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

absolute humidity
A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.

absolute instability
When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated. Related term: instability

absolute temperature
A temperature scale with a freezing point of +273°K (Kelvin) and a boiling point of +373°K. Related term: Kelvin Temperature Scale

absolute zero
Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0° Kelvin, -273.15° Celsius and -459.67° Fahrenheit.

absorption
The process in which incident radiant energy is retained by a substance. The absorbed radiation is then transformed into molecular energy.

abyssal plain
The flat, gently sloping or nearly level region of the sea floor.

adiabatic process
A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In this process, compression will result in warming and expansion will result in cooling.

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

advection fog
Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point. Related terms: Arctic Sea Smoke and sea fog

advisory
Statements that are issued by the National Weather Service for probable weather situations of inconvenience that do not carry the danger of warning criteria, but, if not observed, could lead to hazardous situations. Some examples include snow advisories stating possible slick streets, or fog advisories for patchy fog condition causing temporary res...

air
This is considered the mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are Nitrogen (N₂) at 78.09%, Oxygen (O₂) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide (CO₂) at 0.033%. One of the most important constituents of air and most...

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

air mass thunderstorm
A thunderstorm that is produced by convection within an unstable air mass through an instability mechanism. Such thunderstorms normally occur within a tropical or warm, moist air mass during the summer afternoon as the result of afternoon heating and dissipate soon after sunset. Such thunderstorms are not generally associated with fronts and are le...

air pollution
The soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant, or animal life, or prevent the use and enjoyment of the outdoors.

air quality standards
The maximum level which will be permitted for a given pollutant. Primary standards are to be sufficiently stringent to protect the public health. Secondary standards must protect the public welfare, including property and aesthetics.

alaskan winds
The downslope air flow that blows through the Alaskan valleys. It is usually given local names, such as Knik, Matanuska, Pruga, Stikine, Taku, Take, Turnagain, or Williwaw.

albedo
The ratio of the amount of radiation reflected from an object's surface compared to the amount that strikes it. This varies according to the texture, color, and expanse of the object's surface and is reported in percentage. Surfaces with high albedo include sand and snow, while low albedo rates include forests and freshly turned earth. Related term...

alberta clipper
A fast moving, snow-producing weather system that originates in the lee of the Canadian Rockies. It moves quickly across the northern United States, often bring gusty winds and cold Arctic air.

altimeter
An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.

altimeter setting
The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.

altitude
In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.

altocumulus
Composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. In the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. A defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of 'sheep' or 'woolpack' clouds. Sometimes co...

altocumulus castellanus
A middle cloud with vertical development that forms from altocumulus clouds. It is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. Its formation indicates instability and turbulence at the altitudes of occurrence.

altostratus
This middle cloud genus is composed of water droplets, and sometimes ice crystals, In the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. White to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. It is a good indicator of precipitation, as it often precedes a storm system. Vir...

american meteorological society
An organization whose membership promotes the education and professional advancement of the atmospheric, hydrologic, and oceanographic sciences. For further information, contact the AMS. Related term: National Weather Association

anabatic wind
A wind that is created by air flowing uphill. Valley breezes, produced by local daytime heating, are an example of these winds. The opposite of a katabatic wind.

anemometer
An instrument that measures the speed or force of the wind. Related term: Dave's Dictionary

aneroid barometer
An instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches and millibars....

anomalous propagation
This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. May be referred to as A.P.

antarctic
Of or relating to the area around the geographic South Pole, from 90° South to the Antarctic Circle at approximately 66 1/2°South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstic...

antarctic ocean
Although not officially recognized as a separate ocean body, it is commonly applied to those portions of the Atlantic, Pacific, and Indian Oceans that reach the Antarctic continent on their southern extremes.

anticyclone
A relative pressure maximum. An area of pressure that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. It is the opposite of an area of low pressure, or a cyclone. Related term: high pressure

anvil
The upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes for hundreds of miles downstream from the parent cloud. It may look smooth or fibrous, but in shape, it resembles a blacksmith's anvil. It indicates the mature or decaying stage of a thunderstorm.

aphelion
The point on the earth's orbit that is farthest from the sun. Although the position is part of a 21,000 year cycle, currently it occurs around July, when the earth is about 3 million miles farther from the sun than at perihelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of perihelion.

apogee
The point farthest from the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of perigee.

arctic
Of or relating to the area around the geographic North Pole, from 90° North to the Arctic Circle at approximately 66 1/2 North latitude.

