95% of monsoon rain falls in only a few days, show IMD data.

In 22 cities of sizeable population, 95 per cent of monsoon precipitation occurs over three days to 27 days on average, according to an analysis of rainfall data provided by the India Meteorological Department (IMD).

The southwest monsoon season officially extends from June 1 to September 30, but many cities in the country get almost their entire rainfall of the season in a small window that adds up to just a few days to a few weeks within this four-month period, official data going back more than three decades show.

In 22 cities of sizeable population, 95 per cent of monsoon precipitation occurs over three days to 27 days on average, according to an analysis of rainfall data provided to The Sunday Express by the India Meteorological Department (IMD).

Delhi, for example, receives 95 per cent of its southwest monsoon rainfall in just 99 hours — and 50 per cent in 33 hours — on average, show the data. And Mumbai gets 50% of its annual monsoon rainfall in 134 hours — or just five and a half days — on average. Half of Ahmedabad’s average seasonal rain of 66.3 cm falls in 46 hours, and 95% in 143 hours— roughly six days — on average.

The western cities of Jaipur and Ajmer receive 95% of their share of monsoon rain in about 90 hours. Bengaluru and Lucknow get 95 per cent of their monsoon rain in just over five days. Twelve of the 22 cities have seen a partial decline in southwest monsoon rain hours from 1969 to the early to mid-2000s, the data show.

“This is very important for city and town planning. If you are getting more rain within a short duration, it affects urban flooding,” said K Saikranthi, Inspire faculty at the Department of Earth and Climate Science at Indian Institute of Science Education and Research (IISER), Tirupati, who has collected the data for her research. Her larger study includes data from 108 stations. “If extreme weather events are more, water will get accumulated which will lead to floods. It is very important to study extreme weather events to understand what they lead to, since it affects the public,” Saikranthi said.

In recent years, major urban flooding events have been seen in Mumbai (2005), Srinagar (2014), Chennai (2015), in large parts of Gujarat last year, and in Kerala last month. Ministry of Earth Sciences Secretary Dr M Rajeevan said the data show that “We should not only be considering mean or seasonal rainfall, or only one season’s average rainfall, but we should also for the sake of water management and agriculture, look closely at hourly and daily variations.

“Especially in Mumbai and Delhi, this kind of information is very important for water management and disaster management, when there is intense rain within a very short period of time,” he said.

The decreasing trend of average rain hours “could be an indication of climate change”, Dr Rajeevan said. “It is not a direct link, but it could be interpreted as change caused due to global warming,” he said.

Saikranthi, who studies the microphysics of clouds and precipitation, and is researching precipitation systems using remote sensing techniques in collaboration with Dr Rajeevan, said: “We have found a trend for some parts of the country. We are looking into how significant the trend is. At the moment, we are studying major trends and decadal changes.”