Heat Waves in India — UPSC Environment Notes

Heat waves have become India’s deadliest climate hazard. 2022 was India’s hottest March since 1901, 2024 recorded 40+ consecutive days of heatwave in Delhi, and 2025’s pre-monsoon season broke records across Rajasthan, Haryana, and Bihar. Heat waves now kill more Indians annually than floods and cyclones combined — yet they remain a comparatively under-governed disaster. The Disaster Management Amendment Bill 2024 and the National Action Plan on Heat-Related Illnesses are changing that. For UPSC, heat waves now appear regularly in prelims current affairs and mains GS-III climate change questions.

What is a heat wave?

A heat wave is a period of abnormally high temperature, more than the normal maximum temperature that occurs during the summer season, typically March to June (occasionally July). The extreme temperatures cause physiological stress, and at scale result in excess mortality and morbidity.

IMD criteria

The India Meteorological Department (IMD) uses two pathways to declare a heat wave:

Threshold (must be crossed):

• Plains: ≥ 40°C.
• Coastal stations: ≥ 37°C.
• Hilly regions: ≥ 30°C.

Departure from normal:

• Heat Wave: departure of 4.5°C to 6.4°C.
• Severe Heat Wave: departure ≥ 6.5°C.

Based on absolute temperature (plains only):

• Heat Wave: maximum ≥ 45°C.
• Severe Heat Wave: maximum ≥ 47°C.

A heat wave is declared when these criteria are met at at least 2 stations in a meteorological sub-division for at least 2 consecutive days, announced on the second day.

Favourable conditions

• Prevalence of hot, dry air over a region with appropriate flow patterns.
• Absence of moisture in the upper atmosphere (moisture restricts temperature rise).
• Cloudless skies allowing maximum insolation.
• Large amplitude anti-cyclonic flow over the area.
• Winds generally westerly to north-westerly — heat waves develop over Northwest India and spread eastward and southward (rarely westward).

The heat dome phenomenon

A heat dome is created when high-pressure circulation traps hot air like a lid:

• Still, dry summer conditions build up a warm air mass.
• High pressure from above compresses the air, which heats further.
• As it tries to rise, it is pushed back down, compounding heating.
• Meteorologists call the stagnant pattern an Omega block (from the shape of the jet stream).

Heat domes are linked to climate change — warmer oceans and shifting jet streams make these patterns more frequent and persistent.

Wet bulb temperature

A crucial but under-appreciated concept: wet bulb temperature combines heat and humidity.

• A wet bulb of 35°C is the survivability threshold — the human body cannot cool itself by sweating.
• India's humid coastal and Gangetic plains are particularly vulnerable.
• 2023 Odisha Balasore and 2015 Andhra Pradesh-Telangana events featured lethal wet bulb combinations.

Health impacts

Vulnerable groups:

• Outdoor workers (construction, street vendors, farmers).
• Elderly, infants, pregnant women.
• Urban poor in slums with poor ventilation.
• Persons with pre-existing medical conditions.

Urban heat island (UHI)

Cities run 3-7°C hotter than their rural surroundings due to:

• Concrete and asphalt absorbing and re-emitting heat.
• Reduced green cover.
• Anthropogenic heat from AC, vehicles, industry.
• Reduced wind flow due to tall buildings.

Mumbai, Delhi, Ahmedabad, Nagpur, Chennai all experience severe UHI — making heat waves more lethal within cities.

India's policy response

National Action Plan on Heat-Related Illnesses

Issued by NCDC (Ministry of Health) in partnership with NDMA in 2021 (updated 2023-24):

• Surveillance of heat-related mortality and morbidity.
• Early warning linkage with IMD.
• Capacity building of health workers.
• Cool shelters, hydration stations, work-hour adjustments.

NDMA Heat Wave Guidelines

Original 2019, revised 2023. Prescribe Heat Action Plans (HAPs) at city and state levels.

Heat Action Plans (HAPs)

• Ahmedabad HAP (2013) — India's first, reportedly saved 1,100+ lives/year.
• Now 23+ states and 130+ cities have HAPs.
• Standard elements: early warning, public awareness, cooling centres, inter-departmental coordination, hospital preparedness.

Notification as a disaster

Heat waves are increasingly treated as "notified disasters" under the DM Act for the purposes of SDRF/NDRF relief — states like Odisha, Gujarat, AP, Telangana, UP have formally notified heat waves.

Latest developments (2024-26)

Updated context: figures evolve rapidly — cross-check with IMD, NDMA, and NCDC annual reports.

• 2024 summer — Delhi recorded 52.9°C at Mungeshpur (awaiting official confirmation); over 40,000 suspected heatstroke cases during April-June across India.
• DM Amendment Bill 2024 — new definitions of disaster risk and exposure will help formalise heat wave disaster status.
• Climate attribution studies — World Weather Attribution confirms the 2022 and 2024 heat waves were 30-45 times more likely due to climate change.
• COP29 (Baku, 2024) — India joined the Race to Resilience Cool Communities platform.
• Supreme Court observations (2024) — stressed right to life includes protection from extreme heat; states directed to update HAPs.
• NDMA partnership with NIUA and RMI — expanding UHI mitigation pilots.

Challenges

• No uniform national heat wave declaration criteria.
• Under-reporting of heat deaths — only ~12 states have heat mortality surveillance.
• Urban heat governance gap — cooling access is deeply unequal.
• Occupational safety for outdoor workers poorly regulated.
• Building design still ignores passive cooling principles.

UPSC relevance

GS-III mapping

• Disaster management.
• Climate change.
• Urban issues.

Prelims bullets

• IMD heat wave threshold for plains: 40°C; coastal: 37°C; hilly: 30°C.
• Severe heat wave: departure ≥ 6.5°C or maximum ≥ 47°C.
• Wet bulb survivability threshold: 35°C.
• First HAP: Ahmedabad, 2013.
• Heat dome / Omega block are meteorological phenomena.

Mains angles

• "Discuss the rising frequency of heat waves in India and the institutional response framework."
• "Examine the concept of Urban Heat Island and suggest mitigation strategies relevant to Indian cities."
• "Evaluate the effectiveness of Heat Action Plans across Indian states."

Essay hooks

• Heat as a social justice issue — unequal exposure and protection.
• Climate adaptation in a warming subtropical giant.
• The invisible disaster — governing slow-onset climate hazards.

Quick revision

Bottom line for UPSC: heat waves are India's climate frontier hazard — slow-onset, high-mortality, disproportionately affecting the poor. The combination of IMD early warning, state HAPs, NDMA guidelines, and the DM Amendment Bill 2024 offers a maturing architecture. Implementation at the last mile — wards, villages, worksites — is the work ahead.

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