Geoscience Reference
In-Depth Information
F PRECIPITATION TYPES
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 break-
down 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 conver-
gence in the tropical easterlies (see Chapter 11B.1).
It is usual to identify three main types of precipitation
- convective, cyclonic and orographic - according to
the primary mode of uplift of the air. Essential to this
analysis is some knowledge of storm systems. These are
treated in later chapters, and the newcomer to the subject
may prefer to read the following in conjunction with
them (Chapter 9).
1 'Convective type' precipitation
This is associated with towering cumulus (
cumulus
congestus
) and cumulonimbus clouds. Three subcate-
gories may be distinguished according to their degree of
spatial organization.
1
Scattered convective cells develop through strong
heating of the land surface in summer (Plate 25),
especially when low temperatures in the upper
troposphere facilitate the release of conditional
or convective instability (see B, this chapter).
Precipitation, often including hail, is of the thun-
derstorm type, although thunder and lightning do not
necessarily occur. Small areas (20 to 50 km
2
) are
affected by individual heavy downpours, which
generally last for about 30 minutes to an hour.
3 Orographic precipitation
Orographic precipitation is commonly regarded as a
distinct type, but this requires careful qualification.
Mountains are not especially efficient in causing
moisture to be removed from airstreams crossing them,
yet because precipitation falls repeatedly in more or less
the same locations, the cumulative totals are large. An
orographic barrier may produce several effects, depend-
ing on its alignment and size. They include: (1) forced
ascent on a smooth mountain slope, producing adiabatic
cooling, condensation and precipitation; (2) triggering
of conditional or convective instability by blocking
of the airflow and upstream lifting; (3) triggering of
convection by diurnal heating of slopes and up-slope
winds; (4) precipitation from low-level cloud over
the mountains through 'seeding' of ice crystals or
droplets from an upper-level feeder cloud (Figure 5.14);
and (5) increased frontal precipitation by retarding the
movement of cyclonic systems and fronts. West coast
mountains with onshore flow, such as the Western
Ghats, India, during the southwest summer monsoon;
the west coasts of Canada, Washington and Oregon; or
coastal Norway, in winter months, supposedly illustrate
smooth forced ascent, yet many other processes seem to
be involved. The limited width of some coastal ranges,
with average wind speeds, generally allows insufficient
time for the basic mechanisms of precipitation growth
to operate (see Figure 5.9). In view of the complexity of
2
Showers of rain, snow or soft hail pellets may form
in cold, moist, unstable air passing over a warmer
surface. Convective cells moving with the wind
can produce a streaky distribution of precipitation
parallel to the wind 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.
3
In tropical cyclones, cumulonimbus cells become
organized around the centre in spiralling bands (see
Chapter 11B.2). Particularly in the decaying stages
of such cyclones, typically over land, the rainfall
can be very heavy and prolonged, affecting areas of
thousands of square kilometres.
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 ascent of air through
