More than the amount of rain, it’s the concentration of this rainfall over a few hours on a few days that devastated Chennai and two other affected districts in Tamil Nadu, Cuddalore and Vilappuram—home to a combined 10.7 million people, or nearly 15% of the state’s population.

The visualization below shows how the rains came down on these districts in the 32 days between 1 November and 2 December. There’s an abundance of zero-rain hours and the occasional appearance of the heavy-rain hours. In Chennai, for example, rains were at their severest on two days: 15 November and 1 December.

One quantitative measure of this unevenness is the median rainfall—the amount of rainfall (in mm) at which the number of hours above equals the number of hours below in this dataset of 768 hours for each district. For Chennai, the median is 0 mm. At the 75th percentile, it is just 1 mm. At the 90th percentile, it is 6 mm: only 10% of these 768 hours recorded more than 6 mm of rainfall.

But when it rained, it poured. There are four main reservoirs that feed water to Chennai. Or, if they have extra water, some of them, notably Chembarambakkam reservoir, release water to the Adyar river that winds through the city and empties itself into the sea. The day after the first big burst on 15 November, the authorities released 18,000 cusecs from the Chembarambakkam reservoir into Adyar, or 321 times the average of the previous 16 days.

On 2 December, 29,000 cusecs was released from the same reservoir. That amounts to 28 litres of water per second, or 71 trillion litres in a day. Assuming average consumption of 200 litres a day per household, that’s enough water to supply to 88 million households for a day—about one-third of India. It was concentrated water like this, from the skies and the ground, that brought Chennai to a standstill.

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