Geoscience Reference
In-Depth Information
Plate 5.13
Valley fog from clouds spilling over the mountains north of Katmandu, Nepal, 20 November 1979.
Source:Courtesy of Nel Caine, University of Colorado.
3
In tropical cyclones, cumulonimbus cells
become organized around the center in
spiraling bands (see Chapter 13B.2). Partic-
ularly in the decaying stages of such cyclones,
typically over land, the rainfall can be very
heavy and prolonged, affecting areas of
thousands of square kilometers.
2 'Cyclonic type' precipitation
Precipitation characteristics vary according to the
type of low pressure system and its stage of
development, but the essential mechanism is the
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 air mass 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
Plate 5.14
Mammatus clouds. These form below
cumulonimbus, usually presaging a thunderstorm.
Source:Courtesy of Mark Anderson, University of Nebraska.
direction. Such cells tend to occur parallel to a
surface cold front in the warm sector of a
depression (sometimes as a squall line), or
parallel to and ahead of the warm front (see
Chapter 9D). Hence the precipitation is
widespread, although of limited duration at
any locality.
