A heat wave in India is a period of abnormally high day temperatures that pushes the human body, crops and the power grid beyond safe limits. The India Meteorological Department (IMD) treats it as a hydro-meteorological hazard distinct from a normal hot day, because mortality, hospital admissions and crop damage rise non-linearly once specific thresholds are crossed. For UPSC, the heat wave in India is a recurring GS-I geography and GS-III disaster-management topic that links climate change, the urban heat island, NDMA’s Heat Action Plans (HAPs), and India’s growing wet-bulb stress.
Between 2022 and 2025, the heat wave in India has shifted from an episodic April–June event in the north-western plains to a longer, hotter and more humid hazard reaching the east coast, the northeast and high-altitude towns. The IMD’s seasonal outlook for 2024 forecast 10–20 heatwave days against a normal of 4–8 across many states, and 2024 went on to record the longest spell since 2010. This article unpacks the IMD criteria, the physiology of wet-bulb temperature, vulnerable groups, urban heat island dynamics, and the Heat Action Plan framework — the standard analytical scaffold UPSC examiners expect.
IMD definition of a heat wave
IMD declares a heat wave on the basis of maximum temperature and its departure from the long-period normal, applied separately to plains, coasts and hills.
Threshold criteria
- Plains: maximum temperature must reach 40°C or more.
- Coastal stations: maximum must reach 37°C or more.
- Hilly regions: maximum must reach 30°C or more.
Once the absolute threshold is met, IMD then checks the departure from the climatological normal at that station:
- Heat wave: departure of 4.5°C to 6.4°C above normal.
- Severe heat wave: departure of 6.5°C or more above normal.
Alternatively, irrespective of departure, IMD declares a heat wave when the maximum touches 45°C and a severe heat wave at 47°C. The condition must be met at at least two stations in a sub-division for two consecutive days; the declaration is made on the second day.
Heat wave versus warm night
IMD also issues warm-night advisories when the minimum temperature stays anomalously high. A hot day with a warm night is more lethal than a hot day with a cool night, because the body never gets a recovery window. The 2015 Andhra Pradesh–Telangana event, which killed over 2,000 people, was driven as much by warm nights as by 45°C afternoons.
Recent records: 2022 to 2025
The heat wave in India trend line over the last four years tells the story examiners want.
- 2022: India recorded its hottest March since 1901; large parts of north-west India saw heat-wave conditions from late March, six weeks earlier than the climatological norm. Wheat yields fell 4–6%, prompting an export ban.
- 2023: A severe heat wave hit eastern India in April; Baripada (Odisha) touched 44.2°C, and a single open-air function near Mumbai (Kharghar) saw 13 deaths from heatstroke at a public event.
- 2024: IMD logged 536 heat-wave days across stations between April and June — the longest spell in 14 years. Delhi’s Mungeshpur reportedly recorded 52.9°C in late May (later flagged for sensor error, with the corrected value near 49.1°C). Phalodi (Rajasthan) crossed 50°C.
- 2025: A short but intense pre-monsoon spell pushed Barmer past 48°C in April, and Bengaluru saw its hottest April day in over a decade. Health departments in Maharashtra and Bihar reported sustained heat-stress admissions.
The pattern is clear: earlier onset, longer duration, wider geography, and higher humidity.
Wet-bulb temperature and the 35°C survivability limit
A dry 47°C in Phalodi is dangerous; a humid 35°C in coastal Bengal can be lethal. The metric that captures this is the wet-bulb temperature (Tw).
What wet-bulb measures
Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling at constant pressure. It combines heat and humidity into a single number that directly reflects the body’s ability to shed heat through sweat. The human body cools itself by evaporating sweat; once the ambient wet-bulb temperature approaches skin temperature (~35°C), evaporation effectively stops.
- Tw 26–28°C: heavy exertion becomes unsafe.
- Tw 30–32°C: even resting in shade is dangerous for the elderly.
- Tw 35°C for 6 hours: theoretical survivability limit, even for a fit young adult in shade with unlimited water.
Indian exposure
Recent studies in Nature and IIT-Bombay work flag the Indo-Gangetic Plain, coastal Tamil Nadu, the Konkan and parts of Odisha as zones where Tw could routinely cross 32°C by mid-century under a high-emissions pathway. A 2022 Lancet Countdown report attributed a 55% rise in heat-related deaths among Indians over 65 between 2000–04 and 2017–21. The US CDC similarly warns that wet-bulb stress, not dry heat, is the dominant driver of heat-stroke mortality in humid tropical countries.
Urban heat island and the city penalty
Cities run hotter than their rural surroundings — the urban heat island (UHI) effect — typically by 3–7°C at night.
Causes of UHI
- Concrete and asphalt absorb solar radiation by day and re-radiate it at night.
- Loss of tree cover and water bodies reduces evapotranspirative cooling.
- Anthropogenic heat from air-conditioners, vehicles and industry adds direct warmth.
- Canyon geometry of high-rise corridors traps long-wave radiation.
Delhi’s nighttime UHI intensity routinely exceeds 5°C; ISRO satellite studies of Hyderabad show 6–8°C surface-temperature gaps between concrete cores and green peripheries. UHI is why a “moderate” IMD warning translates into a public-health emergency in slums and informal settlements.
Vulnerable groups
A heat wave is not democratic. The 2015 and 2023 spells killed disproportionately among:
- Outdoor workers: construction labourers, street vendors, rickshaw pullers, farm hands.
- The elderly (60+) with cardiovascular or renal disease.
- Pregnant women — heat stress is linked to pre-term birth and low birth weight.
- Children under five, who have higher surface-area-to-mass ratios.
- Slum dwellers in tin-roofed homes without ventilation.
- Homeless populations and prisoners in unair-conditioned cells.
