| Section A | A index | 461-469 of 917 terms |
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ambient pressure—The air pressure that is characteristic of the atmosphere surrounding a small- scale feature such as a cumulus cloud.
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ambiguity function—In radar, a two-dimensional function of radar range and Doppler frequency, determined by the transmitted waveform, that defines the effective filter response of the radar receiver. The ambiguity function is used to examine the suitability of different transmitted waveforms for achieving accuracy, resolution, freedom from ambiguities, and reduction of unwanted clutter. In particular, it defines 1) the sidelobe response to a complex radar waveform at different Doppler frequencies, and 2) the response of a receiver to targets at other ranges and Doppler frequencies from the desired target. The ambiguity function applies to both point targets and distributed targets.
Nathanson, F. E., 1991: Radar Design Principles, 2d ed., McGraw-Hill, 360–369.
Skolnik, M. I., 1980: Introduction to Radar Systems, 2d ed., McGraw-Hill, 411–420.
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ammonia—A colorless gas, formula NH3, with a sharp, irritating odor, having a density about six- tenths that of air at the same temperature and pressure (0.7720 g cm−3 at STP). A reduced nitrogen gas, NH3 is emitted in large quantities from animal feedstocks, sewerage plants, etc. Ammonia is very soluble in water and is scavenged from the lower atmosphere by clouds. It is the most abundant alkaline gas in the atmosphere and as such plays a large role in neutralizing acidity from sulfuric and nitric acids via formation of the ammonium ion. Large quantities of ammonia gas, and probably even ammonia crystals, occur in the atmospheres of the large planets Saturn and Jupiter. See air.
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ammonification—The conversion of organic nitrogen to ammonia or ammonium ion by heterotrophic bacteria (ammonifiers) involved in the decomposition of organic matter.
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ammonium nitrate—Inorganic salt, formula NH4NO3, formed from the neutralization of nitric acid by ammonia. Ammonium nitrate is a deliquescent solid with a low vapor pressure, which consequently forms in the atmosphere when high concentrations of ammonia and nitric acid occur, for example, in polluted urban areas. It is thought to sometimes make up a considerable fraction of the urban aerosol present in smog episodes. See deliquescence.
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ammonium sulfate—Inorganic salt, formula (NH4)2SO4. A large constituent of tropospheric aerosol, due to neutralization of acidic sulfuric acid aerosol by ammonia. Much of the aerosol is partially neutralized, and the formula NH4HSO4, corresponding to ammonium bisulfate, is generally more appropriate. Ammonium sulfate is a solid, crystalline material that deliquesces at relative humidities greater than 75%, eventually forming liquid droplets. See deliquescence.
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amorphous clouds—Clouds without any apparent structure at all, as may occur in a whiteout in a thick cloud or fog over a snow surface when one loses any sense of direction—up, down, or sideways.
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