Heatwave

A prolonged period of abnormally hot weather, usually lasting several days or weeks. Definitions vary by region.

phenomena

What is a Heatwave?

A heatwave is a prolonged spell of abnormally hot weather, typically lasting at least 2–3 consecutive days. While the precise definition varies by country, all definitions share a common idea: the temperature is significantly above what the local population is used to, and is high enough to cause health, infrastructure or agricultural stress.

In South Asia, heatwaves are now one of the deadliest natural hazards. The Lancet Countdown 2024 report estimates that heat exposure causes more than 24,000 premature deaths annually in India alone, with the number rising as climate change accelerates the trend.

How the IMD defines a heatwave

The India Meteorological Department uses two parallel criteria:

Absolute temperature: Maximum temperature must reach 40°C or more in the plains, 37°C or more in coastal areas, or 30°C or more in hills.
Departure from normal: The maximum temperature is 4.5–6.4°C above the long-term average.

A Severe Heatwave is declared when the temperature exceeds 47°C, or the departure from normal reaches 6.5°C or more.

For comparison, Bangladesh’s BMD declares a heatwave at 36°C, Pakistan’s PMD uses 45°C in interior Sindh, and the World Meteorological Organization’s general definition is “five or more consecutive days when daily maximum is 5°C above the 1961–1990 average.”

How heatwaves form

Heatwaves are caused by stationary high-pressure systems, often called heat domes. The high pressure pushes the air downward, compressing it and warming it adiabatically. Clear skies under the dome allow intense solar heating during the day, and dry air provides little cooling at night.

In South Asia, three factors regularly combine to create devastating heatwaves:

Lack of pre-monsoon rainfall dries out the soil, removing the cooling effect of evapotranspiration.
Loo winds — hot, dry winds blowing from the Thar Desert across northwest India — push temperatures past 45°C.
Delayed monsoon onset prolongs the heat into June, extending the heat-stress window.

Heatwaves in South Asia

The Indian heatwave season runs from late March through June, peaking in May. The most affected regions are:

Rajasthan: Churu, Bikaner, Jaisalmer routinely exceed 48°C in May.
Madhya Pradesh and Vidarbha (Maharashtra): Gwalior, Nagpur, Akola.
Indo-Gangetic plain: Allahabad, Varanasi, Kanpur, Lucknow, Patna, Delhi.
Telangana and Andhra Pradesh: Hyderabad, Vijayawada, Kurnool.
Pakistan’s Sindh: Jacobabad, Sukkur, Sibi, Mohenjo-daro have recorded the highest reliable temperatures on Earth, repeatedly exceeding 50°C.
Bangladesh: Although Bangladesh has more moderate dry-bulb temperatures, the wet-bulb temperature (heat plus humidity) can reach dangerous levels above 30°C, which is harder for the body to tolerate than dry heat.

Health risks

The human body cools itself by sweating. When the wet-bulb temperature reaches 35°C, sweat can no longer evaporate effectively, and the body cannot cool itself — leading to heatstroke and death within hours, even for healthy adults. Several South Asian cities have already crossed this threshold during recent heatwaves.

Common health risks include:

Heatstroke — body temperature above 40°C; cessation of sweating; confusion; coma.
Heat exhaustion — dizziness, nausea, weakness, heavy sweating.
Dehydration — can cause kidney failure if severe and untreated.
Cardiovascular strain — heart attack risk rises during extreme heat.
Worsening of chronic conditions — diabetes, hypertension, respiratory disease.

How to stay safe

Avoid going outside between 11 AM and 4 PM during peak heat.
Drink water frequently — at least 3–4 litres per day, more if working outdoors.
Use ORS (Oral Rehydration Salts) — 6 tsp sugar + ½ tsp salt in 1 L water.
Wear loose, light-coloured cotton clothing and cover your head.
Eat water-rich foods — watermelon, cucumber, lassi, buttermilk, aam panna.
Avoid alcohol, caffeine and sugary drinks which worsen dehydration.
Cool your home — close curtains, use a wet towel in front of a fan.
Check on vulnerable neighbours, especially elderly or those living alone.

Heatwave 2026 and climate change

The IPCC AR6 report finds that South Asian heatwaves are now 30 times more likely than in the pre-industrial era. The 2022 March–April heatwave was the hottest March in 122 years for India. The 2024 season set new May records across Delhi-NCR.

For 2026, IMD has forecast above-normal pre-monsoon temperatures over most of north and central India. See our full guide: Heatwave India 2026 — state-by-state.

Frequently asked questions

When is heatwave season in India? The main heatwave season runs from April to June, with May as the peak month for north and central India. The southwest monsoon (late June) breaks the heat in most regions.

What is the hottest temperature ever recorded in India? The official IMD record is 51.0°C at Phalodi (Rajasthan) on 19 May 2016. Pakistan’s Mohenjo-daro reached 53.7°C in May 2010, one of the highest temperatures reliably measured on Earth.

Why do heatwaves kill so many people? A combination of intense heat, high humidity (raising wet-bulb temperature), inadequate cooling infrastructure for the poor, outdoor labour exposure, and underlying health conditions (cardiovascular, kidney, respiratory).

Is “feels-like” temperature the same as wet-bulb temperature? No. “Feels like” (apparent temperature, heat index) is a comfort metric incorporating humidity and wind. Wet-bulb temperature is a strict thermodynamic measurement of the lowest temperature the air can be cooled to by evaporation — a wet-bulb of 35°C is theoretically lethal for healthy humans.

Where can I check today’s heat in my city? Mausam Online shows live temperature, “feels like” reading and AQI for every Indian, Pakistani and Bangladeshi city: Delhi, Jaipur, Nagpur, Lahore, Karachi, Dhaka.