High Humidity Is Causing Heat Stress In India's 6 Major Cities

The temperature in 17 locations across the northwest breached the 48 degrees Celsius mark amid a severe heatwave which is testing human endurance. Delhi's Mungeshpur reported 48.8 degrees yesterday. Meanwhile, Haryana's Sirsa recorded 48.4 degrees. 

Meanwhile, the number of heatstroke patients in Rajasthan increased from 2809 to 3622 yesterday.

The Centre for Science and Environment (CSE) tracked heat waves in six major cities in India and the dangerous trends contributing to high temperatures. The think tank assessed data from Delhi, Mumbai, Bengaluru, Hyderabad, Chennai, and Kolkata. 

Heat Wave Analysis

Severe heat waves in India have worsened the Urban Heat Island Effect, i.e. when the temperature in urban areas is higher than in rural areas, resulting in a difference. 

Sharanjeet Kaur, a program officer at the Centre for Science and Environment, told NDTV that high temperature and humidity are crucial to understanding cities' heat stress. 

"Usually, heatwaves occur between March and July, but nowadays, we are witnessing more humidity than heat, resulting in heat stress. The temperature is relatively high at night. Pavements and concrete structures absorb the heat during the day, which gets trapped, resulting in warmer nights," she said.

The CSE analysis shows that heat stress results from a deadly combination of air temperature, land surface temperature and relative humidity, which leads to acute thermal discomfort and heat stress in cities. 

The analysis points to a worrying trend in cities - The increasing concretisation and decreases in green cover and water reservoirs. All cities have registered a significant increase in their built-up areas and concretisation, this contributes to the urban heat island effect. 

Concrete absorbs heat during the day but cannot emit infrared heat, causing the heat to be trapped. In rural areas, where relatively more green cover is present, the process of transpiration - Absorption of water by plants and evaporation through leaves and stems, allows water to be released into the atmosphere.

In 2023, Kolkata had the highest percentage of its land under concrete and the lowest green cover among the megacities. Delhi has comparatively the least area under concrete and the maximum green cover. Over the last two decades, built-up area in Chennai has doubled. Meanwhile, Kolkata registered only a 10% increase in its built-up area, making it the slowest as far as concretization is concerned. 

Increase In Relative Humidity

Relative humidity has increased in all zones, the analysis said. This increase has worsened heat stress in warm-humid and moderate climate zones while it nullified the fall in air temperatures in composite and hot-dry climate zones, especially during monsoons. The average relative humidity (RH) has significantly increased in the last 10 summers compared to the 2001-10 average. 

Barring Bengaluru, the average relative humidity has increased by five to 10% in other megacities. In dry zones like Hyderabad, the relative humidity increased by 10% compared to 2001-2010. Meanwhile, in Delhi, it increased by 8%. Mumbai, Kolkata and Chennai are still over 25% more humid than Delhi and Hyderabad.

When the relative humidity is combined with high heat, it adversely impacts human thermal comfort and health, Ms Kaur told NDTV.

The report said high humidity is responsible for adding to heat stress. An average of 3.3 degrees Celsius of heat stress is being added, the report said. Delhi is not cooling down at night, the cooling down of land surface temperature between day and night-time is down by 9%. 

At night, the peri-urban area cools down by 12.2 degrees, while Delhi's core cools down by only 8.5 degrees - thus the city core is cooling down by 3.8 degrees less than its peri-urban areas.

Heat Action Management Plan

Summer temperatures often peak during May, but India Meteorological Department scientist Soma Sen Roy, earlier told Reuters that they are predicting 7-10 heatwave days in northwestern regions this month, compared to the usual 2-3 days.

It was primarily due to fewer non-monsoon thundershowers and an active but weakening El Nino, she said, referring to a climate pattern that typically leads to hot and dry weather in Asia and heavier rains in parts of the Americas.

The report said if climate change projections and urban growth scenarios are combined, future urbanization will amplify the projected increase in local air temperature.

"We have analysed satellite imagery and assessed the change in land covers. We have to reverse the land use by ensuring an increase in green cover and water reservoirs in cities. We have to strengthen scientific monitoring of land use, activate emergency action plans and increase shaded areas and access to drinking water," Ms Kaur said.

"Gig workers like food delivery agents are more vulnerable to heat stress. They should be provided fluids or umbrellas to protect themselves, adding that the poor are more prone to suffer heat stress," she added.