India recorded its warmest March this year since the India Meteorological Department began keeping records 122 years ago. The recent heatwaves began on March 11 and have affected at least 15 Indian states and Union Territories, showed Centre for Science and Environment’s analysis of the meteorological department’s data.

Rajasthan and Madhya Pradesh have suffered the most with 25 days of heatwave and severe heatwave each during this period. The otherwise supposedly cold hilly state is the one that stands third on this with 21 heatwave days – Himachal Pradesh, followed by Gujarat (19) and Jammu and Kashmir (16). These states have recorded temperatures as high as 40 degrees Celsius-42 degrees Celsius and are expected to see a 2 degrees Celsius-3 degrees Celsius rise.

India’s average annual temperature increased at a rate of 0.62 degrees Celsius in the 100 years between 1901 and 2020, according to the World Bank. Heatstroke has killed 11,571 people in the last decade (2011-2020) in India, showed an analysis of the Accidental Deaths & Suicides in India data, published annually by the National Crime Records Bureau.

In fact, heatstroke claimed more lives than floods and cold in the past two decades (2000-2020) and became the second leading cause of death from a natural force in the country after lighting, showed the data. While 20,615 people died of a heat stroke, lightning claimed 49,679 lives in the 20 years.

This phenomenon is not limited to India as across the world more than 1.66 lakh deaths occurred due to heatwaves between 1998 and 2017, according to the World Health Organization.

What’s a heatwave?

There is no universally accepted definition of a heatwave, rather it differs from region to region. In India, a heatwave is a condition of air temperature which becomes fatal to the human body when exposed, defines the India Meteorological Department. The weather agency declares a heatwave for a region when the temperature crosses 40 degrees Celsius in the plains, 37 degrees Celsius in coastal areas and 30 degrees Celsius in hilly regions.

The weather agency has two more ways to keep a tab on heatwaves. One criterion is to declare a heatwave when a place clocks a temperature 4.5 degrees Celsius-6.4 degrees Celsius more than the normal temperature for the region on that day. If the temperature is over 6.4 degrees Celsius more than normal, the India Meteorological Department declares a “severe” heatwave.

Another qualifier is when the temperature crosses the 45 degrees Celsius mark, the weather agency declares a heatwave and when it crosses 47 degrees Celsius, a severe heatwave. If the above criteria is met by at least two monitoring stations in a meteorological sub-division for at least two consecutive days, a heatwave is declared on the second day.

Representational image. Photo credit: Amit Dave/ Reuters

Heatwave warnings

Heatwaves generally develop over northwest India and spread gradually eastwards and southwards but not westwards (since the prevailing winds during the season are westerly to northwesterly). But on some occasions, a heatwave may also develop over any region in situ under favourable conditions, according to the India Meteorological Department.

The meteorological department issues colour codes based on the degree of heat forecasted in a particular region. No cautionary action is required if the colour green is issued. A yellow alert is a heat alert but of moderate temperature.

An orange alert falls in the severe heat alert category which can last up to two days. During this period, there is an increased likelihood of heat illness symptoms in people who are either exposed to sun for a prolonged period or do heavy work.

A red alert signifies extreme heat. There is a high likelihood of developing heat illness and heat stroke in all ages during this period that can extend up to six days.

March heatwave

It is a combination of factors that have led to this anomaly. One of them is anticyclones, which, unlike cyclones, cause hot and dry weather by sinking winds around high-pressure systems in the atmosphere. Experts say that anticyclones over western parts of Rajasthan and the absence of rain-bearing western disturbances caused this early onset.

“An anticyclone (high-pressure system) over the Indian landmass, drier than usual Western disturbances, a persistent La Niña pressure pattern and Arctic warming,” Avantika Goswami, Programme Manager, Climate Change, at CSE, told FactChecker. “These have led to high temperatures and dry conditions as opposed to storms that would typically occur in March.”

Another climate scientist Raghu Murtugudde, from the University of Maryland, explained that a north-south pressure pattern, associated with the La Niña phenomenon in eastern and central India persisted longer than expected and interacted with warm waves coming in from a rapidly warming Arctic region, leading to heatwaves.

While the India Meteorological Department has predicted heatwave conditions in parts of western Rajasthan till April 30 and a severe heatwave on May 1, Goswami said temperatures are expected to be high till June. “The last early heat wave occurred in 2019,” she said. “This particular heatwave is expected to see some relief around May 2, but it will pick up again from May 5, till June approximately.”

Effect on health

Heatwaves are an emerging public health concern. Extreme heat can lead to minor health issues such as rashes, cramps and oedema. But as the temperature soars, heat syncope (fainting occurring from low blood pressure), exhaustion and strokes can occur.

Dr Dileep Mavalankar, Director of the Indian Institute of Public Health Gandhinagar, explained that there are two types of heat strokes caused due to exposure to direct sunlight: exertional or direct heat stroke.

Direct heatstroke cases account for only 10% of heat strokes, said Dr Mavalankar. “Of the heatstroke cases, 90% are indirect or non-exertional.”

“This generally affects people in old age and those with comorbidities,” he explained. “Heat causes further aggravation of their condition because of rapid circulation. To cool the body, the heart functions faster and circulation increases which could lead to the heart’s failure. This leads to an indirect heat stroke.”

The body works best at a wet-bulb temperature of 35 degrees Celsius, found a 2020 study published in Science Advances journal. But it also boils down to humidity. Wet-bulb temperature is a measure of humidity in the air.

It is measured by a thermometer covered in a water-soaked cloth, and it takes into account both heat and humidity. According to the Centre for Science and Environment, “Factoring in humidity along with the heat, called the heat index, helps determine what the temperature actually ‘feels like’.”

Heat impacts much more than physical health. According to a study cited in a March 2022 Intergovernmental Panel on Climate Change report, “Mental health problems increased by 0.5% when average temperatures exceeded 30 degrees Celsius, compared to averages between 25 degrees Celsius-30 degrees Celsius. A 1 degree Celsius warming over five years was associated with a 2% increase in mental health problems.”

Until 2015, India did not have a national-level strategy to combat heatwaves and all disaster-related responsibility rested on state governments. In 2016, the government’s National Disaster Management Authority chalked the first national guidelines for heat waves titled “Preparation of Action Plan-Prevention and Management of Heat Wave”.

The guidelines aim to help stakeholders, such as state governments, district administrations, local self-governments, NGOs, civil society organisations, prepare a Heat Wave Management Plan by providing insight into heat-related illness and necessary mitigation measures.

This article first appeared on, a publication of the data-driven and public-interest journalism non-profit IndiaSpend.