Warmer nights during Indian summers may be more dangerous than heatwaves, and here’s why. While most discussions about extreme heat focus on peak daytime temperatures and heatwaves, recent data reveals a growing concern: rising nighttime temperatures. This issue is particularly relevant for those living in low and middle-income housing units, where indoor thermal exposure could be a significant health risk. The human body needs cooler evenings and nights to recover from daytime heat, but when nighttime temperatures remain high, the body struggles to recoup, leading to prolonged and sustained heat exposure. This is especially problematic in densely populated areas with limited natural ventilation or air conditioning.
A study by Climate Trends in 50 houses in Chennai found that occupants frequently slept in temperatures exceeding 32° Celsius, sometimes even reaching 35° Celsius, similar to peak daytime temperatures. This highlights the need for further research into the health impacts of indoor heat exposure in India, as global studies suggest that nighttime warming could significantly contribute to heat-related mortalities. India's average temperature has increased by about 0.7° Celsius between 1901 and 2018, with both day and nighttime temperatures rising, but at different rates.
In the 30 years between 1986 and 2015, the temperature of the warmest day increased by 0.63° Celsius, while the coldest night increased by 0.4° Celsius. However, future projections indicate a more rapid rise in nighttime temperatures. The temperature of the warmest day could rise by 4.7° Celsius by the end of this century, while the coldest night could jump by as much as 5.5° Celsius. This trend is already evident in recent data from the India Meteorological Department (IMD).
The urban heat island effect, caused by concrete, roads, bricks, and metal absorbing and radiating heat, exacerbates the issue in urban spaces. The lack of vegetation, green spaces, and water bodies, along with dense high-rise buildings, amplifies this effect. The use of air conditioners, which emit hot air outside, further contributes to the problem. Nighttime temperatures in city centers or dense residential areas can differ by 4-6° Celsius or more from the outskirts.
Most urban Indians live in small, poorly designed houses with limited natural ventilation, and those without air conditioning face little nighttime relief. This leads to sleep discomfort, restlessness, exhaustion, and reduced energy, affecting productivity. The health impacts of prolonged heat exposure are still being assessed, but the correlation between higher nighttime temperatures and mortality is evident.
Dileep Mavalankar, former director of the Indian Institute of Public Health, noted that on days when the maximum daytime temperature exceeds 45° Celsius, if the maximum night-time temperature is below 28° Celsius, there is little change in daily all-cause mortality. However, when nighttime temperatures rise to 28-30° Celsius, mortality increases to 165, and above 30° Celsius, it can reach 265. This highlights the critical link between higher nighttime temperatures and mortality.
Heat action plans in at least 23 states and over 200 cities focus on heatwave-like situations, but the trend of rising nighttime temperatures is a constant in Indian summers. Long-term measures to reduce impacts include improving urban infrastructure, low-cost housing, and green spaces. However, immediate relief can be provided through passive cooling solutions like reflective roof coatings, whitewashed roofs and walls, and natural ventilation improvements.
In conclusion, the dangers of warmer nights during Indian summers are becoming increasingly evident, and addressing this issue is crucial for public health. By implementing appropriate measures, we can mitigate the adverse effects of rising nighttime temperatures and ensure a more comfortable and safe living environment for all.
