Introduction

• A heatwave is a period of abnormally hot weather, usually over several days, that is significantly hotter than what is normal for a place at that time of year. The World Meteorological Organization describes it as a period where local excess heat accumulates over a sequence of unusually hot days and nights.
• In India, heatwaves are treated as a major weather hazard because they can affect health, agriculture, water availability, labour productivity, and power demand.

Nature of a heatwave

• A heatwave is not just “hot weather.” It is an anomalous episode of very high temperature lasting for several days and capable of causing serious impacts. IMD’s monograph describes heatwaves as anomalous episodes with extremely high surface air temperatures lasting for several days with serious consequences.
• Heatwaves often become more dangerous when hot days are combined with hot nights, because the body gets less time to recover. WMO’s description specifically refers to unusually hot days and nights.

Why heatwaves are dangerous

• Heatwaves can cause:
  • heat exhaustion
  • heatstroke
  • dehydration
  • kidney stress
  • worsening of heart, lung, and metabolic diseases.
• WHO notes that extreme heat can worsen chronic conditions and can increase the risk of acute kidney injury, while heat stress also puts strain on the heart and kidneys.

People most at risk

• Heatwaves affect everyone, but some groups are more vulnerable, especially:
  • older persons
  • children
  • people with chronic illnesses
  • outdoor workers
  • low-income populations
  • urban residents exposed to heat islands.
• WHO states that older people, children, and those with chronic diseases are at greater risk of complications and death during heatwaves.

Heatwave definition in India

• In India, the India Meteorological Department (IMD) issues official heatwave warnings and guidance.
• IMD’s heatwave framework is based on temperature thresholds and departures from normal, and it distinguishes between heatwave and severe heatwave conditions in different regions. The precise operational definitions are part of IMD’s heatwave guidance and forecasting system.

Heatwave and climate change

• Heatwaves are becoming more important globally because rising temperatures are increasing the frequency and intensity of extreme heat events. WMO states that climate change has increased mortality and morbidity linked to heatwaves, and WHO–WMO guidance in 2025 also noted sharply rising frequency and intensity of extreme heat events.
• This means heatwaves are no longer only occasional weather events; they are increasingly part of the broader climate-risk landscape.

Urban heat island effect

• Heatwaves can be especially dangerous in cities because built-up areas often retain more heat than surrounding rural areas. WHO notes that the effects of heat may be worsened in cities because of the urban heat island effect.
• This is why dense urban settlements often face higher night-time heat stress during prolonged hot spells.

Economic and social impact

• Heatwaves can reduce worker productivity, increase electricity demand, strain health systems, and damage agriculture. WMO notes risks such as mortality, drought and water-quality stress, wildfire risk, power shortages, and agricultural losses.
• WHO–WMO worker guidance from 2025 added that worker productivity drops as temperatures rise and that heat stress has both health and economic consequences.

Heatwaves in India

• India regularly experiences heatwave conditions, especially in northwest, central, and parts of east peninsular India during the hot-weather season. IMD’s operational bulletins and guidance pages continue to issue region-specific heatwave forecasts each year.
• For example, IMD’s 20 April 2026 guidance warned of likely heatwave conditions over isolated pockets of northwest, central, and adjoining east India in the following days.

Heatwave warning and early warning systems

• Heatwaves are now treated as a major hazard in modern early warning systems. WMO lists heatwaves among major extreme-weather hazards addressed through early warning systems.
• In India, IMD provides heatwave guidance and heat-and-hot-humid weather warnings as part of its regular forecast system.

Health response and prevention

• The most basic preventive steps include:
  • staying hydrated
  • reducing exposure during peak daytime heat
  • using shade and ventilation
  • checking official warnings
  • protecting vulnerable persons.
• WHO emphasizes paying attention to official alerts because serious effects can appear suddenly and heat reactions depend on individual ability to adapt.

Heatwave naming

• Unlike cyclones, there is no agreed international system for naming heatwaves. WMO has stated that there is currently no agreed international system or protocol for naming or coordinating the naming of heatwave events.
• WMO has also warned that naming heatwaves can distract attention from the more important issues of risk communication and public response.

Key points to remember

• A heatwave is a period of abnormally hot weather lasting several days.
• It becomes especially dangerous when there are hot nights as well as hot days.
• Main impacts include heatstroke, dehydration, kidney stress, and worsening of chronic disease.
• In India, official heatwave warnings are issued by the IMD.
• Climate change is increasing the seriousness of heatwaves globally.

Conclusion

• Heatwaves are among the most important extreme-weather hazards today because they directly threaten human health, livelihoods, and economic activity. In India and globally, they are becoming more significant due to rising temperatures, urban heat stress, and increasing exposure of vulnerable populations.