Hottest among mega cities, Chennai faces increasing heat stress: study

Chennai's rising temperatures and humidity levels make it the hottest mega city in India, impacting human health and comfort.

“This summer was worse than ever” is a sentiment familiar to every resident of the city, either spoken aloud or silently acknowledged every year between April and June.

Supporting this observation, an analysis conducted by the Centre for Science and Environment (CSE) finds that Chennai not only stands out as the hottest mega city in India, but shows a threefold increase in days where the heat index (what the temperature feels like to the human body when relative humidity is combined with the air temperature) exceeds 41°C, marking the danger zone, over the past two decades.

As a part of the study, the CSE compared summer temperatures from 2001 to 2011 with those from 2014 to 2023 in Delhi, Mumbai, Kolkata, Hyderabad, Bengaluru, and Chennai. The study finds that average relative humidity has significantly increased in the last 10 summers compared to the 2001-10 average, with the decadal summertime average ambient temperature rising by about 0.5°C in Chennai.

Further, the city’s relative humidity is up by 5% now. When relative humidity is combined with high heat, it worsens the heat stress and adversely impacts human thermal comfort and health.

Given the rise of relative humidity during summer, the heat index has risen among mega cities. Chennai’s summer average heat index stood at 37.4°C (impact of humidity: 6.9°C) making it the hottest among the mega cities.

The problem is exacerbated as the night temperature has not come down. Nights in the city are cooler by just 9.7°C, which is 5% down from the 2001-10 level. This does not leave people with much chance to recover from daytime heat.

According to the study, it is not just summers that have become hotter. Both the pre-monsoon period (March, April, and June) and monsoon (June, July, and August) have become thermally more uncomfortable, with the heat index rising by about 2°C. However, it is important to note that the period mentioned as pre-monsoon and monsoon could differ for the city as the southwest monsoon does not bring as much rain here as the northeast, which only sets in during October.