Weather Words - Aa-Am | Weather | WGNTV.com | WGN TV

Weather Words - Aa-Am

-- Aa-Am --

Abbe, Cleveland: (1838-1916) The meteorologist who pioneered in the establishment and growth of the U.S. Weather Bureau (later renamed the National Weather Service). The Weather Bureau was commissioned by Congress on November 1, 1870, as a branch of the Signal Service of the War Department.

Ablation: The loss of snow cover resulting from melting, evaporation or deflation (removal by wind). More generally, the loss from any snow or ice surface (snowpack, glacier, sea ice) by melting, evaporation, deflation or calving.

Abrupt Climate Change: A change in climate occurring on a scale of decades or less, taking place so rapidly and unexpectedly that human and natural systems have difficulty adapting.

Abscission: In botany, the dropping of leaves from a plant. Premature abscission frequently results from excessive exposure to certain air contaminants or to abnormal weather conditions.

Absolute humidity: The actual weight of water vapor contained in a unit volume of air, usually expressed in grams of water per kilogram of air.

Absolute zero: The point at which theoretically no molecular activity exists, or the temperature at which the volume of a perfect gas vanishes; the lowest temperature possible. The value is 0 degrees Kelvin, -273.15 degrees Celsius and -459.67 degrees Fahrenheit.

Acceleration of gravity: During every second of fall, the speed of a body freely falling in a vacuum in Earth's gravity will increase by 32 feet per second.

Accessory clouds: Cloud structures and features accompanying larger clouds, separate from or only partially merged with the main cloud body. Example: Pileus, the cap cloud that sometimes forms above rapidly growing cumulus clouds.

Acclimation: The process by which a living organism becomes adapted to a change of climatic environment, such as adaptation to the climates of polar or tropical regions, or to high altitudes; also called acclimatization.

Accretion: The growth of a small frozen airborne particle, such as an ice crystal, into a larger particle, usually a hailstone, by collision of the particle with supercooled liquid droplets that freeze on contact.

Accuracy: The extent to which the results of readings of a weather instrument approach the true value of the calculated or measured quantities, supposing that all possible corrections are applied.

Acid deposition: The accumulation of an acidic chemical from the atmosphere to the surface of the Earth, or to plants and structures on the surface. Acid deposition is killing trees on the higher peaks of mountains in the Appalachians.

Acid dust: Generally, any dust with a pH less than 5.6. Principal causes: The combining of airborne dust with acidic water droplets in the atmosphere.

Acid fog: Airborne water droplets containing sufficient quantities of sulfuric acid and/or nitric acid to reduce the pH level of the droplets below 5.6, the pH of natural rainwater.

Acid precipitation: Precipitation that has a pH value less than 5.6. The acidity results from excessive concentrations in the atmosphere of sulfuric acid, nitric acid or carbon dioxide.

Acid rain: (1) Rain having a pH lower than 5.6, the pH of natural rainwater. The increased acidity is due mainly to the presence in raindrops of sulfuric acid resulting from atmospheric pollution by sulfur dioxide.

Acid rain: (2) Rainfall with a pH less than 5.6. Principal causes: The combining of rainwater with sulfur dioxide, nitrous oxides and carbon dioxide emissions that are the byproducts of burning of fossil fuels.

Acid rain: (3) Any rainfall that has an acidity level beyond what is expected in non-polluted rainfall. Precipitation that has a pH value less than 5.6 is considered to be acid precipitation by many, but this is not a uniformly accepted standard.

Acid snow: Generally, any snow with a pH less than 5.6. Principal causes: The combining of snowflakes with sulfur dioxide, nitrous oxides and carbon dioxide -- the byproducts of burning fossil fuels.

Acoustic shadow: An atmospheric situation in which nearby loud sounds (such as explosions) that would ordinarily be heard are not heard, caused by strong vertical speed shear or sharp temperature inversions.

Acre-foot: The volume of water required to cover one acre to a depth of one foot, or 328,851 gallons. It is a convenient unit for measuring irrigation water, reservoir capacity and runoff volume.

Actinometer: The general name for any instrument used to measure the intensity of radiant energy, especially that of the sun.

Actinon: A radioactive gas that exists briefly in the atmosphere in trace amounts at the ground as a result of seepage from soil and rocks. Because its half-life is only 3.9 seconds, its minute concentration in the atmosphere is limited to within a few feet of the ground.

Adiabatic lapse rate: (1) The rate at which the temperature decreases in a parcel of air as it is lifted while not exchanging energy with its surroundings. If a cloud is not forming, the parcel cools at 5.4 degrees F. per 1,000 feet. Adiabatic lapse rate: (2) The rate (5.4 degrees F per thousand feet) at which air temperature decreases as air is raised in a vertical ascent, so long as no condensation of water vapor occurs.

Adiabatic process: An atmospheric process that does not involve an exchange of heat with the surroundings. For example, when air rises its temperature drops because of expansion into lower air pressure aloft, not because of heat loss.

Adret: The sunny, south-facing side of a mountain or mountain valley; the opposite slope is known as the ubac. Adrets are warmer, have more sun, less snow cover and a shorter duration of snow cover.

