In middle latitudes, the weather and precipitation are largely controlled by travelling extra- tropical cyclones and fronts. These give plenty of rain in all seasons and in most places except for the regions located in the far interiors of the Asian and North American continents.
Generally summer is the season of more abundant rainfall, but on the west coasts of North America, Europe and North Africa, precipitation is higher during the winter. The mid-latitudes receive a larger part of precipitation from the travelling depressions.
Since this region is the site of the polar front, there is convergence of cold polar air and the warm and moist westerlies. It is along the polar front that temperate cyclones often form.
However, because of the seasonal migration of wind belts, a narrow belt between 30 and 40 degree latitudes experiences a marked seasonal variability in precipitation.
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During winter, this narrow zone is visited by a number of depressions as the polar front moves equator-ward, and these moving depressions produce frontal precipitation. But during summer, this belt is marked by subsiding air associated with subtropical anticyclones and, hence, there is little precipitation.
At high latitudes and especially in the Polar Regions the low precipitation is due partly to subsidence of air in the high pressure belts and partly to the low temperatures of the air which holds little moisture. Throughout the year there is only meager precipitation in this region.
Even in the summer, these snowbound areas are dominated by the anticyclonic conditions that block the movement of pole-ward travelling depressions that are few in number.
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The oceans and continents exercise a great influence on precipitation in case of the Indian monsoon. During the northern hemisphere summer, humid and unstable southwest monsoon moves from the oceans towards the land, and the rainfall is heavy over India and Southeast Asia.
The rainfall is intensified where moisture-laden winds are forced to ascend the windward slopes of the Western Ghats and the Himalayas.
In winter a cool dry wind having its origins over the continent flows southward and produces practically no rain over the land areas. However, after the passage of air over the warm tropical ocean, heavy precipitation occurs over the East Indies.
The combined effects of prevailing winds, mountain ranges, and land and sea are more acutely felt over South America. There the deserts in southern Argentina are located in the rain shadow of the Andes.
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These mountain ranges shut off the arid regions from the rain-bearing wind blowing from the Atlantic. Figure 31.4 shows that there is greater precipitation in the temperate zone of the southern hemisphere than in the northern.
This is because of the preponderance of ocean in the southern hemisphere west wind belt. Moreover, the west coast deserts in the southern hemisphere are situated along the eastern margin of the South Pacific sub-tropical highs where air subsides and has no capacity for giving precipitation.