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WeatherChannel - Weather Terms Category: Meteorology and astronomy > Weather Glossary Date & country: 30/09/2008, USA Words: 751

dry adiabat
The line on a Skew T-Log P chart that depicts the lifting of dry air, or air that is unsaturated. As a parcel rises adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel. When an air parcel is unsaturated and rises, then the temperature decreases at a rate of 1°C per 100 meters (5.5°F per 1,0...

dry bulb thermometer
A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.

dry line
The boundary between the dry desert air mass of the Southwest U.S. and the moist air mass from the Gulf of Mexico. It usually lies north-south across the central and southern High Plains states during spring and summer. The passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to...

dry slot
An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. Best seen on a satellite picture, such as a water vapor image.

dusk
The period of waning light from the time of sunset to dark. Related terms: twilight and dawn

dust
Small particles of earth or other matter suspended in the air. It is reported as 'DU' in an observation and for wide spread dust on the METAR.

dust bowl
The term given to the area of the Great Plains including Texas, Oklahoma, Kansas, Colorado, and New Mexico that was most greatly affected during the Great Drought of the 1930's. Related term: drought

dust devil
A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating. Related term: whirlwind

duststorm
A severe weather condition characterized by strong winds and dust-filled air over a large area. Visibility is reduced to between 5/8ths and 5/16ths statute mile. It is reported as 'DS' in an observation and on the METAR.

earthlight (earthshine)
The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere.

earthquake
A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity.

easterlies
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. Normally, it moves slower than the atmospheric current in which it is embedded and is c...

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

eclipse
The obscuring of one celestial body by another. Related terms: lunar eclipse and solar eclipse

ecliptic
The sun's apparent path across the sky that tracks a circle through the celestial sphere.

ecology
The study of the relationships between living organisms and their environment.

eddy
A small disturbance of wind in a large wind flow, which can produce turbulent conditions. They can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. In oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter t...

electromagnetic radiation
Also called radiation, it is waves of energy propagated though space or through a material media.

electromagnetic spectrum
The band of electromagnetic radiation with components that are separated into their relative wave lengths. The portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. The various types of energy comprising the spectrum are (from longest to shortest) radio, i...

elevation
The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes referred to as station elevation.

el niã`o
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 waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process. Related term...

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

equator
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.

equatorial trough
The quasi-continuous area of low pressure between the subtropical high pressure areas in both the Northern and Southern Hemisphere. Related terms: Intertropical Convergence Zone (ITCZ) and doldrums

equinox
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. Related term: Dave's Dictionary

erosion
The movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind.

evaporation
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.

evapotranspiration
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.

exosphere
This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface. Related term: ionosphere

extratropical cyclone
Any cyclone that is no longer tropical in origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes. An extratropical storm is a cyclone that no longer derives its energy source from the processes involved in sustaining a tropical cyclone, but thrives on baroclinic processes; i.e., the temperature contras...

eye
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 to up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in si...

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.

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

fathom
The common unit of depth in the ocean for countries using the English system of measurement. It is six feet or 1.83 meters. It can also be used in expressing horizontal distance, since 120 fathoms is equal to one cable or nearly on tenth of a nautical mile.

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...

fetch
An area of the water surface over which waves are generated by a wind having a constant direction and speed. Also, it is the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated. One of the ingredients for lake effect snow is the fetch of the water over which cold air can gain moisture....

few
The amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amount for that layer.

filling
Used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. The oppo...

firewhirl
A tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption.

first gust
Another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm. Related term: gust front

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.

