Heat waves are becoming deadlier due to a combination of rising temperatures, increasing humidity & urban expansion
While cities used to cool down considerably at night, the study shows this nighttime cooling has significantly reduced in recent years. Photo for representation: iStock A new study by Delhi-based think tank Centre for Science and Environment (CSE) has revealed a dangerous trend in India’s biggest cities: Heat waves are becoming deadlier due to a combination of rising temperatures, increasing humidity and urban expansion. The research has also uncovered a worrying phenomenon: cities are not cooling down at night 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- April 2024). It found that while air temperatures haven’t risen significantly in some cities, increased humidity is worsening the heat stress people experience.
Read more: Heat index is a critical tool to help beat heat stress
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. Even if air temperatures vary, high relative humidity and land surface temperatures exacerbate discomfort.
The United States’ National Weather Service states that the heat index, which includes humidity, indicates how hot it feels, with a heat index of 41 degrees Celsius being dangerous for human health.
Anumita Roychowdhury, executive director of research and advocacy at CSE, in a statement said:
Assessing the changing trend in heat, relative humidity and land surface temperature along with day and nighttime temperatures is necessary to develop a comprehensive heat management plan for the urban centres.
Further, this is needed to implement emergency measures during heatwaves to protect public health, and also to develop longer term strategies to mitigate heat by increasing green areas and waterbodies, improving thermal comfort in buildings and reducing waste heat from vehicles, air conditioners and industries, she added.
“Addressing the combination of high heat and humidity is particularly important as this can compromise the human body’s main cooling mechanism: sweating. The evaporation of sweat from skin cools our bodies, but higher humidity levels limit this natural cooling,” said Avikal Somvanshi, senior programme manager, Urban Lab, CSE.
Read more: Move over record air temperatures, Delhi’s scorched land surface is making waves
As a result, people can suffer heat stress and illness, and the consequences can even be fatal even at much lower ambient temperatures. Interestingly, night time temperature is remaining elevated in cities, he added.
The culprit behind rising humidity? Uncontrolled urban sprawl. The study found a direct correlation between an increase in built-up area and a rise in urban heat stress in all six cities. For example, Chennai have seen their green cover shrink by nearly 14 per cent in the last two decades, while concretisation has doubled.
This trend is creating a vicious cycle. Concrete absorbs and retains heat, contributing to the “urban heat island effect” where cities are significantly warmer than surrounding areas.
Hotter nights are another worrying finding. While cities used to cool down considerably at night, the study shows this nighttime cooling has significantly reduced in recent years. This means people get little respite from the daytime heat, further impacting their health.
The consequences of this deadly combination are severe. The study highlighted a rise in the number of days with a heat index exceeding 41°C (considered dangerous for human health) in cities like Kolkata and Chennai. Additionally, monsoons are no longer providing relief, with some cities experiencing hotter and more humid conditions during this period compared to pre-monsoon months.
Read more: The reality of living with 50℃ temperatures in our major cities
The CSE study calls for urgent action to mitigate this growing threat, with several key recommendations.
The researchers called for 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.
The study also 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, it pointed out.
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