MUMBAI: The number of 'unusually warm nights' in Mumbai has more than doubled from an average of 12 nights per year during 1981-2010 to 26 nights in 2011-2024, according to an analysis of Indian Meteorological Department (IMD) data by the Council on Energy, Environment and Water (CEEW).
Health Impacts of Warmer Nights
"Poor nighttime cooling disrupts sleep and recovery, especially for people doing physically demanding outdoor work," said Vishwas Chitale, a fellow at CEEW. In coastal cities like Mumbai, high humidity makes it harder for the body to cool itself through sweating. "Warmer nights prevent recovery from intense daytime heat, increasing the risk of heat exhaustion and heat stroke, while worsening conditions such as diabetes and hypertension," Chitale added.
Global and Local Warming Trends
Globally, temperatures are rising due to greenhouse-gas emissions from burning fossil fuels. India's mean temperature has increased by about 0.9°C since 1901. Urbanisation amplifies this warming through the 'urban heat island' effect, where asphalt and concrete trap heat during the day and release it at night.
Seasonal Variations in Mumbai
The CEEW analysis reveals that the rise in warm nights has been greatest in densely populated districts with tier 1 and tier 2 cities across India. In Mumbai, minimum temperatures rose across all seasons, with the sharpest increase in post-monsoon months (October to December). During summer, average minimum temperatures rose by around 0.5°C in 2011-2024 compared to 1981-2010, similar to daytime maximum increase of 0.4°C. However, in post-monsoon months, average minimum temperatures rose by 0.8°C, compared to a 0.1°C rise in daytime maximum.
Reasons for Post-Monsoon Warming
"Global climate science shows that warming is often more pronounced during colder periods and in cooler regions, a pattern emerging in India," said Chitale. Post-monsoon humidity may also play a role, as water vapour traps outgoing heat at night. Climate scientist Raghu Murtugudde noted that changes in monsoon withdrawal and lingering south-westerly winds increase humidity in October, affecting nighttime temperatures.
National Consistency and Future Projections
The Mumbai findings are based on district-level IMD temperature records from CRAVIS, a climate-data platform by CEEW. Chandni Singh, a climate researcher at the Indian Institute of Human Settlements, said, "A lot of previous research has shown that hot night trends are increasing over India from 1975 onwards, with recent years showing a higher rate of increase." A 2018 study from IIT-Gandhinagar found hot day and hot night events increasing between 1975-2015, with hot nights increasing faster.
CEEW's projections for Mumbai show that even in a moderate global warming scenario, unusually warm nights could almost double from 26 nights per year today to 48-49 nights per year by 2031-2050, while unusually hot days remain more stable.
Solutions and Mitigation
"Heat risk is not driven only by daytime temperatures, but also by warm nights, humidity, land-use change, and local socio-economic vulnerabilities," said Chitale. Experts recommend short-term protections for outdoor workers and long-term expansion of green cover and water bodies, known as "blue and green infrastructure." Studies show that Mumbai neighbourhoods with more green cover can be up to 10°C cooler than those without.
