A heat wave is a period of abnormally high temperatures, more than the normal maximum temperature, occurring during the summer season in the North-Western, Central, and South-Central regions of India. It constitutes a severe socio-economic and public health hazard under the Natural Hazards and Disaster Geography of India.
Scientific Criteria for Declaring a Heat Wave
The India Meteorological Department (IMD) classifies heat waves based on geographic terrain and deviation from normal temperatures or absolute temperature thresholds.
Based on Normal Temperature Deviation (For stations where normal maximum is more than 40°C)
- Heat Wave: Departure from normal temperature is 4.5°C to 6.4°C.
- Severe Heat Wave: Departure from normal temperature is greater than 6.4°C.
Based on Normal Temperature Deviation (For stations where normal maximum is less than or equal to 40°C)
- Heat Wave: Departure from normal temperature is 5°C to 6°C.
- Severe Heat Wave: Departure from normal temperature is greater than 6°C.
Based on Actual Maximum Temperature (Regardless of Normal Deviation)
- Heat Wave: When the actual maximum temperature reaches ≥ 45°C.
- Severe Heat Wave: When the actual maximum temperature reaches ≥ 47°C.
Regional Thresholds for Inclusions
- Plains: The maximum temperature must reach at least 40°C.
- Coastal Regions: The maximum temperature must reach at least 37°C.
- Hilly Regions: The maximum temperature must reach at least 30°C.
The Role of Humidity: Warm Night and Wet Bulb Criteria
High humidity amplifies heat stress. IMD also tracks Warm Nights (when nighttime minimum temperature stays 4.5°C or more above normal) and monitors Wet-Bulb Temperature, which combines dry air temperature with relative humidity to measure the human body’s capacity to cool itself through sweat.
Synoptic Mechanisms Driving Heat Waves in India
The formation of heat waves requires specific meteorological and atmospheric conditions working in tandem over the Indian subcontinent.
Subsidence of Air and High-Pressure Systems
Anomalous anti-cyclonic circulations over the upper atmosphere cause the air to sink (subside). As the air descends, it compresses and warms adiabatically, clearing the skies of cloud cover and allowing intense solar radiation to hit the ground.
Advection of Dry, Hot Air Masses
During April to June, northwesterly and westerly winds blow from the arid regions of Pakistan, Baluchistan, and the Thar Desert into Central and Eastern India. This process, known as thermal advection, transports high sensible heat across the plains.
El Niño and Atmospheric Teleconnections
El Niño conditions in the equatorial Pacific Ocean typically correlate with suppressed monsoon activity and altered atmospheric circulation patterns over India. This lengthens the pre-monsoon summer, exacerbating the frequency and intensity of heat wave spells.
Spatial Distribution and Hazard Zones in India
The core heat wave zone (CHZ) covers the north-western, central, eastern, and north-peninsular regions of the country.
High-Vulnerability Zones
- North-West India: Rajasthan, Punjab, Haryana, Delhi, and Chandigarh. This zone experiences intense continental heat.
- Central India: Madhya Pradesh, Chhattisgarh, and Vidarbha (Maharashtra).
- East Coast and Eastern Plains: Odisha, Jharkhand, West Bengal, and western parts of Andhra Pradesh. These areas face a deadly combination of high temperature and high humidity.
Rising Vulnerability in Non-Traditional Regions
Climate change anomalies have caused an expansion of the heat wave footprint into hilly regions like Himachal Pradesh, Uttarakhand, and Jammu & Kashmir, where ecosystems and populations are unaccustomed to extreme thermal stress.
Socio-Economic and Environmental Impacts
Public Health and Excess Mortality
Heat waves cause physiological stress, leading to heat exhaustion, heat cramps, and fatal heat strokes. Dehydration, cardiovascular complications, and renal failure spike during these periods. The vulnerable demographic includes outdoor laborers, children, and elderly citizens.
Economic Productivity and “Heat Stress” Loss
Agriculture suffers due to increased evapotranspiration rates, leading to soil moisture depletion and crop scorching. Dairy yields drop significantly. In urban centers, productivity drops as outdoor working hours decrease, an economic phenomenon termed “heat stress-induced labor capacity loss.”
Energy and Infrastructure Strain
The demand for space cooling creates severe peaks in electricity grids, causing transformers to fail and leading to localized blackouts. Thermal expansion also warps railway tracks and damages road surfaces.
Ecological Imbalances and Forest Fires
Prolonged heat waves dry out forest biomass, transforming dry deciduous forests in Central India and pine forests in the Himalayas into tinderboxes, resulting in widespread forest fires.
Institutional Framework and Management Strategies
India’s approach has transitioned from managing heat waves as a standard summer occurrence to treating them as a major natural disaster under the aegis of the National Disaster Management Authority (NDMA).
Heat Action Plans (HAPs)
Ahmedabad became the first city in South Asia to implement a comprehensive Heat Action Plan in 2013 following a severe heat wave in 2010. Today, multiple states and cities deploy tailored HAPs based on this model.
Key Components of a Heat Action Plan
| Pillar of Action | Implementation Mechanism | Target Outcome |
| Early Warning Systems | IMD issues 5-day color-coded alerts (Green, Yellow, Orange, Red). | Timely administrative mobilization. |
| Public Interventions | Adjusting school and outdoor work hours (NREGS), setting up Pyaas (drinking water kiosks). | Reduced direct exposure to peak noon heat. |
| Urban Infrastructure | Cool roofs initiative, painting roofs with reflective white paint, creating urban green pockets. | Mitigation of the Urban Heat Island (UHI) effect. |
| Medical Preparedness | Dedicated heat-stroke wards in hospitals, training medical staff, stocking ORS packets. | Reduction in heat-related mortality rates. |
Long-Term Structural Adaptation
- Nature-Based Solutions (NbS): Implementing urban forestry (e.g., Miyawaki forests) and restoring urban wetlands to lower ambient micro-climate temperatures.
- Building Bye-Laws: Mandatory incorporation of passive cooling designs, proper ventilation, and insulation materials in public and private housing projects.
Facts and Trivia for UPSC Prelims
- Urban Heat Island (UHI) Effect: Urban areas experience higher temperatures than surrounding rural areas due to the high concentration of concrete, asphalt, and anthropogenic heat emissions, which trap heat.
- Color-Coded Alerts by IMD: * Yellow Alert: Heat wave conditions at isolated pockets for two days;
- Orange Alert: Severe heat wave for two days or heat wave for four or more days;
- Red Alert: Severe heat wave for more than two days or heat wave conditions persisting for more than six days.
- Wet Bulb Temperature vs. Dry Bulb Temperature: Dry bulb temperature measures ambient air temperature. Wet bulb temperature reflects the lowest temperature achievable by evaporative cooling. A sustained wet-bulb temperature of 35°C is considered the absolute limit of human survivability, beyond which the body can no longer shed heat.
- The “Loo”: A striking feature of the Indian hot weather season, the Loo is a strong, dusty, gusty, hot, and dry summer wind that blows during the afternoons across the Indo-Gangetic Plains of North and Northwestern India. Direct exposure to this wind can cause fatal heat strokes.