The distribution of land and water imposes an important influence upon precipitation in the middle and high latitudes. Though situated in the same latitudes, the continents receive less precipitation than the oceans, for the continental air masses are drier than the maritime air masses.
Though 71 per cent of the earth’s surface is water and only 29 per cent land, it has been estimated that 19 per cent of the earth’s total annual precipitation falls on land surfaces and the remaining 81 percent on the oceans.
Interior locations on the continents, being far away from the source of oceanic moisture, receive relatively less precipitation than coastal areas.
The coastal areas which lie directly in the path of moisture- laden on-shore winds naturally get greater rainfall. On the other hand, such coastal areas as have off-shore winds are deprived of the oceanic moisture and so get scanty rainfall.
Wherever the coastal areas are flanked by mountain ranges, the favourable winds yield larger amount of precipitation on the windward slopes than on the leeward side.
Similarly, warm ocean currents are conducive to greater precipitation on the adjacent lands, whereas the cold currents produce only fogs.
The effect of continents on the distribution of precipitation is very distinct in the vast land mass of Asia. Because of their interior locations, the countries of Iran, Afghanistan and Saudi Arabia constitute a long and extensive dry belt extending up to the great desert of Sahara in Africa.
Similarly, the dry areas of North Mexico and south-western U.S.A. are typical examples of the effect of continentally. In the southern hemisphere, the western parts of sub-tropical regions of Australia, South Africa and South America have deserts mainly because of the effect of continentally.
However, the eastern parts of sub-tropical high pressure belts have moist climates, because the moisture-bearing trade winds bring in the oceanic influence and produce plenty of rainfall.