Extreme rain events in July: How unusual are they, what is causing them

North badly hit

By the first two weeks of the monsoon, India as a whole had accumulated a rainfall deficit of more than 50%. Thanks to rains induced by the Biparjoy cyclone in the northwestern and central parts of the country, this deficit had come down to 8% by the end of June.

July has been extremely wet so far in most parts of the country. Overall, India has received 26% more rainfall in July than expected, which has helped in wiping out the entire deficit for the season till now.

The swollen Beas river as it flows above the danger mark following heavy monsoon rains. (PTI Photo/File)

But it is in the northern states — Himachal Pradesh, Delhi, Punjab, Rajasthan, Uttar Pradesh and Uttarakhand — that the rains have come down most heavily. Himachal Pradesh has received rainfall four times than normal for July till now, while in Punjab it is three times higher. Delhi had record-breaking rainfall on a couple of days this month, and Haryana also received more than double of what is expected during the first ten days of the month (see chart 2).

Chandigarh has received 549 mm of rains in just three days, between Sunday and Tuesday, instead of the little more than 30 mm that is expected. This is more than 17 times the normal. Similarly, Ladakh has got seven times the normal rainfall in the last three days.

IMD has attributed this spell to an interaction between the monsoon winds and western disturbances that converged over northern India during the first two weeks of July.

A view of the damaged Shimla-Chandigarh NH following a landslide triggered by heavy monsoon rain, in Himachal’s Solan district, Tuesday. (PTI)

Extremely heavy but not unusual

Such events of extremely heavy rainfall are not unexpected during the monsoon season. IMD defines an extremely heavy rainfall event as one in which more than 205 mm of rainfall happens at any place within a 24-hour period. Hundreds of such events, at different locations in the country, are recorded during the monsoon season every year (chart 3).

Most of these instances of extremely heavy rainfall go unnoticed. But depending on their location, many of these events lead to large-scale urban flooding or, as is being seen in Himachal Pradesh, widespread destruction by raging rivers, landslides, and broken bridges and roads.

Such rainfall-triggered disaster-like situations have become a regular feature during the monsoon. After the 2013 Uttarakhand tragedy, there has not been a single year without at least one major disaster-like situation produced by an extreme rainfall event.

Is it climate change?

Such events are routinely attributed to climate change, but without a proper attribution assessment, scientists are reluctant to describe them as such. In India, only two rainfall-triggered events, the 2013 Uttarakhand disaster and the 2017 Chennai floods, have so far been assessed by attribution studies to have been caused by climate change. The Kerala flood of 2018 is a candidate, but scientific studies so far are inconclusive.

In general, though, climate change is known to increase the possibility, frequency and intensity of extreme weather events, including heavy rainfall and heat waves.

Overall rainfall during the monsoon has remained more or less constant over the last two decades, but the intra-seasonal variation has increased. That means that fewer days are producing a bulk of the seasonal rainfall, while the rest of the days remain dry.