Cyclonic type precipitation
Precipitation characteristics vary according to the type of low-pressure system and its stage of development, but the essential mechanism is ascent of air through horizontal convergence of airstreams in an area of low pressure (see Chapter 6B). In extra-tropical depressions, this is reinforced by uplift of warm, less dense air along an airmass boundary (see Chapter 9D.2). Such depressions give moderate and generally continuous precipitation over very extensive areas as they move, usually eastward, in the westerly wind belts between about 40° and 65° latitude. The precipitation belt in the forward sector of the storm can affect a locality in its path for 6 to 12 hours, whereas the belt in the rear gives a shorter period of thunderstorm-type precipitation. These sectors are therefore sometimes distinguished in precipitation classifications, and a more detailed breakdown is illustrated in Table 10.2. Polar lows (see Chapter 9H.3) combine the effects of airstream convergence and convective activity of category 2 (previous section), whereas troughs in the equatorial low-pressure area give convective precipitation as a result of airstream convergence in the tropical easterlies (see Chapter 11B.1).
Continue reading here: Orographic precipitation
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