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

flood plain
Level land that may be submerged by flood waters.

flood stage
The level of a river or stream where overflow onto surrounding areas can occur.

foehn
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. Related terms: chinook wind and Santa Ana wind

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' ...

fog bank
A fairly well-defined mass of fog observed in the distance. Most commonly seen at sea, over a lake, or along coastal areas.

fogbow
A whitish semicircular arc seen opposite the sun in fog. The outer margin has a reddish tinge, its inner margin has a bluish tinge, and the middle of the band is white. An additional bow with reversed colors sometimes appears inside the first.

forecast
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. Related term: prediction

fractus
The elements of cumulus and stratus clouds that appear in irregular fragments, as if they had been shred or torn. Never appears in cirrus clouds. Also known as scud. Related terms: cumulus fractus and stratus fractus

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 fog
Used to describe the phenomena when fog is present and the air temperature is below 0°C. It is reported as 'FZFG' in an observation and on the METAR.

freezing point/freeze
The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0°C or 32°F. It is the opposite of fusion. In oceanography, the freezing point of water is depressed with increasing salinity.

freezing precipitation
Precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces. Related terms: freezing rain and freezing drizzle

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.

fresh water
Water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity.

friction
In meteorology, it is the turbulent resistance of the earth on the atmosphere. Considered as the resistance of fluids (air and water) to the relative motion of a solid body. The amount is dependent on the size and shape of the body.

friction layer
The thin layer of atmosphere adjacent to the earth's surface. Surface friction is effective in slowing down wind up to approximately 1,500 to 3,000 feet above the ground. Above this level, air tends to flow parallel to the isobars. Wind distribution within this layer is determined by vertical temperature gradient and the physical contours of the un...

front
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. Related terms: cold front and warm front

frontal passage
It is the passage of a front over a specific point on the surface. It is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. Accompanying a passage may be precipitation and clouds. May be referred to as 'fropa.'

frontogenesis
The birth or creation of a front. This occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. It could happen when either air mass, or both, move over a surface which strengthens their original properties. However, it occurs most often along the eastern coasts ...

frontolysis
The destruction or dying of a front where the transition zone is losing its contrasting properties. The opposite of frontogenesis.

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

frozen precipitation
Precipitation that reaches the ground in a frozen state. Examples include snow, snow pellets, snow grains, ice crystals, ice pellets, and hail.

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. Related term: Fujita-Pearson Scale

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. It is reported as 'FC' in an observation and on the METAR.

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 warning
A warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour).

geophysics
The study of the physics or nature of the Earth and its environment. It deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. Areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. By exte...

geosphere
Considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. At their conjunction is the biosphere.

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.

glacier winds
Air flow that descends from glaciers, occasionally at a high rate of speed. Caused by the temperature difference between the air in contact with the glacier and the air at the same altitude, it reaches maximum intensity in the early afternoon. Related term: katabatic wind

glaze
A smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on contact. A storm that produces the accretion of glaze is called an ice storm. Related term: clear ice

gradient wind
A steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure or contour field, assuming there is no friction and no divergence or convergence.

graupel
A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together. Related term: snow pellets

gravitation
The mutual attraction between two masses of matter. The rotation of the earth and the atmosphere modifies this attraction to produce the field of gravity.

gravity
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. Standard Time begins at Greenwich, England, home of the Royal Observatory which first utilized this method of world time. This is also the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of a...

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.

ground fog
Fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. Primarily takes place at night or early morning. Related term: radiation fog

growing season
Considered the period of the year during which the temperature of cultivated vegetation remains sufficiently high enough to allow plant growth. Usually considered the time period between the last killing frost in the spring and the first killing frost of the autumn. The frost-free growing season is between the first and last occurrence of 32Â...

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 Cu...

gully washer
A heavy rain shower that occurs suddenly, possibly creating a flash flood.

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.

gust front
The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary. Related term: first gust

haboob
Sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought.

hail
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 called hailstones. It i...

halo
The ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce this phenomena, the ice crystals must be in a heterogeneous arrangement to refract the sunlight. The most commonly observed is a halo that forms at a 22° radius, although another one at 46° radius may also be seen.

haze
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. It is reported as 'HZ' in an observation and on the METAR.

heat
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 balance
The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. The balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation).

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
The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature. Related term: Heat Index Chart

heating degree day
One heating degree day is given for each degree that the daily mean temperature is below 65°F. It is used as an indication of fuel consumption. Related terms: degree day or cooling degree day

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°F, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical...