A new analysis has found that heat risk in Indian cities is increasingly being shaped not only by daytime heatwaves but also by night-time heatwaves and compound heatwaves, which occur when a daytime heatwave is followed by a night-time heatwave.

A similar study conducted earlier by the Council on Energy, Environment and Water (CEEW) revealed that between 2012 and 2022, the number of very warm nights increased more rapidly than the number of very warm days.

Over 70 per cent of districts witnessed five or more additional very warm nights per summer (March to June) during this period, according to the CEEW analysis.

As night-time temperature continues its upward momentum, it steadily increases difficulties for the human body as it fails to cool down after intense daytime heat, which can lead to a rise in health risks such as heat stroke and worsening of non-communicable diseases like diabetes and hypertension.

Studies like these present before us the downfall of the current Heat Action Plans (HAPs) and urban climate policies, as they mainly focus on daytime heatwaves. A HAP serves as an early warning system and preparedness plan for extreme heat events.

The latest study, 'Night-time and compound heatwaves as emerging climate risks in India's smart cities', was published in the journal Physics and Chemistry of the Earth on March 12.

The analysis was conducted by Imdad, Anubha Yadav, a PhD scholar at PPN PG College, Archana Chaudhary, a researcher at Kanpur's Vikramajit Singh Sanatan Dharma College, and Rayees Ahmed, a researcher at Bengaluru's Indian Institute of Science.

For their study, the researchers developed a diurnal persistence index to analyse 100 Indian smart cities and find where daytime, night-time, and compound heatwaves existed for the period between 2001 and 2024.

"We categorised Indian smart cities into plain, plateau, coastal, and hill areas. In the Indo-Gangetic plain cities, there is a mixed regime (daytime heatwaves are declining), but compound and night-time heatwaves are rising. In southern Peninsular India, daytime heatwaves dominate. In coastal and hill cities, night-time heatwaves dominate," Imdad stated.

Notably, between 2001 and 2024, Srinagar experienced the highest frequency of daytime and compound heatwaves among the 100 smart cities.

While Gujarat's Dahod saw the most intense compound heatwaves, Varanasi experienced the most intense night-time heatwaves, and Imphal the most intense daytime heatwaves.

To address rising night-time temperatures, authorities should develop Heat Action Plans (HAPs) and urban climate policies that focus on reducing heat retention and improving cooling after dark.

Key measures include adopting cool roofs, enhancing natural ventilation and airflow corridors, and increasing urban green spaces.