The ongoing heatwave in north India is pushing daytime temperatures past 40 degrees Celsius daily. However, the real climate story is unfolding after dark: India’s nights are warming nearly five times faster than its days. While daytime maximum temperatures hovered just 0.11 degrees Celsius above average, nighttime minimum temperatures jumped by 0.5 degrees Celsius, robbing millions of the chance to cool down.
India Meteorological Department (IMD) data showed that night temperatures stayed above both the 2020-2025 average and the long-term 1991-2020 normal on most days in April this year, pointing to a sustained rise in overnight heat.
A 2020 climate assessment by the Ministry of Earth Sciences found that between 1986 and 2015, daytime temperatures rose more sharply than nighttime temperatures. The warmest day of the year increased by about 0.63 degree Celsius, while the coldest night rose by 0.4 degree Celsius.
However, future projections showed the trend reversing, with the coldest nights expected to warm even faster than the hottest days by the end of the century. Recent IMD data suggests that shift has already begun, with minimum temperatures increasingly staying above normal across several parts of the country.
What is a warm night?
IMD classifies ‘warm night’ when the minimum temperature stays 4.5 to 6.4 degrees Celsius above normal after a day when the maximum temperature crosses 40 degrees Celsius. A ‘severe warm night’ is when the departure exceeds 6.4 degrees Celsius.
While references to warm nights appeared in IMD’s heatwave FAQs and guidance documents from at least 2020, the terminology has become far more visible in its daily bulletins over the past two years as prolonged heatwaves intensified across India.
In 2024, IMD, for the first time, began publishing data on ‘warm night’ and ‘severe warm night’ conditions in its press releases. A year later, in February 2025, the weather agency started including references to “night temperature above normal” in its regular bulletins, signalling growing concern over rising night-time heat.
Why night temperatures are rising
The IMD attributes three factors driving the trend.
1. Climate change is trapping more heat
Greenhouse gases act like an insulating blanket. During the night, the earth normally releases stored heat back into the atmosphere. However, rising emissions are slowing that cooling process.
2. Cities are becoming heat traps
In cities, concrete roads, glass buildings and dense construction absorb heat during the day and slowly release it at night. This is known as the urban heat island effect.
Areas with shrinking tree cover and rising construction tend to stay hotter after sunset. Studies from cities such as Pune and Kochi have linked rising night temperatures to rapid urbanisation and loss of green cover, as per a Times of India report.
The growing use of air-conditioners is worsening urban heat as they release heat into the surroundings. Night temperatures in dense city centres and residential clusters can be 4-6 degrees Celsius higher than in nearby outskirts, a gap expected to widen as cities continue to expand, according to a report in Indian Express.
3. High humidity worsens discomfort
Humidity prevents sweat from evaporating efficiently, making nights feel even hotter. Coastal and densely populated cities often experience “sultry nights”, where the body struggles to cool down even when temperatures fall marginally. Data suggests that humidity levels have increased over the years, according to IMD studies, conducted between 1981 to 2020.
Numbers game
According to a global study released in 2024, the number of hot nights has increased by around 32 per cent on average due to climatic impacts. The study covered around 300 Indian cities with populations of over 100,000.
Indian cities, on average, had 718 nights during 2014-2023 when the minimum temperature reached 25 degrees Celsius, said the study carried out by non-profit Climate Central and consulting initiative Climate Trends.
The report claimed that “Climate change has had a major influence on night-time temperatures above 25° degree Celsius in India. West Bengal and Assam are the regions that have been most impacted, with cities like Jalpaiguri, Guwahati, Silchar, Dibrugarh and Siliguri experiencing between 80 and 86 additional days each year above the 25°C threshold.”
Roxy Mathew Koll, a climate scientist with the Indian Institute of Tropical Meteorology, Pune, was quoted by DownToEarth as saying, “The urban heat island effect is most visible in the night-time temperatures. The high-rise buildings and concrete setup in the cities do not let the excess heat escape during the night.”
