The northeast monsoon – it provides 30% of the annual rain to the meteorological subdivisions of Tamil Nadu, Puducherry and Karaikal, Coastal Andhra Pradesh, Rayalaseema, Kerala and Mahe, and South Interior Karnataka — began on October 15. Within a day of the weather systems of this season becoming active, several parts of peninsular India were flooded. Does this mean that the season has had an extreme start? An HT analysis shows that while this may not be the case in absolute terms, at every place that was flooded, it is the case in relative terms.

The northeast monsoon – it provides 30% of the annual rain to the meteorological subdivisions of Tamil Nadu, Puducherry and Karaikal, Coastal Andhra Pradesh, Rayalaseema, Kerala and Mahe, and South Interior Karnataka — began on October 15. Within a day of the weather systems of this season becoming active, several parts of peninsular India were flooded. Does this mean that the season has had an extreme start? An HT analysis shows that while this may not be the case in absolute terms, at every place that was flooded, it is the case in relative terms.

In absolute terms, only small parts of peninsular India received rain in the past three days one could call dangerously intense, according to India Meteorological Department (IMD) classification. While IMD does not explicitly call any intensity “dangerous”, it classifies 24-hour rain into nine categories by absolute rainfall: from “no rain” to “extremely heavy rain”. No place in peninsular India received extremely heavy rain on any of the past three days. Even very heavy and heavy intensity rain – the intensities just below extremely heavy rain — was confined to small parts on both coasts and parts of southeastern Karnataka, with the former falling only around Nellore and Prakasam districts of Andhra Pradesh. However, there were reports of flooding from even other places, such as Chennai and Bangalore.

So why are the rains leading to floods if they are not the most intense? One reason is that rain need not be of the highest intensity for floods (more on this later). Moreover, definitions of rain’s intensity based on 24-hour rain may not always be sufficient to predict floods. For example, what is defined as heavy rain for 24 hours (64.5−124.4 mm rain) falling in an hour or less is more intense than the category describes. This can be understood as follows: 124.4 mm rain spread out equally over 24 hours gives an hourly intensity of 5.2 mm per hour, which is a much smaller intensity than 124.4 mm rain of a 24-hour period concentrated in one hour, which is an hourly intensity of 124.4 mm per hour. However, IMD’s 24-hour definition would classify both kinds of rain in the same category. To be sure, IMD does have one way of accounting for intensity at an hourly frequency. It calls 100 mm rain falling at a station as a cloud burst. However, this definition also misses intensities lower than the most disastrous.

The gist of the discussion above is that absolute rain data either does not show peninsular India rain to be extreme in the past three days or is insufficient to capture extreme intensity. In relative terms, however, the rain in the past three days does appear extreme in parts of peninsular India in the past three days. HT calculated the 99th percentile (values above this would fall in the top 1%) of daily rain for the October-December period (the official northeast monsoon season) for the past century (1901-2000). A comparison of the rain in the past three days with the 99th percentile shows that several places crossed this threshold in peninsular India on at least one of the past three days.

While the places that overshot the 99th percentile (or came close to it) are the same places that received rather heavy to very heavy rain (see the first map for thresholds), the comparison highlights an important aspect of flooding. Even lower-intensity rain (in absolute terms) can cause floods if a place is not used to it. The comparison with the 99th percentile shows that even intensities lower than the highest two are unusual in many places in peninsular India. This is why rain has a role to play in the floods. This also makes intuitive sense because places like Cherrapunji do not get flooded with rain of similar intensities because geography and the settlements on it are more used to it.

To be sure, the part the rains played in the floods in peninsular India does not take away the fact that concretisation and expansion of settled areas have a role to play in floods. If urban development over natural land does not make up for the loss of natural drainage and seepage, floods are an expected outcome even at relatively lower intensities of rain.

Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.