Context: For the past few days, Rajasthan, Delhi, Uttar Pradesh, Madhya Pradesh, and Maharashtra have been experiencing severe to very severe heatwave conditions.

More about the news:

  • In its very first spell this summer, this heatwave has pushed day temperatures significantly above normal.
  • Recently Churu in Rajasthan reported 50 degrees temperature and was the highest temperature recorded in the world on that particular day.

Phenomena of Heat Wave


The heatwave is a period of abnormally high temperatures, more than the normal maximum temperature

Period of occurrence

Heatwaves typically occur between March to June, and in some rare cases even extend till July. 

Range of temperature

  • The heatwave is considered if the maximum temperature of a station reaches at least 40°C or more for Plains, 37°C or more for coastal stations and at least 30°C or more for Hilly regions. 
  • Following criteria are used to declare heatwave:

a) Based on Departure from Normal

Heat Wave: Departure from normal is 4.5°C to 6.4°C

Severe Heat Wave: Departure from normal is >6.4°C

b) Based on Actual Maximum Temperature (for plains only)

Heat Wave: When actual maximum temperature ≥ 45°C

Severe Heat Wave: When actual maximum temperature ≥47°C

  • To declare a heatwave, the above criteria should be met at least in 2 stations in a Meteorological subdivision for at least two consecutive days.

Mechanism of formation

  • Heatwaves can form in many ways. They often form when high pressure aloft strengthens and remains over a region for several days up to several weeks.
  • This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. 
  • Summer time weather patterns are generally slower to change than in winter. As a result, this mid-level high pressure also moves slowly.
  • Under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome capping the atmosphere. This cap helps to trap heat instead of allowing it to lift.
  • Without the lift, there is little or no convection and therefore little or no convective clouds (cumulus clouds) with minimal chance for rain. The lack of clouds means that an affected area is struck with strong sunlight.
  • The end result is a continual build-up of heat at the surface that we experience as a heatwave.


  • Health hazards - According to the National Health Service based in the UK, heatstroke and heat exhaustion are the two major risks posed by high-temperature conditions.
    • Continuous and constant exposure to high temperatures could result in nausea and heat cramps, resulting in a rapid rise of the body temperature.
    • Dehydration (absence of adequate water within the body) could also aid in heat exhaustion.
  • Adverse Effects On Mental Health - Research has revealed that exposure to high temperatures over a sustained period of time can have a negative impact on the psychology of a person.
    • It has also been observed that crime rates go up when the temperature rises.
    • Also, higher temperatures lead to lesser income as people are unable to devote sufficient time to work due to the heat-associated stress.
  • Infrastructural Damage - As heat causes the metal to expand, heat waves can lead to major infrastructural defects.
    • Power transformers can detonate causing fires.
    • Water lines can burst to cause the loss of water and water shortage.
    • Heatwaves can also induce the kinking or buckling of railroads.
    • Highways can melt or develop cracks in extreme heat. For example, two traffic lanes in Oklahoma City, US, had to be closed during the 2006 North American heat wave after they buckled under the heat.
  • Trigger Devastating Wildfires - When a heatwave is accompanied by an episode of drought that dries out the vegetation, it creates the ideal environment for the break-out of a wildfire or a bushfire.
  • Power Outages - The sudden spike in electricity consumption challenges the available electricity supplies of the area.

Occurrence of heatwaves in India

  • Past experiences
    • The longest recorded heatwave spell, in recent years, was between 18 – 31 May 2015. 
    • This spell had severely affected parts of West Bengal along with Odisha, Andhra Pradesh, and Telangana. A similar spell in 2014 was reported from June 2 – June 11.
    • The heat waves of 2015 and 2016 had killed 2,040 and 1,111 people across the country respectively. 
    • In nine out of the last 10 years (till 2017) India has suffered from heat waves that have killed close to 8,000 people. 
      • Andhra Pradesh and Telangana seem to be particularly vulnerable to deaths by heatwave conditions.
  • Number of days it usually lasts
    • Heatwave conditions occurring in May have been observed to last longer earlier, as the season reaches its peak this month. 
    • On the other hand, those reported in June often die down sooner, often due to the onset of Southwest monsoon over the location or in its neighborhood.
  • Core Heat Zone areas in India
    • Heatwaves are common over the Core Heatwave Zone (CHZ) 
      • Those areas are Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra, parts of Gangetic West Bengal, Coastal Andhra Pradesh and Telangana.
    • Several studies have indicated that CHZ experience more than six heatwave days per year during these four months from March to June. 
    • Also many places in the northwest and cities along the southeastern coast report eight heatwave days per season. 
    • On the other hand, the regions in the extreme north, northeast, and southwestern India are lesser prone to heatwaves.

About the current unusual heatwave

  • At unusual timing
    • Summer season reaches its peak by May 15 in India, when the day temperatures across north, west, and central India cross 40 degrees and hover close to 45 degrees then on. 
    • However this year, north India did not experience such peak temperatures till May 21.
  • Reasons behind unusual timings
    • Unusual presence of Western disturbances
      • It was mainly because of the continuous inflow of Western Disturbances that influenced the weather in the north till as late as April. 
      • Since last winter, there was frequent passing of Western Disturbances over the north, appearing after every five to seven days.
      • Usually, a significant influence of Western Disturbances is experienced during December to February. But this year, its influence persisted till early May.
      • When Western Disturbances interact with weather systems heading warm winds blowing in from the Bay of Bengal or the Arabian Sea, they cause snowfall or rainfall over the north. 
      • The recent Western Disturbances got support from easterly winds blowing over from the Bay of Bengal.
    • Role of Cyclone Amphan
      • It was a massive Super Storm covering 700 km.
      • The storm managed to drag maximum moisture from over the Bay of Bengal, the entire South Peninsula, parts of Central India, and to some extent, even from the Arabian Sea.
      • All the moisture that was otherwise built during the thunderstorm and rainfall, got gradually depleted from over vast areas as the storm advanced towards West Bengal and Bangladesh between May 16 and 20.
      • It has now triggered dry north-westerly winds to blow which is considered to be responsible for causing severe heatwaves over western India.

Measures to Deal with phenomena of heatwaves:

  • Sensitizing States to the need of preparing and implementing specific Heat Action Plans in line with NDMA’s national guidelines on Heat Wave.
  • Establish Early Warning System and Inter-Agency Coordination to alert residents on predicted high and extreme temperatures.
  • Capacity building/training program for health care professionals at the local level to recognize and respond to heat-related illnesses, particularly during extreme heat events.
  • Public Awareness and community outreach: Disseminating public awareness messages on how to protect against the extreme heat-wave through print, electronic and social media and Information, Education and Communication (IEC) materials.
  • Collaboration with non government and civil society in building temporary shelters, wherever necessary, improved water delivery systems in public areas and other innovative measures to tackle Heatwave conditions.
  • Knowledge of effective prevention and first-aid treatment, besides awareness of potential side-effects of prescription drugs during hot weather, is crucial for physicians and pharmacists.
  • Sharing experiences and best practices to help other stakeholders prepare and implement their Heat Action Plans.


Western Disturbances

  • A western disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent.
  • It is a non-monsoonal precipitation pattern driven by the westerlies
  • Extratropical storms are a global phenomenon with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere. 
  • In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas. 
  • They are more frequent and strong in the winter season.
  • They are important for the development of the rabi crop, which includes the locally important staple wheat.