Q.Discuss the salient features of the world distribution of precipitation.

The world distribution of precipitation is far from uniform, and its pattern is governed by a few dominant controls: latitude, the global wind belts, distance from the sea, and the presence of mountain barriers.

• Equatorial regions receive the heaviest rainfall, often exceeding 200 cm annually, because of intense convection and the rising air of the Intertropical Convergence Zone.
• Subtropical high-pressure belts (around 20°–30° latitude) are dry, with many of the world's great deserts, because descending air suppresses cloud formation.
• Mid-latitude westerlies bring moderate rainfall, especially on the western coasts of continents, while the interior of large continents at these latitudes is dry due to distance from moisture sources.
• Monsoon regions (notably South and Southeast Asia) receive heavy seasonal summer rain, while their winters are dry.
• Windward slopes of mountains receive heavy orographic rainfall, whereas leeward slopes lie in rain shadows and are arid.

Think of the Earth’s rainfall map as a story written by the atmosphere. The sun heats the equator most fiercely, so air there rises, cools, and sheds its moisture as heavy rain. That is why the equatorial belt — roughly between 5°N and 5°S — receives the highest annual precipitation on the planet, often exceeding 200 cm. This is the zone of the great rainforests, where convectional rainfall occurs almost daily, and there is no true dry season. The air that rises here does not stay put; it spreads poleward, cools, and eventually sinks around 30° latitude, creating a belt of high pressure where descending air suppresses cloud formation. The result is the great desert belt of the world — the Sahara, the Arabian deserts, the Thar, and the Australian outback — where annual rainfall is often below 25 cm. This is the single most striking feature of global precipitation: the alternation of a wet equatorial trough and dry subtropical highs.

Between these two extremes lie the trade wind belts, where the weather is more variable. On the eastern sides of continents in the tropics, the trade winds blow from the sea, bringing moisture and orographic rainfall along coastal ranges — think of the eastern coasts of Brazil, Madagascar, and the Philippines. On the western sides, the same winds blow from the interior, so they arrive dry, reinforcing the desert conditions. This is why deserts like the Atacama and the Namib sit on the western edges of South America and Africa, while lush coasts face east. The pattern is not random; it is a direct consequence of the planetary wind system and the position of the subtropical high-pressure cells.

Moving into the mid-latitudes, roughly between 35° and 60° in both hemispheres, precipitation becomes more moderate and more evenly distributed through the year. Here the westerlies dominate, carrying moist air from the oceans onto the western coasts of continents. This is the realm of the Mediterranean climate on the western edges — with winter rains and summer dryness — and the marine west coast climate further poleward, where rainfall is frequent but not torrential. The interiors of continents at these latitudes, however, tell a different story. Distance from the sea becomes the controlling factor: the further inland you go, the drier it gets. Central Asia, for instance, receives very little rain because moist ocean air rarely penetrates that far, and what does arrive has often lost its moisture crossing thousands of kilometres of land. This is why the steppes and semi-deserts of Kazakhstan and Mongolia exist.

The polar regions, above 60° latitude, are the driest of all in terms of actual precipitation, even though they are covered in ice and snow. Cold air holds very little moisture, so snowfall is light — often less than 25 cm of water equivalent per year. The Antarctic interior is technically a polar desert. The ice sheets persist not because of heavy snowfall but because the little snow that falls never melts. This is a crucial distinction: precipitation amount and the appearance of a landscape can be very different things.

Two more factors complicate the simple latitudinal picture. First, mountain barriers create rain shadows: the windward side of a range receives heavy orographic rain, while the leeward side is parched. The Western Ghats in India and the Andes in South America are classic examples. Second, the seasonal migration of the sun and the pressure belts causes many regions to have distinct wet and dry seasons — the monsoon lands of South and Southeast Asia being the most dramatic case, where a complete reversal of winds brings torrential summer rain and nearly dry winters.