| Section A | A index | 221-229 of 917 terms |
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advisory—Statements issued by a weather service that discuss 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 restrictions to visibility. See also special weather statements.
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aeolian tones—Sound, usually in the band of audible frequencies, associated with wake-eddy, vortex- produced pressure fluctuations resulting from air flow around obstacles, such as wires and twigs. Although many such sounds are irregular noises, other familiar sounds involve fairly clear musical notes or humming sounds. The latter sounds were called aeolian tones by Rayleigh. Their pitch is controlled by the frequency with which eddies are formed and detached in the wake region on the lee side of the obstacle. The tones produced by wind flowing over a cylinder, including stretched wire, were shown by Strouhal in 1878 to be of frequency (pitch) f given by  where u is the cross-axis wind velocity (m s−1) and d is the cylinder diameter (m).
Lord Rayleigh, 1878: The Theory of Sound, Vol. II, 412–413.
Humphreys, W. J., 1940: Physics of the Air, 3d ed., 442–448.
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aeolian—(Also spelled eolian.) Pertaining to the action or the effect of the wind, as in aeolian sounds or aeolian deposits (of dust); derived from the name of the Greek god of the winds, Aeolus, whose harp was held responsible for the murmur of the gentle breezes and whose conch-shell trumpet was regarded as the source of the gale's howl.
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aeration—In general, any process whereby a substance becomes permeated with air or other gas; can be used synonymously with “ventilation.” This term refers to the formation and renewal of soil air. See normal aeration.
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aerial perspective—Reduced contrast of distant objects caused by airlight. In artistic practice, aerial perspective is the painting of this reduced contrast to suggest an object's distance.
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AERO code—(Short for aeronautical code.) An international synoptic code in which selected observable meteorological elements of particular interest to aviation operations are encoded and transmitted in groups (“words”) of five numerical digits in length.
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aerobiology—The study of the distribution of living organisms freely suspended in the atmosphere and some consequences of this distribution. It includes microorganisms and some insects, seeds, and spores. Dispersion is by the wind, aided in some species by special adaptations and in some by flight. Small organisms are lifted and maintained in the air by eddy diffusion and rising thermal currents, sometimes reaching heights exceeding 16 km (10 miles) in air currents and by lateral mixing. They may be collected by adhesive-coated slides exposed horizontally, or by culture media, but for quantitative work, volumetric sampling at the surface or by aircraft is required.
Jacobs, W. C., 1951: Compendium of Meteorology, 1103–1111.
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