Rising ‘heat stress’ in six metros as humidity up, nights warmer: Study

“Urban heat island” effect refers to the trapping of heat due to an increase in built-up area, declining green cover, congestion, heat absorption by urban structures, and heat generated by human activities. This causes the heart of megacities to be much warmer, especially at night, than its outskirts and neighbouring towns.

“Hot nights are as dangerous as mid-day peak temperatures. People get little chance to recover from day-time heat if temperatures remain high overnight,” said Avikal Somvanshi, senior programme manager, Urban Lab, CSE.

Heat index is a measure of discomfort felt due to high heat and humidity.

The combination of rising air and land surface temperatures and high relative humidity is increasing the heat index and heat stress in these cities, the study said. Heat index is a measure of discomfort felt due to high heat and humidity.

The CSE study comes at a time when the country is seeing prolonged heatwaves this summer. Odisha recorded 18 heatwave days, while West Bengal recorded 16 in April. A long heatwave is currently prevailing over Rajasthan and parts of Haryana and Delhi. The India Meteorological Department has forecast no respite for at least three more days.

With the exception of Bengaluru, all the other metros have seen a 5-10 per cent rise in average relative humidity during summer, the study noted. According to data, in the last decade, summers in Hyderabad were 10 per cent more humid on an average – the highest among all metros. In Delhi, Mumbai, Kolkata and Chennai, humidity increased by 8 per cent, 7 per cent and 5 per cent, respectively.

“The combination of high heat and humidity can compromise the human body’s main cooling mechanism: sweating. The evaporation of sweat from skin cools our bodies, but higher humidity levels limit this natural cooling,” the study said. The combination of these two factors makes people sick and, in some cases, can also prove fatal, even at lower ambient temperatures, it added.

The analysis stated that in Delhi, there was a direct correlation between increase in built-up area and increase in urban heat stress. “Increase in green cover shows impact on daytime temperatures but has no impact on nighttime temperature and increasing heat index in the city,” the analysis said. Delhi’s built-up area increased from 31.4 per cent in 2003 to 38.2 per cent in 2022, contributing to the rise in urban heat stress, it noted.

The heat index in the six cities was also found to be higher in the monsoon period than the pre-monsoon period of March to May, the analysis said. The monsoon period has become hotter in Delhi, Mumbai and Kolkata, while in Chennai, the marginal cooling during monsoon has disappeared. In Bengaluru and Hyderabad, the monsoon was still a bit cooler than pre-monsoon.

An assessment of heat trends is necessary to develop a comprehensive heat management plan for urban centres, said Anumita Roychowdhury, executive director, research and advocacy, CSE. “This is needed to implement emergency measures during heatwaves to protect public health, and also to develop longer term strategies to mitigate heat by increasing green areas and waterbodies, improving thermal comfort in buildings, and reducing waste heat from vehicles, air conditioners and industries,” she said.