Heat wave in India: Why is Delhi unbearable and Bengaluru better? Study explains a worrying phenomena

Delhi, Mumbai, Chennai, Bengaluru, Kolkata, and Hyderabad are facing worsening "heat stress" due to rising relative humidity over the past two decades, says a study by the Centre for Science and Environment (CSE).

The study blames it on "urban heat island" effect, where built-up areas trap heat, reduce green cover, cause congestion, absorb heat, and generate additional heat from human activities. 

This effect makes city centers much warmer, especially at night.

High heat and humidity compromise the body's cooling mechanism, making people sick and potentially causing fatalities even at lower temperatures. 
The combination of rising temperatures and humidity is increasing the heat index, a measure of discomfort.

Bengaluru the exception

Prolonged heatwaves this summer have intensified the issue, with Odisha recording 18 heatwave days and West Bengal 16 in April. Rajasthan, Haryana, and Delhi are currently experiencing long heatwaves, with no relief expected for at least three more days, according to the India Meteorological Department.

The study noted a 5-10% rise in average summer humidity in most metros over the last decade, with Hyderabad seeing the highest increase at 10%. Mumbai, Kolkata, and Chennai saw increases of 8%, 7%, and 5%, respectively. 

Bengaluru was the exception.

Monsoon periods are now hotter in Mumbai and Kolkata, while Chennai has lost its marginal cooling during this season. Bengaluru and Hyderabad remain slightly cooler during monsoon compared to the pre-monsoon period.

Why is Delhi looking unbearable

The analysis shows that Delhi's extreme heat is directly linked to the increase in built-up areas. As the city has expanded, with built-up areas growing from 31.4% in 2003 to 38.2% in 2022, urban heat stress has intensified. While more green cover helps lower daytime temperatures, it doesn't affect nighttime temperatures or the increasing heat index. This urban expansion traps heat, making the city hotter, especially at night.