The intersection of class, occupation and gender means heat-wave mortality data is consistently under-reported, especially among informal workers.
NDMA Heat Action Plans
India’s institutional response is anchored in the NDMA National Guidelines for Preparation of Action Plan – Prevention and Management of Heat Waves, first issued in 2016 and updated in 2019.
What a Heat Action Plan does
- Early-warning chain: IMD colour-coded forecasts (yellow / orange / red) cascade to states, districts and ULBs 24–72 hours in advance.
- Cool-roof programmes: white-painted or solar-reflective coatings on slum roofs can cut indoor temperatures by 2–5°C.
- Adjusted work hours: MGNREGA and construction work shifted to pre-noon and late-evening windows.
- Water and shade points: piau stalls, ORS distribution, public drinking-water access at bus stops.
- Hospital surge capacity: dedicated heat-stroke beds, ice-pack protocols, ambulance triage.
Ahmedabad’s HAP (2013) was the first in South Asia and has since been replicated in over 100 cities. Independent evaluations credit it with averting an estimated 1,100 deaths annually. Telangana’s plan integrates Aanganwadi workers as front-line responders; Odisha’s plan reroutes school timings statewide.
Gaps
A 2023 Centre for Policy Research audit of 37 Indian HAPs found:
- Most are not legally binding documents.
- Few have dedicated budgets; financing is ad hoc.
- Vulnerability mapping is coarse, missing ward-level data.
- Long-term measures (tree cover, cool roofs, building codes) are weak.
Mitigation and adaptation
A long-horizon response goes beyond emergency advisories.
- Cool-roof and reflective-paint subsidy schemes for low-income housing.
- Urban greening: city-level tree-canopy targets, water-body restoration, sponge-city design.
- Passive building codes: ECBC and the Eco-Niwas Samhita mandate insulation and shading.
- Occupational safety: a National Heat Stress Standard under the Code on Occupational Safety, Health and Working Conditions (2020) — still not notified.
- Crop calendars: shifting wheat sowing earlier, drought-tolerant rice varieties.
- Insurance: parametric heat-insurance pilots (e.g., SEWA-Arsht in Ahmedabad, 2023) pay informal women workers when temperatures cross a threshold.
The broader lesson is that heat is a slow-moving disaster requiring the same seriousness as a tsunami or earthquake — and a permanent response architecture similar to the NDRF.
Climate change link
Attribution science is now strong enough for examiners. The World Weather Attribution group concluded that the 2022 South Asian heat wave was made 30 times more likely by anthropogenic warming. India’s mean temperature has risen by about 0.7°C since 1901, with the warming concentrated in summer minima. Under a 2°C global-warming scenario, the IPCC AR6 projects that current 1-in-50-year Indian heat events would recur every 4–5 years.
Frequently Asked Questions
What is the IMD definition of a heat wave in India?
IMD declares a heat wave when the maximum temperature reaches 40°C in plains, 37°C in coastal areas or 30°C in hills, and the departure from the long-period normal is 4.5–6.4°C; a severe heat wave needs a departure of 6.5°C or more. Alternatively, 45°C absolute is a heat wave and 47°C absolute is a severe heat wave, irrespective of departure.
Why is wet-bulb temperature important during a heat wave in India?
Wet-bulb temperature combines heat and humidity into the single value that decides whether the body can shed metabolic heat through sweat. A sustained wet-bulb of 35°C is considered the upper survivability limit for healthy adults. India’s coastal and Indo-Gangetic belts are increasingly approaching wet-bulb conditions where outdoor work becomes physiologically unsafe.
Which Indian heat-wave events have caused the highest mortality?
The 2015 Andhra Pradesh–Telangana heat wave killed over 2,000 people; the 1998 Odisha event killed about 1,300; the 2003 Andhra event over 1,200. The 2023 Kharghar open-air event in Maharashtra and the prolonged 2024 spell in north India also recorded large clusters of deaths, especially among outdoor workers and the elderly.
What is the urban heat island effect?
The urban heat island is the tendency of cities to be 3–7°C warmer than surrounding rural areas, especially at night. Concrete and asphalt store solar heat; loss of trees and water bodies reduces cooling; air-conditioners and vehicles add anthropogenic heat. UHI converts an IMD u0022moderateu0022 warning into a public-health emergency for slum dwellers and homeless populations.
What is a Heat Action Plan and which Indian city pioneered it?
A Heat Action Plan (HAP) is a multi-agency response document covering early warning, work-hour shifts, cool roofs, water and shade points, and hospital readiness. Ahmedabad released India’s first HAP in 2013 after the 2010 heat wave that killed about 1,344 residents; it has since been replicated in over 100 Indian cities under NDMA’s national guidelines.
Who are the most vulnerable to heat waves in India?
Outdoor workers, the elderly with cardiovascular disease, pregnant women, infants, slum dwellers and the homeless are the highest-risk groups. Informal workers — construction labour, street vendors, agricultural hands — bear the largest burden because they cannot stop work during peak hours and rarely have access to cooled shelter.
How does climate change affect heat waves in India?
Climate change has roughly doubled India’s heat-wave frequency since the 1980s and made events such as the 2022 spell about 30 times more likely, according to World Weather Attribution analyses. The IPCC AR6 projects that what is today a 1-in-50-year heat event in India will recur every 4–5 years under 2°C warming, with sharper rises in humid coastal belts.
What mitigation steps reduce heat-wave mortality?
The proven measures are cool-roof coatings on low-income housing, urban tree planting and water-body restoration, ECBC-compliant passive building design, shifted MGNREGA and construction work hours, ORS and shade points at transit nodes, dedicated heat-stroke wards in hospitals, and parametric heat-insurance for informal workers. Long-term, deep emission cuts to limit global warming are essential.
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