Unusually warmer nights increasing faster in Mumbai than hotter days: CEEW analysis

Mumbai: Hot days and warm nights have both increased in Mumbai, driven by a combination of global warming and urbanisation. But warm nights have increased much more sharply, according to an analysis of IMD data by the Council on Energy, Environment and Water (CEEW).

The number of ‘unusually warm nights’ in a year more than doubled from an average of 12 nights in 1981–2010 to 26 in 2011–2024, according to CEEW’s analysis.

Unusually hot days also rose in this period, but not quite as sharply, increasing from an average of 12 days a year to 18 days a year in the same period.

An unusually hot day or warm night is defined as one in which local temperatures are higher than on 90% of days or nights recorded during 1981–2010.

The trend has implications for the city’s health and productivity.

“Poor nighttime cooling disrupts sleep and recovery, especially for people doing physically demanding outdoor work,” said CEEW analyst Vishwas Chitale. In coastal cities like Mumbai, high humidity also makes it harder for the body to cool itself through sweating, he noted. “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.”

Globally, temperatures are rising as greenhouse-gas emissions from burning coal, oil and gas warm the atmosphere. India’s mean temperature has increased by about 0.9°C since 1901.

But urbanisation amplifies this warming. That’s because the asphalt and concrete of cities trap heat during the day and release it at night — part of a phenomenon known as the ‘urban heat island’ effect.

Across India, the rise in warm nights has been greatest in densely populated districts housing tier 1 and tier 2 cities like Mumbai, Delhi, Chennai and Jaipur, according to CEEW.

Interestingly, CEEW’s analysis also reveals a seasonal trend for the city. While minimum temperatures rose across all seasons, the increase was sharpest in the post-monsoon months between Oct-Dec.

In summer, average minimum temperatures rose by around 0.5°C in 2011-2024, compared with 1981-2010, similar to the daytime maximum increase of 0.4°C. But in the post-monsoon months, average minimum temperatures rose much more sharply — by 0.8°C, compared to a 0.1°C rise in daytime maximum.

There could be several reasons for this. For one, “global climate science shows that warming is often more pronounced during colder periods and in cooler regions, a pattern that is also emerging in India,” said Chitale.

Post-monsoon humidity may also play a role, along with urbanisation. “After the monsoon, the atmosphere retains significant moisture, and water vapour traps outgoing heat during the night, reducing cooling,” said Chitale.

Climate scientist Raghu Murtugudde said that changes in monsoon withdrawal and lingering south-westerly winds are also increasing humidity in Oct and thus affecting nighttime temperatures.

The Mumbai findings are based on district-level IMD temperature records available through CRAVIS, a climate-data platform recently launched by CEEW.

The local trends are consistent with national ones, said experts. “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,” said Chandni Singh, a climate researcher at the Indian Institute of Human Settlements.

A 2018 study from IIT-Gandhinagar found hot day and hot night events across India increasing between 1975-2015, with hot nights increasing faster. Most projections for the future similarly point to much warmer nights, Singh said.

CEEW’s projections for Mumbai show that even in a moderate global warming scenario, unusually warm nights could almost double from 26 nights a year today to 48 to 49 nights a year by 2031-2050. Meanwhile, the number of unusually hot days would remain more stable, according to these projections.

The trends also point to some of the solutions. The main lesson, said Chitale, is that “heat risk is not driven only by daytime temperatures, but also by warm nights, humidity, land-use change, and local socio-economic vulnerabilities”.

To reduce heat, experts said cities should implement a range of measures from short-term protections for outdoor workers to long-term expansion of green cover and water bodies, or what is often called “blue and green infrastructure”.

Studies have shown that Mumbai neighbourhoods with more green cover are as much as 10°C cooler than those without. Green spaces are especially important for low-income communities who can’t afford air conditioning. In fact, Singh said, “access to green cover might just be as important to heat management as PDS is to food security — it has to be accessible to the poorest in our cities”.