arctic air mass
An air mass that develops around the Arctic, it is characterized by being cold from surface to great heights. The boundary of this air mass is often defined by the Arctic front, a semi-permanent, semi-continuous feature. When this air mass moves from its source region, it may become more shallow in height as it spreads southward.

arctic jet
The jet stream that is situated high in the stratosphere in and around the Arctic or Antarctic Circles. It marks the boundary of polar and arctic air masses.

arctic sea smoke
A type of advection fog that forms primarily over water when cold air passes across warmer waters. Related term: steam fog

argon (a)
A colorless, odorless inert gas that is the third most abundant constituent of dry air, comprising 0.93% of the total.

arid
A term used for an extremely dry climate. The degree to which a climate lacks effective, life-promoting moisture. It is considered the opposite of humid when speaking of climates.

astronomical twilight
The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's location, to allow astronomical work to proceed. It ends when the center of the sun is 18° below the horizon. Related term: twilight

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

atmospheric pressure
The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). Related term: barometric pressure

aurora
It is created by the radiant energy emission from the sun and its interaction with the earth's upper atmosphere over the middle and high latitudes. It is seen as a bright display of constantly changing light near the magnetic poles of each hemisphere. In the Northern Hemisphere, it is known as the aurora borealis or Northern Lights, and in the Sout...

autumn
The season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the North Hemisphere and the months of March, April, and May in the Southern Hemisphere. Astronomically, this is the period be...

aviation weather center
As one of the National Centers for Environmental Prediction, it is the national center for weather information that is used daily by the Federal Aviation Administration, commercial airlines, and private pilots. It is entering a new phase of service, growing to accept global forecasting responsibilities. For further information, contact the AWC, loc...

backing
A counterclockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is clockwise. This can either happen in the horizontal or the vertical (with height). For example, the wind shifts from the northeast to the north to the northwest. It is the opposite of veering.

backscatter
A radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave. Also the scattering of radiant energy into space before it reaches the earth's surface.

ball lightning
A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air. Related term: globe lightning

baroclinity
The state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of constant density. Also known as baroclinicity. An example is the tight temperature gradient along the East Coast of the United States during the winter that gives rise to intense cyclogenesis.

barograph
An instrument that continuously records a barometer's reading of atmospheric pressure. Related term: aneroid barometer

barometer
An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer.

barometric pressure
The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). Related term: atmospheric pressure

barotropy
The state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. It is considered zero baroclinity.

barrier winds
Refers to the westerly flow of air along the northern slope of the Brooks Range in northern Alaska that precedes the arrival of colder air from the north.

bathythermograph
A device used to obtain a record of temperature against depth (pressure) in the ocean. May be referred to as a B.T.

beaufort wind scale
A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

bellot winds
Refers to the winds in the Canadian Arctic that blow through the narrow Bellot Strait between Somerset Island and the Boothia Peninsula, connecting the Gulf of Boothia and Franklin Strait.

bernoulli's theorem
A statement of the conservation of energy for a steady, nonviscous, incompressible level flow. It is an inverse relationship in which pressures are least where velocities are greatest. Theorized by Daniel Bernoulli (1700-1782), a Swiss mathematician and physicist.

biosphere
The transition zone between the earth and the atmosphere within which most terrestrial life forms are found. It is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere.

black blizzard
A local term for a violent duststorm on the south-central Great Plains that darkens the sky and casts a pall over the land. Related term: black roller

black ice
Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.

blizzard
A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours. A severe blizzard is characterized by temperatures near or below 10°F, winds exceeding 45 mph, and visibility ...

blocking high
The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. Such a high will move very slowly, tending to move westward during intensification and eastward during dissipation. It prevents the movement of migratory cyclones across its latitude...

blowing dust
Dust that is raised by the wind to heights of six feet or greater. It is reported as 'BLDU' in an observation and on the METAR.

blowing sand
Sand that is raised by the wind to heights of six feet or greater. It is reported as 'BLSA' in an observation and on the METAR.

blowing snow
Snow that is raised by the wind to heights of six feet or greater. It is reported as 'BLSN' in an observation and on the METAR.

blowing spray
Salt spray that is raised by the wind to heights of six feet or greater. It is reported as 'BLPY' in an observation and on the METAR.

blue norther
Refers to a swift-moving cold frontal passage in the southern Great Plains, marked by a dark, blue-black sky with strong wintery winds from the northwest or north and temperatures that may drop 20°F to 30°F in a few minutes. Related term: Texas Norther

boiling point
The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. The boiling point of pure water at standard pressure is 100°C or 212°F.

boulder wind
A local name referring to an extremely strong downslope wind in the Front Range of the Rocky Mountains near Boulder, Colorado.

