World Distribution of Rainfall

Rainfall is not spread evenly across the planet. Some places get drenched year-round, others barely see a drop, and the pattern is far from random. The distribution follows a few broad rules tied to latitude, distance from the sea, and the direction of prevailing winds.

The most basic pattern is latitudinal: as you move from the equator toward the poles, annual rainfall steadily decreases. The equator itself is a zone of intense heating and rising air, so it receives the most rain. Beyond that, two other generalisations hold. Coastal areas get more rain than the interiors of continents, because oceans are vast sources of moisture. And for the same reason, rainfall over the oceans exceeds rainfall over landmasses.

The direction of the prevailing winds reshapes this simple picture. Between latitudes 35° and 40° north and south of the equator, rain is heavier on the eastern coasts and decreases steadily toward the west. But between 45° and 65° north and south, the westerlies dominate, so the opposite happens: rain first falls on the western margins of the continents and decreases toward the east. Mountains add a further twist. Wherever a mountain range runs parallel to the coast, the coastal plain and the windward side receive more rain, while the leeward side gets progressively less.

On the basis of total annual precipitation, the world can be divided into four broad rainfall regimes:

• Heavy rainfall (over 200 cm per year): the equatorial belt, the windward slopes of mountains along the western coasts in the cool temperate zone, and the coastal areas of monsoon lands.
• Moderate rainfall (100–200 cm per year): the interior continental areas.
• Moderate to low rainfall (50–100 cm per year): the coastal areas of continents, the central parts of tropical lands, and the eastern and interior parts of temperate lands.
• Very low rainfall (less than 50 cm per year): areas lying in the rain shadow zone of the interior of continents, and high latitudes.