Why has it been feeling so hot even at night? The phenomenon was studied in India’s bigger cities where it was found that a combination of rising temperatures, humidity and urban expansion is making heatwaves deadlier.
According to a study conducted by Delhi-based think tank Centre for Science and Environment (CSE), cities are not cooling down as much as they did during 2001-2010.
The study, ‘Decoding the Urban Heat Stress among Indian Cities, analysed data from six major cities – Delhi, Mumbai, Kolkata, Hyderabad, Chennai and Bengaluru – over a period of 23 years (January 2001 to April 2024).
The study highlighted while some cities have not seen a significant rise in temperatures but the humidity levels have left people stressed.
Heat stress results from a combination of air temperature, land surface temperature, and relative humidity, causing “acute thermal discomfort and increased heat-related diseases in cities”, as mentioned in a report by Down to Earth. Even if air temperatures vary, high relative humidity and land surface temperatures exacerbate discomfort.
According to the US National Weather Service, the heat index is a measure of how hot it really feels when humidity is factored in with the actual temperature. It is considered that a heat index of 41°C is dangerous to human health.
How Hot Has Delhi Become
According to the CSE analysis, Delhi’s ambient air temperature “shows little change” over the past 10 summers compared to the 2001-2010 decade. However, the “nature of heat is changing” with a significant increase in relative humidity over the past 10 summers.
The last 10 summers (2014-23) in Delhi have been 8% more humid on average compared to the average relative humidity from 2001-2010. While the average relative humidity for the summers from 2001 to 2010 was 52.5%, in the summer of 2023, this was as high as 60.9%. Similarly, in 2020, it was 61.4%, while it was 57.3% in 2021, and 53.5% in 2022, going by the analysis.
Delhi recorded 14 days in the summer of 2023 when the daily average heat index crossed the 41-degree mark, which is considered dangerous to human health.
The analysis also said Delhi is “not cooling down at night”. During the summers of 2001-10, the land surface temperature would fall by 12.3 degrees on average from the daytime peak to the nighttime low. Over the past decade, 2014-23, the nighttime cooling has fallen to an average of 11.2 degrees Celsius, and has been even lower in recent years – 9.8 degrees in 2023.
With urban expansion and high population density, people get a little chance to recover from daytime heat if the temperatures remain high overnight. Delhi’s ‘core,’ with the higher built-up area and population, is 2.9 degrees Celsius hotter than its peripheries, the study adds.
Heat Stress In Other Cities
Mumbai: As per the CSE study, Mumbai’s air temperature and humidity have increased, worsening the heat stress. The summer-time in the city has registered a 0.6°C increase in decadal average ambient air temperature; relative humidity has gone up by 7% between 2001-10 and 2014-23. Cooling down of land surface temperature between day and night time is down by 24%. Mumbai’s built-up area has increased from 38.4% in 2003 to 52.1% in 2023. Green cover has decreased from 35.8% in 2003 to 30.2% in 2023.
Kolkata: The city’s summer-time has registered insignificant change in average ambient air temperatures, but the relative humidity has increased by 5% between 2001-10 and 2014-23. Days with daily heat index exceeding 41°C (danger mark) have tripled compared to 2001-10 in Kolkata. Unlike other metros, Kolkata cools down at night at the same rate as in 2001-10. Built-up area has increased from 70% in 2001 to 80.1% in 2023. Green cover has decreased from 15.2% in 2001 to 14.5% in 2023.
Hyderabad: Summers have registered a 0.9°C drop in decadal average ambient air temperatures, but relative humidity has increased by 10% between 2001-10 and 2014-23. Hyderabad has 30-90 days in summer when the daily ambient temperature exceeded the 37°C mark. Built-up area has increased from 20.6% in 2003 to 44% in 2023. Green cover has also gone up from 8.9% in 2003 to 26.5% in 2023. This rise in green cover indicates an impact on day-time temperatures, but none on night-time temperatures and the increasing heat index in the city.
Bengaluru: The city has registered a 0.5°C increase in decadal average ambient air temperature in summers, while relative humidity has remained stable over the last two decades. March-April 2024 was significantly hotter (about 3°C) compared to the average of 2014-23. Cooling down of land surface temperature between day- and night-time is down by 15%. Built-up area has increased from 37.5% in 2003 to 71.5% in 2023. Green cover has also increased from 18.8% in 2003 to 26.4% in 2023.
Chennai: Summer temperatures have seen a rise of 0.4°C in decadal average ambient air temperature; relative humidity has increased by 5% between 2001-10 and 2014-23. March-April 2024 was 1°Chotter compared to average of 2014-23. Heat Index of the city has increased by over 5%. Days with daily heat Index exceeding 41°C (danger mark) has tripled compared to 2001-10. Built-up area has increased from 30.7% in 2003 to 73.5% in 2023. Green cover has decreased from 34% in 2003 to 20.3% in 2023.
Deaths Due to Heatwave to Rise
Not been able to control climate change impacts can result in an increase of heat wave-related deaths in the world by up to 2,000% by 2080, shows a recent study by researchers at Monash University in Melbourne, Australia.
The researchers developed a model to predict the number of heat wave deaths in 412 communities in 20 countries on four continents between 2031 and 2080. Mortality was calculated on the basis of greenhouse gas emissions, population density and adaptation strategies under different scenarios.
The study says countries near the equator are projected to see a 2,000% increase in heat wave-related fatalities from 2031 to 2080, compared to the 1971 to 2020 span.
This year, India has recorded 60 heat-related deaths. Government data shows since March, 32 people have died due to heatstroke and 28 from suspected heatstroke, with the latest two suspected deaths reported in Andhra Pradesh and Rajasthan’s Kota on May 22.
The country has seen 16,344 suspected heatstroke cases since March 1, 2024, with 486 suspected heatstroke cases on May 22 alone.
Muscle cramps, abdominal pain, generalised weakness, nausea, vomiting, and recurrent episodes of loose stools are common symptoms of heat exhaustion.
Doctors recommend people to increase the intake of water, cover themselves with a scarf when stepping out, dress in cotton clothes as preventive measures against a heatwave.
How to Mitigate Heat Effects
The CSE study has called for reversing land use by increasing green cover and water bodies in cities to create a cooling effect. Strengthening scientific monitoring of heat-related factors like land use and surface temperatures and developing emergency action plans to protect vulnerable populations during heat waves are also important.
Implementing city-specific heat management plans that go beyond just emergency response during heat waves and focusing on reducing heat generation from buildings, vehicles, and industries can help control the growing heat wave threat.
As per CSE, it is critical to develop emergency healthcare systems for heat-related disease burden, expand the shaded areas in cities, ensure availability of drinking water in public spaces, and reduce heat exposure for vulnerable and occupationally exposed groups in cities.