Besides the mean annual precipitation, its seasonal variation is important for classifying the climate of a particular region.
As can be seen in Figures 31.5 and 31.6, there are certain areas in the equatorial regions where rainfall is heavy and occurs throughout the year, and other areas within the tropics with alternate wet and dry seasons.
In the temperate regions, the west coasts of the continents receive most of their precipitation in winter, and the summer is mainly dry.
The interiors of the continents have a summer maximum of precipitation in the form of thundershowers, but their winters are almost dry. The eastern coasts of the continents get a fairly even distribution of rainfall throughout the year with the maximum usually in summer.
A simplified picture of the seasonal concentration of precipitation is shown on a hypothetical continent. It shows four distinct levels of seasonal rainfall concentration: (a) precipitation deficient in winter, (b) precipitation deficient in summer, (c) precipitation sufficient in all seasons, and (d) precipitation deficient in all seasons.
Seasonal variation of precipitation is referred to as precipitation regime in climatological terminology. According to Haurwitz and Austin, there are six major precipitation regimes which are described here briefly:
(1) Equatorial regime:
This regime has two precipitation maxima after the vernal and autumnal equinoxes. On the other hand, there are two precipitation minima after the winter and summer solstices. Equatorial rains are caused by the convergence of trade winds in the doldrums which follow the sun in its yearly shift on both the sides of the equator.
Therefore the zone of maximum precipitation follows the sun also. Besides, the maximum amount of insolation is received from the sun at the time of solstice, which gives rise to strong vertical convectional currents. These currents contribute substantially to the equatorial rains.
This precipitation regime is found between 10° north and south of the equator. As the distance from the equator increases, the two precipitation maxima come closer and ultimately at the extreme limit of this belt, there is only one precipitation maximum. Seasonal wind changes also play an important role in this precipitation regime.
(2) Tropical regime:
This precipitation regime has only one rainfall maximum. The maximum amount of precipitation is recorded just after the summer solstice in each hemisphere.
At this time the doldrums is located closest to the belt of maximum insolation. This results in the greatest convective activity and heavy precipitation. In winter, when the sun is on the opposite side of the equator, the doldrums moves far away.
At this time, under the influence of subtropical high- pressure belt, the amount of precipitation recorded is the least. In exceptional cases, there may be areas where the mountain barriers force the on-shore trade winds to rise and produce heavy rainfall on their windward sides.
Since trade winds are stronger in winter than in summer, such areas have the maximum precipitation in the low-sun period. Such areas lack dry season.
(3) Monsoon regime:
The seasonal variation of precipitation in the monsoon regions is more distinct than anywhere else. The moisture-laden prevailing winds from the warm oceans produce the summer maximum of precipitation.
On the contrary, in winter the cool and dry off-shore winds that are continental in origin give little or no precipitation.
In the northern hemisphere, the rains fall from June to September in India and along the east coast of China and the Southeast Asia. The distribution of precipitation in the monsoon region is largely controlled by the mountain barriers.
(4) Mediterranean (subtropical) regime:
Because of the seasonal shifting of the permanent wind belts, the polar margins of subtropical belts come under the influence of the prevailing westerlies in winter and trade winds during summer.
Therefore this regime has winter precipitation. Summer is the dry season. However, owing to the monsoon effect, the east coasts of the continents do not have this type of regime. There summer is the wet season.
(5) Continental regime:
It has precipitation in summer when the convective activity due to surface heating is at its maximum. This regime is found in the interior of the continents. In winter, the anticyclonic conditions do not favour the release of precipitation.
Another important factor is the low temperature of the air which reduces the air’s capacity to hold more moisture. The maximum of precipitation occurs in the high sun period and the minimum in winter.
However, the dry period during winter is not so pronounced over the continental interiors in middle latitudes as in the monsoon regions. Winter gets a little precipitation from the travelling depressions.
(6) Maritime regime:
Since winter is the period of maximum cyclonic activity, the temperate regions record the winter maximum of precipitation over the oceans and the adjoining coastal regions.
In winter, the contrast between the temperature and moisture content of different air masses is greatest, therefore, the maximum number of cyclones develop along the polar fronts.
Since the maritime air masses have higher temperature and moisture content during late autumn than the midwinter, so the autumn is rainier than the winter.
This type of precipitation regime is found in the most developed form along the west coasts of the temperate zone. The east coasts in the same latitudes get a larger proportion of precipitation in summer. Prevailing westerlies are the primary control of maritime precipitation regime.
The above account of the seasonal distribution of precipitation presents an oversimplified and generalized picture. In fact, each precipitation type is locally modified in various ways and because of various climatic factors.