Daily cycle of air temperature - When hourly air temperature readings are taken throughout a 24-hour period and plotted on a graph, the curve for a clear day typically shows one low point near sunrise and one high point in mid afternoon with a fairly smooth curve through out. This is rhythmic rise and fall of air temperature cycle.
Temperature inversion and frost - Although air temperature typically falls with increasing elevation, weather conditions at night in the lower air on land are typically such that, instead of falling, the temperature first rises with increasing height above ground before beginning to drop off into the normal lapse rate. This condition is termed temperature inversion and signifies that warmer air overlies colder air.
Low-level temperature inversion, or ground inversion, commonly results at night from rapid heat loss by radiation from the ground surface and basal air layer upward into space. Rapid radiation loss is favored by the presence of clear skies. Over snow-covered surfaces on clear winter nights, inversion is particularly marked. If the heat loss is great during a night in spring or early fall, the air temperature close to the ground may drop below freezing, resulting in a killing frost, which damages sensitive crops. There are other causes and varieties of both temperature inversions and killing frosts, but these involve movements of masses of air and are not phenomena of heat radiation.
Killing frost may be prevented in orchards and citrus groves by causing a circulation of air that mixes the warmer air above with the cold-air layer near the ground. Where the cold-air layer is thin this effect may be accomplished by the use of powerful motor-driven propellers that circulate air much as does a fan in a room.
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