During the winter season, the northeast trade winds prevail over the country.…

Concept — A wind always moves from an area of higher air pressure towards an area of lower air pressure, and the moisture it carries is decided by the surface it has just travelled across. Air that has spent its journey over open sea reaches its destination laden with water vapour; air that has spent its journey over a continent reaches it with very little.

Application — In the cold weather season the landmass of northern India and Central Asia loses heat rapidly and builds up a high pressure cell over the interior, while the neighbouring ocean cools far more slowly and remains comparatively low in pressure. The pressure gradient therefore points outward, from the interior towards the ocean, and the northeast trade winds of this season move off the continent and out over the water. Because the stretch of their track that crosses India runs over land and not over water, they carry almost no water vapour while they are over the country, and the greater part of it stays rainless.

Cross-check — Test the rule where it appears to break. The same northeast winds that travel over the Bay of Bengal do acquire a sea leg, arrive moist, and give the Coromandel coast of Tamil Nadu its October to December rainfall. The exception shows up precisely where a stretch of open water is added to the track, which confirms that it is the outward land to sea direction that keeps the rest of the country dry.

Result — The northeast trade winds of the winter season blow from land to sea.