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. Related terms: surface boundary layer and friction layer

bow echo
A radar echo signature often associated with severe thunderstorms, especially those that produce wind damage. It is bent outward in a 'bow' shape.

boyle's law
States that when the temperature is held constant, the volume of a gas is inversely proportional to its pressure. Therefore, if the pressure increases, the volume decreases and visa versa. For example, if the volume if halved, then the pressure is doubled. If the temperature is held constant, it becomes an isothermal process. Discovered by Robert B...

bright band
A narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest.

broken
The amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer.

bubble high
A small high that may be created by precipitation and vertical instability associated with thunderstorm activity. A product of downdrafts, it is relatively cold and often has the characteristics of a different air mass. Convergence along the leading edge of a bubble high may help form additional thunderstorms. Related term: meso high

buys ballot's law
Describes the relationship of the horizontal wind direction to the pressure distribution. In the Northern Hemisphere, if one stands with one's back to the wind, the pressure on one's left is lower than the pressure on one's right. It is reversed in the Southern Hemisphere. This law was named after the Dutch meteorologist, Buys Ballot, who developed...

calm
Atmospheric conditions devoid of wind or any other air motion. In oceanic terms, it is the apparent absence of motion of the water surface when there is no wind or swell.

calorie
In meteorology, it is the amount of heat required to raise the temperature of one (1) gram of water one (1) degree Celsius. It is a unit of heat energy.

cap
Composed of a layer of warmer, dryer air aloft which may suppress or delay the development of thunderstorms. As an air parcel rises, it becomes cooler relative to the ambient, or surrounding, air in the cap and therefore, less buoyant and unable to rise further. Also referred to as a lid.

cape verde islands
A group of volcanic islands in the eastern Atlantic Ocean off the coast of West Africa. A Cape Verde hurricane originates near here.

carbon dioxide (co2)
A heavy, colorless gas that is the fourth most abundant constituent of dry air, comprising 0.033% of the total.

catalina eddy
A weak low pressure circulation that may form off the Southern California coast.

ceiling
The lowest cloud layer that is reported as broken or overcast. If the sky is totally obscured, then it is the height of the vertical visibility. Related terms: measured ceiling and variable ceiling

ceiling light
An instrument consisting of a drum and an optical system that projects a narrow vertical beam of light onto a cloud base.

ceilometer
An instrument that is used to measure the angular elevation of a projected light on the base of a cloud. It measures the angle of the cloud base included by the observer (or machine), the ceiling light and the illuminated spot on the cloud.

celestial equator
The projection of the plane of the geographical equator upon the celestial sphere.

celestial sphere
The apparent sphere of infinite radius having the earth as its center. All heavenly bodies (planets, stars, etc.) appear on the 'inner surface' of this sphere and the sun moves along the ecliptic.

celsius temperature scale
A temperature scale where water at sea level has a freezing point of 0°C (Celsius) and a boiling point of +100°C. More commonly used in areas that observe the metric system of measurement. Created by Anders Celsius in 1742. In 1948, the Ninth General Conference on Weights and Measures replaced 'degree centigrade' with 'degree Celsius.' Rela...

central pressure
The atmospheric pressure at the center of a high or low. It is the highest pressure in a high and the lowest pressure in a low, referring to the sea level pressure of the system on a surface chart.

centrifugal force
The apparent force in a rotating system that deflects masses radially outward from the axis of rotation. This force increases towards the equator and decreases towards the poles.

centripetal force
The force required to keep an object moving in a curved or circular path. It is directed inwards toward the center of the curved path.

charles' law
States that when the pressure is held constant, the volume of a gas varies directly with the temperature. Therefore, if the pressure remains constant, the volume of a gas will increase with the increase of temperature. It was developed by Jacques Charles and is also known as the Charles-Guy-Lussac Law.

chemosphere
A vaguely defined region of the upper atmosphere in which photochemical reactions take place. It includes the top of the stratosphere, all of the mesosphere, and sometimes the lower part of the thermosphere.

cheyenne fog
An upslope fog formed by the westward flow of air from the Missouri River Valley, producing fog on the eastern slopes of the Rockies.

chinook
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 temperature could rise by 20°F to 40°F in a matter of minutes. Related term: Snow Eater and Dave's Dictionary

chocolatta north
A West Indian gale that blows from the northwest.

chromosphere
A thin layer of relatively transparent gases above the photosphere of the sun. It is observed best during a total eclipse of the sun.