Wind, Rain and the Monsoon Vector

Most of the world's climates have a prevailing wind. India has two — and they point opposite ways.

For roughly half the year the south-west monsoon (June–September) drags moist air off the warming ocean onto the land: it carries both the cooling breeze every building wants and the driving rain every building fears. For the other half — the retreating north-east monsoon and the dry winter (October–May) — the flow runs the other way, land-to-sea, bringing dry air, dust and the winter chill.

This seasonal flip is the monsoon reversal — the subcontinent's defining climatic rhythm. A building on the coast must answer to both winds, because they often arrive from the same compass quarter at opposite times of year.

A wind rose is a radial diagram: petals reach out from a centre toward each compass direction, their length showing how often the wind blows from there and their banding showing how strongly. It is the working drawing for any ventilation decision.

But for monsoon India there is a trap. An *annual-average* wind rose for a monsoon city is worse than useless — it smears two opposite winds into a meaningless smudge in the middle, hiding the very signal you need. The June south-westerly and the December north-easterly cancel each other on the page, and you end up designing for a wind that never actually blows.

The discipline is simple: always split the rose. Draw one for the monsoon months and one for the dry months. Read each on its own. The prevailing arm swings between them — and your openings have to live with both positions of that arm.

The instinct is to pick the dominant wind and orient to it. On a monsoon coast that fails, because the dominant wind brings the rain too. So you don't choose — you design openings that answer the breeze *and* shed the storm on the same line.

Aim the main openings at the monsoon breeze, then protect them: deep overhangs, a wrapping verandah, louvres set to the local driving-rain angle. When that wind turns to water, the geometry sheds the rain while air still slips through underneath. This is exactly the logic of the Mangalore-tiled coastal house — deep eaves, a shaded verandah on the weather side, never bare glass facing the sea.

And ventilation needs a path, not just an inlet. A windward opening with no leeward escape barely moves air at all — it just presses on the glass. Cross-ventilation lives or dies on the outlet.