The India Meteorological Department has warned of severe hot weather conditions across large parts of India this month. Maximum temperatures (as on 18/03/2022) were recorded at 40°C and above over most parts of West Rajasthan, Marathawada, Vidarbha in Maharashtra, many parts of Gujarat, some parts of West Madhya Pradesh, Rayalaseema and at isolated pockets over Telangana and Odisha.

The maximum temperatures were above normal by 4.5⁰C over most parts of Jammu-Kashmir Ladakh-Gilgit-Baltistan-Muzaffarabad, Himachal Pradesh, Punjab and West Rajasthan; over many parts of Haryana-Chandigarh-Delhi, East Rajasthan and Assam-Meghalaya; over some parts of West Madhya Pradesh, Arunachal Pradesh and Sub-Himalayan West Bengal-Sikkim and at isolated pockets over Vidarbha and Gujarat. The highest maximum temperature of 43.2°C was reported at Barmer (West Rajasthan).

IMD predicts moderate to severe heat wave conditions in isolated pockets over West Rajasthan, some pockets over Vidarbha and West Madhya Pradesh, West Rajasthan and in isolated pockets over Gujarat over the next five days.

India remains one of the most vulnerable countries to extreme weather events reveals the recent Intergovernmental Panel on Climate Change report. Heat extremes have increased while cold extremes have decreased, and these trends will continue over the coming decades, warns the report.

What is a heat wave?

A heat wave is generally defined as a prolonged period of excessively hot weather.

A heat wave is declared by India Meteorological Department (IMD) when:

• Based on departure from normal temperature, if maximum temperature of a station reaches at least 40°C or more for  the plains and at least 30°C or more for hilly regions, the station is declared to be experiencing a heat wave.
• When the departure from normal is 4.5°C to 6.4°C, this is described as a normal heat wave while when the departure from normal is >6.4° C, it is declared as an extreme heat wave.
• Based on actual maximum temperature, a heat wave is declared when the actual maximum temperature is ≥ 45°C while a severe heat wave is declared when the actual maximum temperature is ≥47
• If above criteria meet at least in 2 stations in a meteorological sub-division for at least two consecutive days then a heat wave is declared on the second day.
• For heat waves for coastal stations in India, when maximum temperature departure is 4.5°C or more from normal, heat wave may be declared provided actual maximum temperature is 37°C or more.

The peak month of the heat wave over India is May. Heat waves occur over plains of northwest India, Central, East & north Peninsular India during March to June and cover Punjab, Haryana, Delhi, Uttar Pradesh, Bihar, Jharkhand, West Bengal, Odisha, Madhya Pradesh, Rajasthan, Gujarat, parts of Maharashtra & Karnataka, Andhra Pradesh, Telengana and at times  Tamilnadu & Kerala.

IMD has a network of surface observatories over the country that measure various meteorological parameters like temperature, relative humidity, pressure, wind speed & direction etc. The data is regularly monitored and predictions are made based on this information.

The risk of heat waves is rising in India

The annual mean temperature over India has increased by 0.85°C from 1901 to 2015 and the surface temperature is expected to rise substantially by the end of this century. Increasing mean temperatures can lead to more intense heat waves that last longer and/or occur more frequently informs this paper titled 'Anthropogenic influence on the changing risk of heat waves over India' published in Nature Scientific Reports.

The paper discusses the findings of a study that examined the occurrence of heat waves in India during the twentieth and twenty-first century using CMIP5 climate model simulations. The study finds that land areas in India affected by intense heat are increasing in recent decades. The frequency and intensity of heat waves has drastically increased due to anthropogenic activities and these are posing a significant risk to human health.

Human induced anthropogenic factors such as greenhouse gas emissions, increase in land use, and land cover are predicted to lead to a two-fold increase in severe heat waves in central and mid-southern India in the twentieth century and a tenfold increase during the twenty-first century.

The study also finds a significant relationship between heat waves and deficits in precipitation showing that concurrent heat waves and droughts will occur in most places in India during the twenty-first century. The IPCC report too predicts that the frequency and intensity of heat waves will rise in South Asia thus increasing the likelihood of drier conditions and droughts in the years to come.

Drier conditions can lead to stronger heat waves putting substantial pressure on the environment, water resources, human health, agriculture, and the energy sector if adaptation and mitigation measures are not put in place, warns the study.

Another study finds that global ocean temperatures are also rising due to excessive absorption of heat trapped by greenhouse gas emissions. Marine heat waves are rising in the Indian ocean triggering changes in the monsoon patterns in India - leading to decrease in monsoon and increase in dry weather conditions over the central Indian subcontinent. The study notes an increase in the rainfall pattern over south peninsular India due to heatwaves in the North Bay of Bengal.

Heat waves and health

According to the World Health Organisation (WHO) the health impacts of heat depend on the timing, intensity and duration of temperature rise, the level of acclimatisation, and the adaptability of the local population and infrastructure available. The threshold at which temperature can pose as a hazard is variable and depends on the region and factors such as humidity and wind, local levels of human acclimatisation and preparedness for heat like conditions.

Exposure to extreme heat can challenge the body’s ability to regulate temperature and can result in heat cramps, heat exhaustion, heatstroke, and hyperthermia. Deaths and hospitalisations from heat can occur rapidly and result in death or illness among vulnerable people such as the old and frail. Heat conditions can alter human behavior, the transmission of diseases, health service delivery, air quality, and critical social infrastructure such as energy, transport, and water.

Coping with heat waves

The India Meteorological Department (IMD) issues warnings based on colour codes that indicate the severity of the weather phenomena expected. This is to forewarn relevant officials and the disaster management authority "about the impact of the weather expected so as to keep them ready for necessary action related to disaster risk reduction". The four colour codes include Green - No Action needed; Yellow - Watch and stay updated; Orange - Be prepared; Red - Take action.

The IMD recommends avoiding exposure to heat and preventing dehydration by taking in plenty of water and other fluids such as ORS, homemade drinks like lassi, torani (rice water), lemon water, buttermilk, etc. to maintain hydration levels and preventing exposure of the vulnerable to the sun in cases of extreme heat.

The World Health Organisation too recommends prevention of dehydration and of exposure to extreme heat to prevent heat strokes. It suggests:

• Seeking help in case of symptoms such as dizzyness, weakness, anxiety, intense thirst and headache by moving to a cool place as soon as possible and measuring body temperature.
• Resting immediately in a cool place in case of painful muscular spasms (particularly in the legs, arms or abdomen, in many cases after sustained exercise during very hot weather), and drinking oral rehydration solutions containing electrolytes. Medical attention is needed if heat cramps last more than one hour.
• Calling an ambulance immediately in case of unusual symptoms such as hot dry skin and delirium, convulsions and/or unconsciousness.

More details can be viewed in the WHO factsheets on Heat and Health and the IMD heatwave guidance section.