“Why the Monsoon Almost Always Turns Up in June” described the engine: land heats faster than the sea, warm air rises over the subcontinent, and moist ocean air flows in to replace it. On a map, the ocean that feeds it lies to the south, so you might expect the monsoon to blow straight up from the south. It doesn’t. Everyone who lives on the west coast knows which way the rain comes from: the south-west. The summer monsoon is officially called the south-west monsoon, and the reason it swings in from that angle is that the planet underneath it is spinning.
Picture the Earth from above the North Pole. It turns anticlockwise, once a day. Every point on its surface is being carried eastward, but not at the same speed. A point on the equator has the furthest to travel in a day, the whole circumference, about forty thousand kilometres, so it moves eastward at about 1,670 kilometres an hour. Delhi, further north, is on a smaller circle and moves at about 1,450. Near the poles, the ground hardly moves at all.
Now imagine some air near the equator starting to drift north. It begins with the equator’s eastward speed. As it travels north over ground that is moving east more slowly, it keeps much of that extra eastward speed. So, seen from the ground, it seems to curve off to the east, to its right. Air moving south from the north does the opposite: it’s moving east more slowly than the ground it arrives over, so it seems to lag behind and curve west, which is again to its right. In the Northern Hemisphere, moving air is always deflected to the right. In the Southern Hemisphere, everything is mirrored, and moving air is deflected to the left. This is called the Coriolis effect, after the French scientist who worked out the mathematics in the 1830s.
Here is where it matters for the monsoon. The air that becomes the monsoon starts far to the south, over the southern Indian Ocean. Pulled north towards the low pressure over India, it moves north-west, and south of the equator the Coriolis effect deflects it to the left. That’s why the steady winds there, the trade winds, blow from the south-east. When this air reaches the equator, the Coriolis effect, which is zero at the equator itself, fades away. And when it crosses into the Northern Hemisphere, the deflection switches sides. Now it bends to the right. The air that was heading north-west is swung round until it is heading north-east, blowing from the south-west, straight at the coast of Kerala, the Western Ghats and Mumbai.
Much of this happens in a remarkable river of air close to the coast of East Africa. As the trade winds cross the equator, the mountains of East Africa help funnel them into a narrow, fast-moving stream a kilometre or so above the sea, sweeping north past Somalia and then curving east across the Arabian Sea towards India. Meteorologists call it the Somali jet, or the Findlater jet, after the scientist who mapped it in the 1960s. It’s among the fastest low-level winds on Earth, and it carries a large share of the moisture that falls on India each summer.
In winter, as the original piece described, the pattern reverses: the land cools faster than the sea, the pressure pattern flips, and the wind blows from the land towards the ocean. Coming from the north and deflected to the right, it blows from the north-east. That winter north-east monsoon is dry over most of India, but not everywhere: by the time it reaches Tamil Nadu, it has crossed the Bay of Bengal and picked up moisture, which is why Chennai gets much of its rain between October and December, when most of the country is dry.
The same rule shapes the biggest storms on the planet. Air rushing in towards the low pressure at the centre of a cyclone is deflected to the right in the Northern Hemisphere, so instead of flowing straight in, it swirls round the centre, anticlockwise. In the Southern Hemisphere, cyclones spin clockwise. And right on the equator, where the Coriolis effect vanishes, cyclones almost never form, because there’s nothing to set the air spinning.
So when the first monsoon clouds roll in over the Arabian Sea from the south-west, they are tracing a curve drawn by the Earth’s spin. The rain that falls on Kerala in June began its journey south of the equator, as a south-east trade wind, and only became a south-west monsoon because it crossed into a hemisphere where everything bends the other way.