The distribution of rainfall across the globe is highly uneven, governed by the global pressure belts, planetary winds, the arrangement of land and water bodies, and topographical features. On average, the Earth receives approximately 970 mm of annual precipitation, but this varies from near-zero in polar and subtropical deserts to over 10,000 mm in equatorial and monsoonal hotspots.
Factors Influencing Global Distribution
- Latitiude: Rainfall is generally higher near the equator and decreases toward the poles.
- Pressure Belts: High-pressure zones (Subtropical and Polar) are associated with aridity, while low-pressure zones (Equatorial and Sub-polar) are associated with high precipitation.
- Continentality: Coastal areas receive more moisture than the interiors of continents.
- Mountain Barriers: The windward side of mountains receives heavy orographic rain, while the leeward side remains a rain shadow zone.
Zonal Distribution of Rainfall
The world can be divided into distinct rainfall zones based on the annual amount of precipitation received.
| Zone | Annual Rainfall | Geographic Regions |
| Equatorial Zone | > 2000 mm | Amazon Basin, Congo Basin, Southeast Asian Islands. |
| Monsoon/Sub-tropical | 1000–2000 mm | Coastal South Asia (India), parts of East Africa, Northern Australia. |
| Temperate Zone | 500–1000 mm | Interior of Europe, North America, and Eastern Asia. |
| Semi-Arid Zone | 250–500 mm | Steppes of Central Asia, edges of major deserts. |
| Arid Zone (Deserts) | < 250 mm | Sahara, Atacama, Gobi, Thar, and Polar regions. |
Major Latitudinal Patterns
The Equatorial Belt (0° to 10° N/S)
This region is characterized by the Inter-Tropical Convergence Zone (ITCZ). High temperatures lead to constant evaporation and daily convectional rainfall, often referred to as “4 o’clock rain.” There is no distinct dry season here.
The Trade Wind Belt (10° to 30° N/S)
Rainfall in this belt occurs primarily on the eastern margins of continents because the Trade Winds blow from east to west (offshore on western margins). This explains why major hot deserts (Sahara, Namib, Atacama) are located on the western sides of continents in these latitudes.
The Subtropical Highs (20° to 35° N/S)
These are regions of descending air, which prevents cloud formation. Consequently, these latitudes are the most arid regions of the world, hosting the world’s great “Horse Latitude” deserts.
The Westerlies Belt (35° to 60° N/S)
In this zone, the western margins of continents receive more rainfall than the eastern margins due to the on-shore flow of the Westerlies. This rain is primarily cyclonic/frontal in nature.
Planetary Anomalies and Special Cases
Orographic Barriers
The most striking example is the Himalayan range and the Western Ghats in India. The windward slopes receive torrential rain, while the Tibetan Plateau and the Deccan Plateau receive significantly less, creating sharp longitudinal contrasts in rainfall.
Cold Currents and Deserts
Cold ocean currents on the western coasts of continents (e.g., Benguela, Peru/Humboldt, Canaries) stabilize the air and inhibit precipitation, contributing to the extreme aridity of coastal deserts like the Atacama and the Namib.
Global Rainfall Maxima and Minima
- Wettest Place on Earth: Mawsynram, Meghalaya (India), followed by Cherrapunji. Both benefit from the “funneling effect” of the Khasi Hills.
- Driest Place on Earth: Atacama Desert (Chile). Some parts have recorded no rain for decades due to the double rain-shadow effect of the Andes and the influence of the cold Humboldt Current.
Key Facts for UPSC Prelims
- Maximum Rainfall over Oceans: In total volume, more rain falls over the oceans than over land, but land surfaces show more extreme variations.
- Inversion of Rainfall: Generally, rainfall increases with altitude up to a certain point (the Line of Maximum Precipitation), after which it decreases due to the loss of moisture in the rising air.
- Isohyets: These are the lines on a map that connect points of equal rainfall. Observing the shift in isohyets over decades is a primary method for tracking climate change.
- The “Desert Diagonal”: A continuous belt of aridity stretching from the Atlantic coast of North Africa (Sahara) across the Middle East to the Gobi Desert in Central Asia.