Explained: What is a Heat Wave & Why is India Experiencing One?

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

India's average annual temperature increased at a rate of 0.62°C in the 100 years between 1901 and 2020, according to the World Bank. Heat stroke 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, heat stroke 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 heat waves between 1998 and 2017, according to the World Health Organization.

What is a heat wave?

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

The weather agency has two more ways to keep a tab on heat waves. One criterion is to declare a heat wave when a place clocks a temperature 4.5-6.4°C more than the normal temperature for the region on that day. If the temperature is over 6.4°C more than normal, the IMD declares a 'severe' heat wave.

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

Heat wave warnings

Heat waves generally develop over Northwest India and spread gradually eastwards & southwards but not westwards (since the prevailing winds during the season are westerly to northwesterly). But on some occasions, heat wave may also develop over any region in situ under the favourable conditions, according to the IMD.

The IMD 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's 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.

Why did heat wave begin early this time?

It's 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. These have led to high temperatures and dry conditions as opposed to storms that would typically occur in March," Avantika Goswami, Programme Manager, Climate Change, at CSE, told FactChecker.

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 heat waves.

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

How much heat can a human body endure?

Heat waves 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 heat stroke cases account for only 10% of heat strokes, said Dr Mavalankar. "90% heat strokes are indirect or non-exertional. This generally affects people in old age and those with comorbidities. Heat causes further aggravation of their condition because of rapid circulation," he explained. "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°C, 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's measured by a thermometer covered in a water-soaked cloth, and it takes into account both heat and humidity. According to CSE, "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°C, compared to averages between 25-30°C; a 1°C 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 heat waves 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.