The study utilised the HI framework provided by the National Weather Service of the National Oceanic and Atmospheric Administration, categorising any HI exceeding 41 degrees Celsius as falling into the “danger” category. This method has also been employed by other research groups including the Indian Institute of Science, Bengaluru, and aligns with the HI thresholds set by the Union earth sciences ministry in their July 2023 notification.

The India Meteorological Department (IMD) describes the HI as an experimental measure that combines air temperature and relative humidity to provide a "feels like" temperature. This index is designed to give a more accurate representation of how hot the conditions feel to the human body, taking into account the increased discomfort caused by high humidity levels and the temperature.

IMD identifies heat stress in terms of heatwave conditions, which are declared based on specific temperature thresholds. A heatwave is declared when the temperature in the plains crosses 40 degrees Celsius, 37 degrees Celsius in coastal areas, and 30 degrees Celsius in the hills. Additionally, the temperature of a particular day must be above normal by at least 4.5 degrees Celsius for two consecutive days to qualify as a heatwave.

According to the analysis, in April 2024 alone, 27 cities reported experiencing heat stress, with most of these cities situated along the east coast. This is a significant jump from the previous year, where only six cities (Vijayawada, Bhubaneshwar, Visakhapatnam, Chennai, Puducherry, and Thrissur) experienced the highest jump in heat stress hours, with each recording over 100 hours in April 2024. The largest increases were observed in Vijayawada, Bhubaneswar, and Visakhapatnam. Heat stress was more prevalent during morning hours for most cities than during evening hours. For example, Vijayawada, Bhubaneshwar, and Visakhapatnam had their peak heat stress hour between 11 am to 12 noon while cities like Bhilai and Raipur had their peak heat stress hour at 5 pm.

“The number of consecutive days of extreme heat stress in many of the east coast cities of India has increased at an unprecedented rate. This rapid change in environmental conditions due to climate impacts requires immediate and urgent attention,” said Ronak Sutaria, founder and chief executive officer, Respirer Living Sciences.

Experts identify reasons for this extreme heat stress

The weather department and experts identified key factors contributing to this extreme heat stress observed across the east coast. “Generally during April, the formation of an anticyclonic circulation (a weather pattern characterised by clockwise rotating winds around a centre of high atmospheric pressure) forms over the Bay of Bengal that provides moisture to the east coast resulting in thunderstorms. This year, however, this weather pattern was not observed over the Bay of Bengal during April resulting in unfavourable weather conditions, with dry winds moving from the land to the sea resulting in heatwave conditions and heat stress,” said Mrutunjay Mohapatra, director general, IMD.

Private weather forecasting agency Skymet’s vice president (meteorology and climate change) Mahesh Palawat highlighted the broader reasons for the lack of this weather system. “Some of the impacts are accelerated in a warming world, especially due to active El Niño (the periodic warming of sea surface temperatures in the equatorial Pacific),” he said.

Mohapatra added that over the past week, there has been a wholly opposite trend than what was witnessed on the east coast during April 2023. “Now, there is heavy thunderstorm activity as there is an anticyclonic circulation off the Andhra Pradesh coast resulting in a change in wind direction from sea to land bringing in moisture that led to rains for the east coast over the past week and may continue for some areas,” he said.

Analysis reveals how hot it got for cities in April 2024

In April 2024, all cities that had experienced heat stress in 2023 saw a rise in heat stress hours, the RLS analysis found. The most significant increases were in Vijayawada (138 hours), Bhubaneswar (130 hours), and Visakhapatnam (110 hours). “Most of these increases occurred between 10 AM and 1 PM, highlighting that heat stress is most intense during the morning,” said Sutaria.

Heat stress poses severe health risks, including heat cramps, exhaustion, and even heat strokes, affecting vulnerable populations such as children, pregnant women, and the elderly. "Rehydrating and avoiding substances like alcohol, caffeine, and aerated drinks is crucial during these times," said Sutaria.

In April this year, the cities with the highest average heat indices in India were Vijayawada (36.82 degree Celsius), closely followed by Puducherry (36.70 degree Celsius), Kozhikode (36.65 degree Celsius), Malappuram (36.24 degree Celsius), and Kannur (36.14 degree Celsius). Chennai ranked sixth with an average heat index of 36.06 degree Celsius, while Visakhapatnam had 35.77 degree Celsius. Tiruchirappalli and Bhubaneswar recorded average heat indices of 35.47 and 35.33 degree Celsius respectively, with Thrissur completing the top ten at 35.26 degree Celsius.

Across India,14 cities (Kozhikode, Kochi, Kannur, Thiruvananthapuram, Surat, Kollam, Jamshedpur, Asansol, Salem, Solapur, Vadodara, Dhanbad, Rajkot, and Nagpur) had no heat stress in April 2023 but all have had a significant increase in heat stress this year. “Kozhikode jumped from zero to nearly 77 heat stress hours. All of this points to a rapidly changing environment - which requires urgent attention,” said Sutaria.

Is seasonal climate migration on the cards for India?

The analysis also hints at broader socio-economic impacts due to these climate conditions, suggesting potential shifts in real estate, insurance markets, and agricultural outputs. "Severe air pollution in the winter months for northern cities and extreme heat stress in the summer months for east coast cities are creating conditions that could lead to seasonal climate migration across India," Sutaria said.

The movement of people from one location to another due to factors such as rising sea levels, increased frequency of extreme weather events including heat stress, and changes in conditions that affect agricultural productivity, making their current location less habitable is attributed to climate migration.

“We're seeing altered weather patterns like delayed monsoons and winters over the past two years, increasing severity of heat, expanding drought regions, and frequent intense rains causing floods and landslides. These changes threaten agriculture, food production, and livelihoods, and could drive migration. However, further studies over the next three to four years are essential to confirm if climate migration has started in India,” said Palawat.

Regional weather outlook for coming months

The IMD on Monday said that the southwest monsoon was likely to make its way to Andaman and Nicobar Islands by May 19. In the meantime, the analysis by RLS states that severe heat stress might continue during May, June, and July across India.

“The recent power sector shortfall predictions are indicating that peak nighttime power shortfall, which is usually triggered by peak air conditioning usage to counter heat stress, is expected to happen in mid-June and will even extend to July — which is largely triggered by rapidly rising humidity without adequate fall in temperature. Hence, going by night-time power shortfall proxies, severe heat stress can be expected to persist and might most likely rise from May to July,” said Sutaria.

Mohapatra added that presently northwest India is likely to witness heatwaves and severe heat stress through the rest of May. “This region is not likely to witness any more strong western disturbances (a weather pattern originating in the Mediterranean that brings sudden rain and snow to the northwestern parts of the Indian subcontinent) in the foreseeable future. While this is seasonal, dry winds will further increase heat stress for the region,” he said.

RLS’ latest analysis highlights the critical areas and times for intervention to mitigate the adverse effects of rising temperatures on India's vast population. With the increasing frequency of such extreme weather conditions, the need for comprehensive monitoring and management is more pressing than ever.