| Section A | A index | 501-509 of 917 terms |
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anchor balloon—A super-pressure balloon suspended below a zero-pressure balloon to permit the zero-pressure balloon to survive night-time cooling without the expenditure of ballast and daytime warming without release of lifting gas. The lift decreases during ascent and decreases during descent, providing stability to the zero- pressure balloon.
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anchor ice—Ice attached to the beds of streams, lakes, and shallow seas, irrespective of its nature of formation. On clear, cold nights in relatively still water, anchor ice may form directly on submerged objects. It also develops in supercooled water if turbulence is sufficient to maintain uniform temperature at all depths, in which case a spongy mass of frazil accumulates on objects exposed to rapid flow, and later deposition fills in the pores and creates solid ice. When the water temperature increases to above 0°C, the ice rises to the surface, often carrying with it the object on which it had accumulated. Sometimes anchor ice is erroneously called ground ice, a term which should be reserved for bodies of more or less clear ice in frozen ground.
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anelastic approximation—An approximate system of equations for deep and shallow atmospheric convection. The equations are derived under the assumptions that the percentage range in potential temperature is small and that the timescale is set by the Brunt–Väisälä frequency. Acoustic waves are thereby filtered—hence the term anelastic, meaning elastic energy is not allowed. If the vertical scale of motion is small compared to the depth of an adiabatic atmosphere, the anelastic equations reduce to the Boussinesq equations for shallow convection.
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anemometer level—1. The height above the surface at which an anemometer is actually exposed. 2. The ideal exposure height of an anemometer; usually designed to be above the wakes that form behind individual surface roughness elements such as nearby trees, houses, or ship structures. 3. The desired or standard exposure height of an anemometer, as specified by national or international agreement. This height is usually taken as 10 m.
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anemometer—The general name for instruments designed to measure either total wind speed or the speed of one or more linear components of the wind vector. These instruments may be classified according to the transducer employed; those commonly used in meteorology include the cup, propeller, Pitot-tube, hot-wire or hot-film, and sonic anemometers.See also current meter, wind vane.
Middleton, W. E. K., and A. F. Spilhaus, 1953: Meteorological Instruments, 3d ed., rev., Univ. of Toronto Press, 135–165.
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anemometry—The study of measuring and recording the direction and speed (or force) of the wind, including its vertical component. Systematic records of wind direction as indicated by wind vanes were begun in Italy in 1650 and in England in 1667. The speed of the wind was first observed by noting the rate at which light substances are carried along by the air; it is now determined by means of anemometers.
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anemoscope—An instrument for indicating the existence and direction of the wind.
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