India’s Tryst With Heatwaves Begins

The heatwave season in India has made quite an early appearance this year. The West Coast has been bearing hot and humid weather conditions, especially parts of Coastal Maharashtra and Goa.

Mumbai and adjoining areas have been reeling under heatwave conditions, with high humidity levels increasing the discomfort. According to the Climate Shift Index (CSI) by US-based weather agency Climate Central, the last three-day temperatures (Feb 25-27) in Panaji, Goa were made at least 5 times more likely by human-caused climate change. Similarly, temperatures in Mumbai during the same time frame were at least 3 times more likely due to global warming. The heatwave might take a back seat but hot and sultry weather conditions would persist during the next few days.

India has kicked off its heatwave season rather early in the year.The West Coast has been dealing with hot and humid weather conditions, especially parts of Coastal Maharashtra and Goa. The coastal city of Mumbai was issued a heatwave warning for February 25 and 26.

On February 26, Mumbai recorded a maximum temperature at 38.7°C, which was 5.9 degrees above the normal average.

Some other regions in Coastal Maharashtra and Goa have also reported heatwaves, with the mercury soaring beyond 37 degrees°C. Meanwhile, other parts like coastal parts of Karnataka and Gujarat also witnessed hot and sultry weather conditions, with temperatures just short of ticking the threshold of heatwave and settling in the range of 35°C to 37°C.

Heatwaves typically occur between March and June, and in some rare cases even extend till July. The criteria for declaring heatwave for coastal stations in India is when the maximum temperature departure is 4.5°C or more above normal, provided the actual maximum temperature is 37°C or more.

Data Courtesy: Ogimet

The country’s nodal weather agency, India Meteorological Department (IMD), has predicted that hot and humid weather conditions will persist over the next couple of days, even if the heatwave recedes.

According to meteorologists, the ongoing hot and humid weather is a result of extremely rain deficit winter season. “It would not be wrong if we say that this year, the country has been witnessing one of the most dry winter seasons. In the absence of any weather systems, rains have remained in hiding. An anti-cyclone has been persisting over Madhya Pradesh, which had been pushing warm easterly winds across the West Coast. These inland winds delayed the setting in of sea breeze and prolonged land breeze led to a rise in mercury.

Besides this, high humidity levels due to the proximity of the coast increased the discomfort manifold, resulting in heatwave-like conditions,” said Mahesh Palawat, VP- Meteorology and Climate Change, Skymet Weather.

During the day, when the land heats up more quickly than the water, cooler air from the sea (sea breeze) moves inland to restore balance. Similarly, due to the high specific heat of water, the sea takes longer to cool after sunset. At night, cooler air from the land (land breeze) flows toward the sea, helping to maintain equilibrium.

Palawat further added, “The heatwave has arrived very early in the season. While the waxing and waning of rainfall during any particular season can be considered normal, we cannot rule out the role of climate change in the gradual rise in average temperatures over the years. It has been established that global warming has affected the winter rainfall in India. Summers have expanded and the winter season has downsized, with erratic rainfall patterns impacting temperature profiles across the country.”

“Anthropogenic climate change is escalating both meteorological and hydrological extremes worldwide, resulting in frequent weather record-breaking events. Currently, warmer-than-usual temperatures are being observed across all continents, indicating a relatively uniform global warming pattern. Unless we limit global warming by rapidly cutting the emission of greenhouse gases, weather records will continue to break frequently,” said

Dr Akshay Deoras, Research Scientist, National Centre for Atmospheric Science and the Department of Meteorology, University of Reading, UK. told that – According to the Climate Shift Index (CSI) by US based weather agency Climate Central, the last three day temperatures (Feb 25-27) in Panaji, Goa were made at least 5 times more likely by human-caused climate change. Similarly, temperatures in Mumbai during the same time frame were at least 3 times more likely due to global warming.

CSI is a system that quantifies the influence of climate change on local daily temperatures around the world. The Climate Shift Index ranges from -5 to +5. Positive levels indicate temperatures that are becoming more likely due to climate change (negative scores indicate conditions that are becoming less likely).

India battled an intense and prolonged heatwave season in 2024, also making it the hottest year on record. As per the data given by IMD, the country witnessed 536 heatwave days during Summer 2024. This has been the highest since 2010, when we registered 578 heatwave days.

May, the peak heatwave month, recorded the fourth-highest mean temperature in 2024 since 1901. Northwest India was worst affected, with the third highest maximum ever with an anomaly of 1.81ºC during the month.

Recent Heatwave and Wet-Bulb Temperature Trends in India

Heatwaves in India have become longer, more intense, and more frequent due to climate change. Heat waves have claimed more lives in India than other natural hazards, with the exception of tropical cyclones. According to a report, ‘An assessment of long-term changes in mortalities due to extreme weather events in India,’ mortality due to heat waves has increased by 62.2% between 2000 and 2019.

The heatwave phenomenon can be categorised into: Dry Heatwave and Humid Heatwave. During hot weather conditions, the human body normally cools itself by perspiration, or sweating, in which the water in the sweat evaporates and carries heat away from the body.

However, during the humid heatwave, which is currently the case, where the relative humidity is high, the evaporation rate of sweat is reduced. It means heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. This combination of land surface temperature, air temperature and relative humidity is also known as heat index and wet bulb temperature (TW). The adverse effects of high temperatures on the human body starts when, the saturation temperature is below 35°C and atmospheric temperatures are greater than 40°C, and the air conditioning ability of our body in the form of sweat reduces as the sweat does not evaporate. This builds up excess heat in the body, disrupting its normal functioning and, in the worst cases, leading to dehydration and death.

A wet bulb temperature of 35°C can be considered an upper limit for human survivability. Based on an ensemble of high-resolution climate change simulations, a study found that wet bulb temperature extremes in South Asia are likely to approach and exceed this critical threshold in a few places by the end of the 21st century under the business-as-usual scenario of future greenhouse gas emissions.

The writer of this article is Dr. Seema Javed, an environmentalist & a communications professional in the field of climate and energy