1.3. Physical and geographical backgrounds of precipitation
Major weather patterns producing rainfall and the geographical distribution
of rainfall are explained below.
1.3.1. Frontal disturbances
When air is cooled as a result of the converging of two contrasting air masses
it can produce widespread rainfall.
Warm air is made to rise and hence cools, and the
condensation of water vapour forms characteristic clouds (stratus,
stratocumulus) and rainfall. Precipitation usually occurs along the fronts,
which
may be several hundreds of kilometres long. On average, precipitation
produced
by a warm front is usually prolonged with moderate intensity (the amount
of
rain that has precipitated within a certain period); at the cold front, the
rainfall
is usually more heavy and short-lived (and thus more intense). The
development of these frontal disturbances is typical for extratropical
cyclones
(depressions), the major weather pattern for producing precipitation in the
temperate regions, where cold air from the pole meets warm air originating
in
the tropics. For example, more than 60% of the annual rainfall in the British
Isles
comes from such disturbances.
1.3.2. Convectional precipitation
A great deal of the precipitation in the tropics is caused by convectional
activity.
When a tropical maritime air mass moves over land at a higher temperature,
the
air is heated and forced to rise by convection. Deep cumulus clouds form,
becoming cumulonimbus (heavy rain-clouds) extending up to the tropopause
(the top of the troposphere, the lower part of the atmosphere from the
surface to
around 10-15 km), sometimes accompanied with thunder and lightning; the
horizontal scale of convection is typically 10km x 10km. The development of convection is a regular daily feature of
the
weather throughout the year in many parts of the tropics. A large supply of
moisture is carried upwards. When vertical velocities are large enough, the
air
cools to condensation temperatures, and rainfall of great intensity occurs.
Convectional activity is not confined to the tropics; it is a common local scale
rain-forming phenomenon in higher latitudes, particularly in summer.
1.3.3. Orographic rainfall
Precipitation (rain or snow) can come directly from a maritime air mass that
cools when forced to rise over mountains. It is an important feature of the
western mountains of the British Isles, which lie along the track of the
prevailing
winds that carry moisture from the Atlantic Ocean. A range of mountains
may
delay the passage of a front and cause long periods of rainfall.
1.3.4. Monsoon
The marked predominance of summer rainfall over much of Asia is
associated
with the summer monsoon circulation. It carries moist air northwards from
the
Indian Ocean. A low pressure field above the southeastern continent, due to
intense warming, generates this circulation. The seasonal movements of the
Intertropical Convergence Zone (ITCZ) determine the development of the
weather
conditions in the monsoon areas. The ITCZ is a varying boundary between air
masses originating in the Northern and Southern Hemispheres. The
regularity of
the onset of the rainy seasons is a marked feature of the monsoon. The
precipitation is caused by the confrontation of differing air masses,
convection,
and orographic effects. The summer period is characterised by moist
southwesterly winds, causing the transport of warm, moist air from the
oceans to
the subcontinent. Quantities of rain vary but intensities are high. For
example,
India receives 80% of its annual rainfall during the monsoon period.
1.3.5. Global distribution
The geographical distribution of annual average precipitation is unequally
distributed over the earth. At the middle
latitudes,
rainfall amounts are moderate with precipitation originating primarily from
frontal disturbances, summer convection and orographic influences
(mountains).
Heavy precipitation is observed over large areas of the tropical continents.
Predominantly convectional rain occurs in this area around the equator. The
major dry areas (deserts) of the world correspond to subtropical latitudes.
