| Section A | A index | 371-379 of 917 terms |
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airways shelter—A small instrument shelter designed for use at supplementary airways weather reporting stations (SAWRS). It is a white, louvered box mounted on a single post.
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Aitken dust counter—(Or Aitken nucleus counter.) An instrument developed by John Aitken for determining the dust content of the atmosphere. A sample of air is mixed in an expandable chamber with a larger volume of dust-free air containing water vapor. Upon sudden expansion, the air in the chamber cools adiabatically below its dewpoint, and droplets form with the dust particles as nuclei. A portion of these droplets settle on a ruled plate in the instrument and are counted with the aid of a microscope. See dust counter.
Aitken, J., 1923: Collected Scientific Papers, 236–246.
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Aitken nucleus—Any one of the many microscopic particles in the atmosphere that serve as condensation nuclei for droplet growth in the large supersaturations (greater than a few hundred percent over water) produced during the rapid, near adiabatic expansion produced in an Aitken dust-counter. These nuclei, often numbering many tens of thousands per cubic centimeter in city air, are both solid and liquid particles with diameters on the order of tenths of microns or smaller. Because of the excessive supersaturations that accompany expansions of the air sample in an Aitken dust- counter, the nucleus spectrum observed with this instrument does not correspond to that observed in natural cloud condensation processes, where supersaturations larger than one per cent over water are probably rare. On the other hand, Aitken nuclei play an important role in determining the local electrical conductivity of the air, because they capture small ions, becoming large ions with much lower mobility in the earth's fair-weather electric field. In air containing large numbers of Aitken nuclei, the small ion population is small, the large ion population is large, and the air conductivity is low. Either nucleus may also be a protoparticle for larger particles such as cloud condensation nuclei, the subset of Aitken nuclei responsible for the formation of cloud droplets.
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alarm level—Likelihood of or confidence in a forecast or report of dangerous, threatening, or damaging weather-related phenomena. For example, a warning or watch may be issued based on either a forecast or observation of severe thunderstorms, flash flooding, river flooding, high winds, winter storms, etc. The National Weather Service does not wait for the event to occur before warning the public; there must be some lead time associated with it, such as a flood warning prior to reaching flood stage.
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Alaska Current—The eastern semi of the North Pacific subpolar gyre. It is a shallow current carrying relatively warm water northward and thus has a climate influence similar to that exercised by the North Atlantic and Norwegian Currents on the climates of northwestern Europe, though on a smaller scale. It flows cyclonically around the Gulf of Alaska, feeding into the Alaskan Stream. Freshwater from the many rivers of Canada and Alaska reduces the water density near the coast; the result is a pressure gradient normal to the coast that constrains the current geostrophically to the coastal region and increases its speed to 0.3 m s−1.
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Alaskan Stream—The continuation of the Alaska Current along the southern side of the Aleutian Islands. The distinction between the Alaskan Stream and the Alaska Current is gradual, and the two currents are sometimes regarded as one. They are, however, of different character, the Alaska Current being shallow and variable but the Alaskan Stream reaching to the ocean floor. Despite its modest speed of 0.3 m s−1, it is a western boundary current. Most of the water of the Alaskan Stream feeds directly into the Oyashio. Some of its flow enters the Bering Sea between the Aleutian Islands (most of it between 168° and 172°W) and follows a cyclonic path before feeding into the Kamchatka Current, thus eventually also contributing to the Oyashio.
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albedo—The ratio of reflected flux density to incident flux density, referenced to some surface. Albedos commonly tend to be broadband ratios, usually referring either to the entire spectrum of solar radiation, or just to the visible portion. More precise work requires the use of spectral albedos, referenced to specific wavelengths. Visible albedos of natural surfaces range from low values of ∼0.04 for calm, deep water and overhead sun, to > 0.8 for fresh snow or thick clouds. Many surfaces show an increase in albedo with increasing solar zenith angle. See also plane albedo, planetary albedo, spherical albedo, directional-hemispherical reflectance, bihemispherical reflectance.
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albedometer—An instrument used for the measurement of the reflecting power (the albedo) of a surface. A pyranometer adapted for the measurement of radiation reflected from the earth's surface is sometimes employed as an albedometer.
Fritz, S., 1948: The albedo of the ground and atmosphere. Bull. Amer. Meteor. Soc., 29, 303–312.
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Alberta clipper—A low pressure system that is often fast-moving, has low moisture content, and originates in western Canada (in or near Alberta province). In the wintertime, it may be associated with a narrow but significant band of snowfall, and typically affects portions of the plains states, Midwest, and East Coast.
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