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Chennai, hottest among major cities, faces increasing heat stress, study shows

There is a direct correlation between the increase in built-up area and the increase in urban heat stress, the study said. Photo: Archives

There is a direct correlation between the increase in built-up area and the increase in urban heat stress, the study said. Photo: File | Photo Credit: SR RAGHUNATHAN

“This summer is worse than ever” is a sentiment familiar to every city dweller, one that people either say out loud or silently admit every year between April and June.

Supporting this observation, an analysis conducted by the Center for Science and Environment (CSE) found that not only did Chennai become the hottest major city in India, but the number of days with high heat index (felt temperature) increased threefold. Mid-year, when relative humidity is combined with air temperature, the body temperature exceeds 41°C, marking the danger zone.

As part of the study, CSE compared summer temperatures in Delhi, Mumbai, Kolkata, Hyderabad, Bengaluru and Chennai from 2001 to 2011 with those from 2014 to 2023. The study found that average relative humidity over the past 10 summers has increased significantly compared to the average relative humidity from 2001-10, and Chennai’s ten-year average summer ambient temperature has increased by about 0.5°C.

Additionally, the city’s relative humidity has now increased by 5%. When relative humidity is combined with high temperatures, it can exacerbate heat stress and adversely affect human thermal comfort and health.

warm night

As relative humidity rises in summer, heat index increases in large cities. The average summer heat index in Chennai is 37.4°C (humidity effect: 6.9°C), making it the hottest among major cities.

The problem is exacerbated by the fact that nighttime temperatures have yet to drop. The city’s nighttime temperature dropped by only 9.7°C, a 5% decrease from 2001-10. This leaves people without much opportunity to recover from the heat of the day.

According to the study, it’s not just summer that’s getting hotter. Both the pre-monsoon period (March, April and June) and the monsoon period (June, July and August) become more uncomfortable, with the heat index rising by about 2°C. However, it is worth noting that the mentioned pre-monsoon and monsoon periods may be different for the city as the southwest monsoon does not bring as much rainfall here as the northeast monsoon, which only occurs in October.

Built-up area

It is worth noting that studies have pointed out that the increase in floor area may be a factor causing severe heat island effect. In Chennai, the built-up area increased from 30.7% in 2003 to 73.5% in 2023.

Therefore, there is a direct correlation between the increase in built-up area and the increase in urban heat stress, the study said.

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