| Section S | S index | 251-259 of 1376 terms |
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semiannual oscillation—That component of the annual cycle of a variable that consists of a sinusoidal oscillation with a period of six months. Strong semiannual signals in thermal and momentum fields of the troposphere are found in both the Tropics (especially in the eastern hemisphere) and in the oceanic regions of Southern Hemisphere middle and high latitudes.
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semiarid climate—In Thornthwaite's 1931 climatic classification, a humidity province in which the principal plant life is drought-resistant short grasses. Köppen called these conditions the steppe climate. Semiarid regions are very susceptible to severe drought.
Thornthwaite, C. W., 1931: The climates of North America according to a new classification. Geogr. Rev., 21, 633–655.
Köppen, W. P., and R. Geiger, 1930–1939: Handbuch der Klimatologie, Berlin: Gebruder Borntraeger, 6 vols
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semiarid zone—Aridity is a climatological condition in which the amount of precipitation received (supply) is exceeded, on average, by potential evapotranspiration (demand). A number of physically based indices have been proposed to describe this deficiency that relate precipitation to temperature and humidity. For practical purposes, in the temperate and tropical zones, semiarid climates generally receive between 200 and 500 mm of precipitation per year on average, and arid climates receive less than 200 mm. However, definitions based on mean precipitation are not always satisfactory, because they do not express variability or the likelihood of drought. Rainfall in arid climates is extremely variable, with coefficients of variation for arid climates exceeding 50%, and coefficients for semiarid climates ranging between 30% and 50%. In broad economic terms, variability restricts the potential use of these lands. Arid climates are unsuitable for growing crops using rainfall alone, because crops may fail three or more years out of ten. Semiarid climates often support grasses that are suitable for grazing animals. Thus, livestock raising is often more appropriate, less risky, and more common than rainfed agriculture in semiarid areas.
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semigeostrophic motion—Atmospheric motion that fulfills the principles and requirements of semigeostrophic theory.
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semipermanent anticyclone—A high pressure circulation feature on monthly mean charts that is often evident in the same location during different years. Examples include the North American high and the Siberian high.
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semipermanent depression—A low pressure circulation feature on monthly mean charts that is often evident in the same location during different years. Examples include the Aleutian low and the Icelandic low.
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