In the humid tropics of Southeast Asia, afternoon thunderstorms roll through almost every day.

Daytime heat causes towering, fluffy cumulonimbus clouds to form and swell before unleashing torrential rain on cities such as Singapore, Hong Kong and Bangkok. In these places, drainage systems are designed to cope with short, heavy downpours of rain.

But the rains in Bangkok over the last week were far from typical.

September is the wettest month in Thailand’s bustling capital, with a monthly average of 344 millimetres usually spread across 18 rainy days. But just last week, roughly 300mm of rain fell in Bangkok over 48 hours, overwhelming the city’s drainage systems.

This extraordinary volume of water prompted authorities to declare a city-wide disaster zone. As stagnant, brown water has inundated the city, at least 31 people have died and thousands have been forced to evacuate.

What caused all this rain?

This sudden and extreme downpour was caused by a low-pressure system that stayed over the Bangkok region for about three days. In a low-pressure system, air rises and moisture in the air condenses to form clouds and rain. If a low-pressure system stalls, it can dump rain over the same location........

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