Advection: Horizontal movement of an atmospheric property such as temperature, moisture or clouds entirely by the wind. Advection usually refers to movement over considerable distance, often hundreds of miles.

Advection fog: (1) A type of fog formed by the movement of moist air over a cold surface and subsequent cooling of that air to below its saturation point. Advection fog commonly forms when moist air blows over snow-covered ground.

Advection fog: (2) A type of fog formed by the movement of moist air over a cold surface and subsequent cooling of that air to below its saturation point. Advection fog commonly forms in the spring when mild, moist air blows over Lake Michigan's cold waters.

Advisory: A forecast issued by the National Weather Service to highlight conditions that require caution or might cause considerable inconvenience, but are not thought to be immediately life- or property-threatening.

Aeolian: (also eolian) Pertaining to the action or the effect of the wind, as in aeolian sounds or aeolian deposits of sand. The word is derived from the name of the Greek god of the winds, Aeolus.

Aeolian sounds: Sounds produced by eddying motions of air in the lee of obstacles such as wires, twigs, and even the ear itself, when wind blows over those obstacles.

Aeolus: In Greek mythology, the god of the winds. His harp was believed to be responsible for the murmur of gentle breezes, and his conch-shell trumpet was regarded as the source of the gale's howl.

Aeration: Any active or passive process by which intimate contact between air and water is assured: by spraying water into the air, by bubbling air through water or by mechanical agitation of water.

Aeroallergens: Any of a variety of allergens such as pollens, grasses, or dust carried by winds.

Aerobic: Characterizing organisms able to live only in the presence of air or free oxygen; also, characterizing conditions that exist only in the presence of air or free oxygen. Contrast anaerobic.

Aerobiology: The study of the distribution of living things (such as microorganisms and some insects, seeds and spores) freely suspended in the atmosphere, and some consequences of this distribution.

Aerodynamics: The study of the forces and reactions arising from the motion of bodies through air.

Aeroembolism: The formation of bubbles of nitrogen in the bloodstream when the body experiences a rapid decrease of air pressure. Symptoms are discomfort, pain in the joints and loss of mental efficiency.

Aerology: An older term for meteorology; a study of the atmosphere through its entire vertical extent, as distinguished from studies of the atmosphere near the Earth's surface.

Aeronomy: The study of the physics and chemistry of the upper atmosphere of the Earth (or other planet).

Aerophobia: The irrational or morbid fear of wind and drafts, or of fresh air.

Aeroplankton: Any of the tiny life forms (such as bacteria, pollen and viruses) that float and drift in the air, carried by currents of wind. It is the atmospheric analogue to oceanic plankton.

Aerosol: (1) A system consisting of dispersed, suspended particles and the gaseous medium that contains them. The atmosphere and its suspended load of haze, smoke and (small) cloud particles is an aerosol. It is poor usage to refer to the dispersed particles alone as aerosols, though this is often done.

Aerosol: (2) A system consisting of dispersed, suspended particles and the gaseous medium that contains them. The atmosphere and its suspended load of haze, smoke and (small) cloud particles is an aerosol.

Aestival: Pertaining to the summer season.

Africa's lowest temperature: On Monday, Feb. 11, 1935, the temperature at Ifrane, Morocco, plunged to -11 degrees F, the lowest temperature ever recorded on the continent of Africa.

Afterglow: A broad, high arch of radiance seen occasionally in the western sky above the highest clouds in deepening twilight.

Agglomeration: The process in which precipitation particles (like raindrops or snowflakes) grow larger by collision with other cloud particles or other precipitation particles.

Agricultural drought: A shortage of precipitation (relative to the normal precipitation for that place) sufficient to have an adverse effect on crop production or range production.

Air: The mixture of gases that forms the atmosphere of the Earth. By volume, dry air is composed of nitrogen (78.09 percent), oxygen (20.95 percent), argon (0.93 percent), carbon dioxide (0.03 percent), and several trace gases. Water vapor varies between zero and four percent by volume.

Air burst: An above-ground nuclear explosion. The amount of radioactive fallout is reduced (compared to a ground blast), but the blast-damaged area is increased and the blast's electromagnetic pulse is possibly capable of damaging electronic equipment hundreds of miles outward from the blast.

Air conditioning: The artificial control of temperature, humidity, purity and motion of the air within buildings and other enclosed spaces. The objective is usually to secure either maximum human comfort or the best environment for a given industrial or commercial operation.

Air density: If the density of water is defined as 1.000, the density of air at sea level is 0.001293, or slightly more than one thousandth (1/1000) the density of water.

Air drainage: The gravity-induced downslope flow of relatively cold air; usually responsible for surprisingly chilly temperatures often observed in certain low sites in suburban Chicagoland.

Air mass: A widespread body of air that contains rather uniform characteristics, especially as regards temperature and moisture content. Air masses over land are usually hundreds of miles in extent, occasionally thousands.

Air pocket: An expression used in the early days of aviation for a downdraft. Pioneer pilots at first imagined these downdrafts to be pockets in which there was insufficient air to support the plane.

