (SELS) unit of the National Severe Storms Forecast Center .

Storm Winds
On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour).

Straight-Line Winds
Any surface wind that is not associated with rotation. An example is the first gust from a thunderstorm, as opposed to tornadic winds.

Clouds composed of water droplets that exhibit no or have very little vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. Bases of these clouds are generally no more than 6,000 feet above the ground. They are classified as low clouds, and include all varieties of stratus and stratocumulus.

A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. It is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them.

The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by a decrease in temperature with increasing altitude.

The layer of the atmosphere located between the troposphere and the mesophere, characterized by a slight temperature increase and absence of clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. It is the location of the earth's ozone layer.

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.

The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water).

A sinking or downward motion of air, often seen in anticyclones. It is most prevalent when there is colder, denser air aloft. It is often used to imply the opposite of atmospheric convection.

The region between the tropical and temperate regions, an area between 35 and 40 degrees North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.

Subtropical Jet
Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subtropical air and the tropical air. It is found approximately between 25 and 35 degrees North latitude and usually above an altitude of 40,000 feet. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer.

Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the South Hemisphere.

A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes.

The reduction of the temperature of any liquid below the melting point of that substance's solid phase. Cooling a substance beyond its nominal freezing point. Supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. The smaller and purer the water droplets, the more likely they can become supercooled.

Surface Boundary Layer
The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.

The increase in sea water height from the level that would normally occur were there no storm. Although the most dramatic surges are associated with hurricanes, even smaller low pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. It is estimated by subtracting the normal astronomic tide from the observed storm tide.

Ocean waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch.

Synoptic Chart
Any map or chart that depicts meteorological or atmospheric conditions over a large area at any given time.

Synoptic Scale
The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more.


The measure of molecular motion or the degree of heat of a substance.

Terminal Doppler Weather Radar (TDWR)
Doppler radar installed at major airports throughout the United States to detect microbursts.

Terrestrial Radiation
Long wave radiation that is emitted by the earth back into the atmosphere. Most of it is absorbed by the water vapor in the atmosphere, while less than ten percent is radiated directly into space.

Thermal Low
Also known as heat low, it is an area of low pressure due to the high temperatures caused by intensive heating at the surface. It tends to remain stationary over its source area, with weak cyclonic circulation.

An instrument used for measuring temperature. The different scales used in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.

A thermal classification, it is the layer of the atmosphere located between the mesophere and out space. It is a region of steadily increasing temperature with altitude.

THSW (Temperature/Humidity/Sun/Wind) Index
Temperature measurement that uses humidity and temperature like the Heat Index, but also includes the heating effects of sunshine (solar radiation) and the cooling effects of wind (like wind chill) to calculate an apparent temperature of what it “feels” like out in the sun.

The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 degrees Celsius in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.

Thunder Snow
A wintertime thunderstorm from which falls snow instead of rain.

Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.

The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave throught the atmosphere and along the surface of the earth's waters.

A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.

Tornado Alley
A geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa . In terms of sheer numbers, this section of the United States receives more tornadoes than any other.

Towering Cumulus
Another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clouds whose height exceeds its width. Its distinctive cauliflower top often mean showers below, but lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm.

Generally, an unmeasurable or insignificant quantity. A precipitation amount of less than 0.005 inch.

Trade Winds
Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. Primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.

The process by which water in plants is transferred as water vapor to the atmosphere.

Triple Point
The point at which any three atmospheric boundaries meet. It is most often used to refer to the point of occlusion of an extratropical cyclone where the cold, warm, and occluded fronts meet. Cyclogenesis may occur at a triple point. It is also the condition of temperature and pressure under which the gaseous, liquid, and solid forms of a substance can exist in equilibrium.

Tropic of Cancer
The most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees North latitude.

Tropic of Capricorn
The most southern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees South latitude.

Tropical Air Mass
An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.

Tropical Cyclone
A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized cirulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.

Tropical Depression
A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.

Tropical Disturbance
An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.

Tropical Prediction Center (TPC)
A division of the National Centers for Environmental Prediction, the Center issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics for both domestic and international communities. The National Hurricane Center is a branch. For further information, contact the TCP, located in Miami , Florida .

Tropical Storm
A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.

Tropical Wave
Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitation during part of the year.

The boundary zone or transition layer between the troposphere and the stratosphere. This is characterized by little or no increase or decrease in temperature or change in lapse rate with increasing altitude.

The lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude.

An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. The opposite of a ridge.

