What is causing severe heatwaves in Delhi, Mumbai, other big cities? New study explains factors causing heat stress

livemint.com:

Increasing relative humidity across all climate zones has worsened the heat index (combination of air temperature and relative humidity) and heat stress in every city," said a trend-analysis report released by the Center for Science and Environment. The study was conducted by the CSE between the summers of January 2001 and April 2024.The CSE explained in its analysis that the heat stress is not just about rising temperatures.

"It is a deadly combination of air temperature, land surface temperature and relative humidity that leads to acute thermal discomfort and heat stress in cities," the report said.1. Rising Humidity: As per the report, the increase in relative humidity in all zones "has made heat stress worse in warm-humid and moderate climate zones".

This increase has also "nullified the fall in air temperatures in composite and hot-dry climate zones, especially during monsoons".The report highlighted the city-wise trends and added that the increased humidity in Delhi and Hyderabad, which are located in climate zones known for the driest and harshest summers, "somewhat nullifies their marginal drop in ambient air temperatures".2. Spike in built-up area: An increase in the built-up area in cities is another factor contributing to the urban heat stress.

"All megacities have become more concretised in the last two decades; this has contributed to the rise in heat stress," the report said.It added that though an increase in green cover can moderate day-time heat, it is not that ineffective in arresting night-time heat."Built-up area has increased from 31.4 percent in 2003 to 38.2 per cent in 2022. Green cover has increased from 32.6 percent in 2003 to 44.2 percent in 2022.