Air pollutants: Substances in the atmosphere that are directly or indirectly harmful to living things or damaging to property, and usually applied only to substances in concentrations larger than occur naturally.

Air pollution: (1) The presence in the atmosphere of substances directly or indirectly harmful to living things or damaging to property, and usually applied only to substances in the air in concentrations larger than occur naturally.

Air pollution: (2) The presence in the atmosphere of substances, usually directly or indirectly harmful to living things, that do not occur naturally or are in concentrations greater than occur naturally.

Air pollution and obesity: The average American now weighs ten pounds more than in 1990 -- extra weight that causes airlines to use additional fuel that annually generates 3.8 million tons of carbon dioxide.

Air pollution episode: (1) The most severe air pollution episodes of modern times are brought about by inversions (warm air lying over colder air at the surface) that prevent the surface air, and its pollutants, from rising.

Air pollution episode: (2) A period of time, usually several days, during which air pollution standards are exceeded, or during which pollutant levels are significantly greater that normal.

Air pressure: (1) The force exerted per unit area by the weight of the atmosphere. The weight of a column of air extending to the top of the atmosphere. At sea level, air pressure is 14.7 pounds per square inch.

Air pressure: (2) The weight (14.7 pounds per square inch at sea level) of a column of air extending to the top of the atmosphere.

Air temperature: The temperature indicated by a thermometer exposed to freely circulating air but sheltered from precipitation and direct sunshine, ideally located in a shelter about five feet off the ground.

Aircraft icing: The accumulation of ice on the exposed surfaces of aircraft when flown through supercooled water drops. Icing is a significant hazard to the safe operation of aircraft.

Airglow: Faint, ever-present brightness in the night sky caused by excited atoms of sodium and oxygen in the upper (50+ miles) atmosphere constantly emitting energy in the form of orange and green light.

Air mass thunderstorm: A type of thunderstorm, not associated with fronts or storm systems, that forms in mid or late afternoon in an air mass that has become unstable because of low-level heating by the sun; rarely becomes severe.

Alaskan permafrost: Permafrost (permanently frozen ground) underlies about 85 percent of the state of Alaska, and in northern Alaska it reaches its greatest thickness, about 2,000 feet.

Alaska's glaciers: The greatest concentration of glaciers in the United States is found in Alaska. Glaciers and ice fields cover 29,000 square miles of Alaska, or five percent of the state. Nearly all of the state's glaciers are in retreat.

Alaska's greatest single snowstorm: 175.4 inches (that's 14 1/2 feet) at Thompson Pass, Alaska. It was a six-day storm, beginning on Dec. 26, 1955, and finally ending on Dec. 31.

Alaska's highest temperatures: At Anchorage, the all-time highest is 86 degrees F recorded on June 25, 1953. The state record is 100 degrees F on June 27, 1915, at Fort Yukon.

Alaska's temperature extremes: A range of 180 degrees F from 100 at Fort Yukon (elevation 420 feet) on June 27, 1915, to -80 at Prospect Creek Camp (elevation 1,100 feet) on January 23, 1971.

Albedo: Of the light falling upon a body, the proportion that is diffusely reflected. The albedo of the planet Earth is about 0.4, which means that four-tenths of the sun's radiation is reflected back into space.

Alberta clipper: During the winter, a fast-moving low pressure system that sweeps southeast from Alberta, Canada, across the northern Great Plains and Midwest of the United States. These storms usually bring a few inches of low water-content snow, and, when intense, can produce blizzards.

Aleutian low: The low pressure system located near the Aleutian Islands (west of Alaska) during the winter. It represents one of the main centers of action in Northern Hemisphere atmospheric circulation.

Algorithm: A computer program that solves a frequently occurring problem. National Weather Service Doppler radars use algorithms to analyze radar data for determining storm motion, accumulated rainfall and many other kinds of weather information.

Almanac: A calendar to which astronomical information and weather data are added. The word is of Arabic origin, originally meaning a camp, and subsequently the weather at that place. In Arab nations the weather is largely seasonal, and it was associated with positions of the stars. Almanacs were compiled of positions of the stars and the weather to be expected on each day.

Alpenglow: The illumination (usually reddish) from sunlight on the peaks of mountains or the tops of vertically built clouds like thunderheads just after evening sunset or before morning sunrise.

Altimeter: An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of air pressure values.

Altitude: Height expressed as the vertical distance above a reference point, normally sea level or ground level.

Alto: Middle height, as in altocumulus or altostratus.

Altostratus: A medium-height cloud (altitude 8-20 thousand feet) characterized by a uniform, sky-covering gray or bluish (never white) sheet or layer, sometimes thin enough to reveal the position of the sun.

Ambient air: The air that surrounds us and within which we live; background, environmental or surrounding air.

American Meteorological Society: The professional, scientific organization through which the collective interests of the science of meteorology have been represented since 1919. The Chicago chapter, organized in 1938, meets monthly at the Chicago National Weather Service office in Romeoville.

Ammonia: A colorless gas with a sharp, irritating odor, having a density about 0.6 that of air; it is emitted in large quantities by animal feedstocks and sewer treatment plants. Ammonia is very soluble in water and is removed from the atmosphere by clouds.