Seasonal heating changes regional pressure and winds. Rainfall depends on the air's route over water and the terrain it meets.
In Northern Hemisphere summer, Asia's land surface warms faster than the surrounding oceans. Strong heating over the continent and elevated terrain helps establish low surface pressure and a northward shift of the tropical rain belt. Moist air flows towards this region from the Indian Ocean.
Air crossing the equator towards South Asia is deflected to its right in the Northern Hemisphere. This helps produce southwesterly summer monsoon winds. Convergence and uplift over mountains cool the moist air, forming cloud and heavy rain. Winds, pressure systems and upper-air circulation act together; the monsoon is more than an enlarged daily sea breeze.
In winter, the Asian landmass cools rapidly and a continental high-pressure system develops. Air flows outwards towards warmer surrounding seas, commonly as northeasterly winds in tropical Asia. Air arriving directly from the continent is relatively dry. It can gain moisture over the ocean and then bring heavy rain to an exposed coast.
The Western Ghats enhance summer rainfall on India's western slopes. During the northeast monsoon, winds crossing the Bay of Bengal can bring rain to India's southeast coast. Thus, a winter monsoon is not universally dry. Mountain rain shadows, distance inland, depressions and year-to-year variability also change rainfall totals.
Step by step
Compare land and sea
Seasonal heating and cooling change regional pressure differences.
Trace the wind
Include its ocean crossing and Coriolis deflection.
Locate the rain
Identify moisture supply, convergence and mountain uplift.
Worked example: Opposite Indian coasts
Southwesterly summer flow reaches India's west coast after crossing the Arabian Sea. Northeasterly winter flow can reach Tamil Nadu after crossing the Bay of Bengal. The moisture source explains why both seasonal wind regimes can produce rain.
Watch out for this
Winter monsoon winds are dry everywhere.
Their continental origin favours dry air, but an ocean crossing can replenish moisture before landfall.
Check your understanding
A northeast monsoon wind crosses a warm sea before reaching a windward coast. What is most plausible?
- It must stay dry because it began over land.
- The Coriolis effect creates water vapour.
- It can bring rain after moisture uptake and uplift.