An ocean wave with a long period that is formed by an underwater earthquake or landslide, or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin and builds up to great heights over shallower water. Also known as a seismic sea wave, and incorrectly, as a tidal wave.

The irregular and instantaneous motions of air which is made up of a number of small eddies that travel in the general air current. Atmospheric turbulence is caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure.

Often called dusk, it is the evening period of waning light from the time of sunset to dark.

A slang term used in the United States for a tornado.

The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean . This same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean .


Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. Although it accounts for only 4 to 5 percent of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone.

United States Weather Bureau
The official name of the National Weather Service prior to 1970.

Universal Time Coordinate
One of several names for the twenty-four hour time which is used throughout the scientific and military communities.

Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own.

A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development.

Upper Air/Upper Level
The portion of the atmosphere which is above the lower troposphere. It is generally applied to the levels above 850 millibars. Therefore, upper level lows and highs, troughs, winds, observations, and charts all apply to atmospheric phenomena above the surface.

Upslope Effect
The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable, stratiform clouds and precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development.

UV Index
An international standard measurement of how strong the ultraviolet (UV) radiation from the sun is at a particular place on a particular day. It is a scale primarily used in daily forecasts aimed at the general public. Its purpose is to help people to effectively protect themselves from UV light, of which excessive exposure causes sunburns, eye damage such as cataracts, skin aging, and skin cancer (see the section health effects of ultraviolet light). Public-health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat) when the UV index is 3 or higher; see the table below for complete recommendations.

Recommendations for protection when the day's predicted UV index is at various values are:

UV Index Description Media Graphic Color Recommended Protection
0–2 Low danger to the average person Green Wear sunglasses; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
3–5 Moderate risk of harm from unprotected sun exposure Yellow Wear sunglasses and use sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.
6–7 High risk of harm from unprotected sun exposure Orange Wear sunglasses and use sunscreen having SPF 15 or higher, cover the body with sun protective clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 11:00 AM to 4:00 PM during summer in zones that observe daylight saving time).
8–10 Very high risk of harm from unprotected sun exposure Reddish-purple Same precautions as above, but take extra care — unprotected skin can burn quickly.
11+ Extreme risk of harm from unprotected sun exposure Violet Take all precautions, including: wear sunglasses and use sunscreen, cover the body with a long-sleeve shirt and pants, wear a broad hat, and avoid the sun from two hours before to three hours after solar noon.



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.

Vernal Equinox
Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Vertical Temperature Profile
A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

Vertical Wind Profile
A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere ove a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground.

A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight.

Visible Light
The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 miles per second. It has a wave length longer than ultraviolet light and shorter than x-rays.

Visual Flight Rules (VFR)
Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles.

Any circular or rotary flow in the atmosphere that possesses vorticity.

The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.

Vorticity Maximum
A center of vorticity, or the maximum of the vorticity field fo a fluid.


Wall Cloud
An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visible precipitation underneath. Forming in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. It often develops before strong or violent tornadoes.

Warm Advection
The horizontal movement of warmer air into a location.

Warm Front
The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage.

Warm High
A high pressure system that has its warmest temperatures at or near the center of circulation.

Warm Low
A low pressure system that has its warmest temperatures at or near the center of circulation. Also referred to as a warm core low.

A forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

A forecast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, or heavy snows.

A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the water equivalent of a landspout.

The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes.

Weather Surveillance Radar (WSR-88D)
The newest generation of Doppler radars, the 1988 Doppler weather radar. The radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm.

Usually applied to the broad patterns of persistent winds with a westerly component. It is the dominant persistent atmospheric motion, centered over the midlatitudes of each hemisphere.

Wet Bulb
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.

Wet Bulb Depression
Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

Wet Bulb Thermometer
A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista.

Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

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.

Wind Run
A measurement of how much wind has passed a given point in a period of time. For example, a wind blowing at three miles per hour for an entire hour would give a wind run of three miles. Used to calculate evapotranspiration as well as sea wave height.

Wind Shear
The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane.

Wind Vane
An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position.

The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.

Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere.

World Meteorological Organization (WMO)
From weather prediction to air pollution research, climate change related activities, ozone layer depletion studies and tropical storm forecasting, the World Meteorological Organization coordinates global scientific activity to allow increasingly prompt and accurate weather information and other services for public, private and commercial use, including international airline and shipping industries. Established by the United Nations in 1951, it is composed of 184 members.


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.


Zonal Flow
The flow of air along a latitudinal component of existing flow, normally from west to east.

Zulu Time
One of several names for the twenty-four hour time which is used throughout the scientific and military communities.