Why does it rain like this in Mangaluru? The monsoon, explained from my window

In Mangaluru we basically have two kinds of weather: monsoon, and waiting for monsoon. Schools reopen in the rain, umbrellas die in the rain, and my amma hangs clothes on a rope across the living room from June to September and calls it normal. I've lived through fifteen or sixteen of these seasons and never really knew why it happens. "It rains because it's June" is not an explanation. So I read about it, asked my geography teacher too many questions, and drew a lot of arrows. This is the explanation I wish someone had given me.

Step 1: the monsoon is a giant sea breeze

You know how at the beach the wind blows from the sea to the land in the afternoon? Land heats up faster than water, the air over land warms and rises, the pressure there drops, and cooler air from the sea rushes in to fill the gap. The monsoon is the same idea, scaled up to a whole continent and a whole season.

Why does land heat up faster? Specific heat. Water needs a lot of energy to warm up, about $4200\ \text{J kg}^{-1}\text{K}^{-1}$, while dry soil and rock are around a fifth of that or less. From $Q = mc\Delta T$, for the same heat and the same mass, a smaller $c$ means a bigger temperature rise. Also, the sea mixes heat downward into deeper water, while land just bakes on top. So by April and May the northern plains and the north-west become furnaces, and a big low-pressure zone forms over north-west India. The Indian Ocean to the south is cooler, so it has higher pressure. Air moves from high pressure to low pressure.

Step 2: why south-west, and not just south?

The winds that cross the equator from the southern hemisphere start as south-easterly trade winds. Once they cross into the northern hemisphere, the Coriolis effect (caused by Earth's rotation) deflects moving air to its right. A wind heading north gets bent towards the east, so it ends up blowing from the south-west. That's why it's the south-west monsoon. The word "monsoon" itself is said to come from the Arabic mausim, meaning season.

Step 3: moisture and the wall

These winds travel over the Arabian Sea and the Bay of Bengal and pick up huge amounts of water vapour. When moist air is forced to rise, it cools, the vapour condenses into clouds, and condensation releases latent heat. That warms the air, so it rises more, pressure falls further, and even more moist air gets pulled in. It's a feedback loop, and I think this is part of why the monsoon seems to "burst" rather than arrive slowly.

Now my coast enters. The Western Ghats run down the west of India like a wall. The moist south-westerlies slam into that wall and are forced upward (called orographic lift), cool, and dump rain on the windward side, which is us, along with the Konkan coast and Kerala. On the other side of the Ghats, the air has already lost most of its moisture and descends, getting warmer and drier. That's a rain shadow, and it's why a place a couple of hours inland can be much drier than the coast when both are under the same monsoon.

The timeline

• The monsoon normally arrives over Kerala around 1 June. Mangaluru usually gets it a few days after.
• Through June it spreads north in two branches: the Arabian Sea branch (up the west coast) and the Bay of Bengal branch (over the north-east and then along the Gangetic plain). In a normal year the whole country is covered by around early to mid July.
• Within the season there are "breaks", dry spells of a week or two on the plains, when the monsoon trough shifts toward the Himalayan foothills and the rain moves with it.
• From around September the monsoon starts to withdraw from the north-west, and the winds begin to reverse. Then Tamil Nadu and parts of the south-east coast get their main rain from the north-east monsoon in October to December.
• El Niño years are often (not always) linked with a weaker monsoon in India. The word "often" is doing a lot of work there, it's not a rule.

What I take from this

Before, rain was something that just happened to us. Now when I stand on the balcony in July and the sky is that heavy slate grey, I picture an entire ocean's worth of moisture being shoved up the Ghats, climbing, cooling and falling onto my umbrella. It makes me a little more patient with the wet laundry. A little.

The Kannada word for rain is male, by the way, and my grandmother says "male barthide" (the rain is coming) the way other people say "the guests are coming". It's the same feeling, honestly. Everything gets rearranged around it.

If I got something wrong, tell me in the comments. I've only read about this, not measured it.