No relief from heat even after sunset

CHENNAI: If hot sultry days have zapped your energy, be prepared to sweat even in the evenings as a recent study by WRI India (World Resources Institute) has flagged a worrying trend across Indian cities: nights are no longer offering relief from extreme heat.

As temperatures remain elevated even after sunset, experts caution that effects of ‘urban heat island’ are intensifying health risks, especially among vulnerable populations in cities like Chennai.

As the India Meteorological Department (IMD) continues to issue heat stress warnings, with several regions recording temperatures above 40°C, a less visible but equally serious pattern is emerging cities are failing to cool down at night. According to a study by WRI India, rising night-time temperatures are turning heat waves into a 24-hour phenomenon, with significant consequences for public health and urban living.

The study highlights how urbanisation, particularly dense construction and land use patterns, is driving what scientists call the ‘urban heat island’ effect. This phenomenon causes cities to retain heat during the day and release it slowly at night, preventing temperatures from dropping to comfortable levels.

Explaining this trend, VR Durai, director, Area Cyclone Warning Centre, Chennai, told DT Next that the impact was closely tied to the built environment. “We call this the urban heat island effect. In cities dominated by concrete structures, heat absorbed during the day is gradually released at night. As a result, night time temperatures remain higher than normal,” he said. “In Chennai, the maximum temperature recorded on May 1 (Friday) is 35.7°C, while the minimum night time temperature stood at 27.8°C. If the minimum temperature rises by more than 3°C above normal, we classify it as a ‘warm night’.”

Data from 15 monitoring stations across Tamil Nadu cities like Tiruchy, Madurai and Coimbatore indicate that night temperatures are currently 1- 2°C above normal levels in several urban centres. While northern India has been experiencing more intense impacts so far, meteorologists warn that Tamil Nadu could see a similar escalation during May.

The implications of warmer nights extend beyond discomfort. The WRI India study points out that sleep disruption is one of the most immediate effects. In densely populated urban cores, especially in informal settlements and houses with tin roofs, retained heat creates stifling indoor conditions long after sunset.

This lack of adequate cooling disproportionately affects low-income communities that do not have access to air conditioning or other cooling systems. As a result, the health burden is unevenly distributed. The study further links poor sleep to reduced productivity and increased vulnerability to chronic illnesses, particularly among individuals with pre-existing conditions such as cardiovascular disease.

Bupathi Raj from the Indian Medical Association told DT Next that elevated night time temperatures can significantly impact human health. “Temperature variation at night definitely affects sleep quality. Poor sleep disrupts metabolism and digestion. Over time, continuous sleep disturbance can lead to reduced attention span, impaired brain function, and in severe cases, mental health issues,” he explained.

On an individual level, doctors recommend simple measures such as wearing loose cotton clothing and improving ventilation. “For those without access to AC, increasing green cover around homes can help reduce ambient temperatures,” he added.

Another consequence of rising night temperatures is increased energy demand. As households rely more on fans, coolers, and ACs throughout the night, electricity consumption rises, putting additional strain on urban infrastructure. Experts emphasise that the discourse around heatwaves must evolve.

“We cannot focus only on peak daytime temperatures any more. It is equally important to understand how cities trap and re-radiate heat,” the study notes.