What Is a Heatwave and How to Prevent Heatstroke: An Explainer

As large parts of India face intense summer heat, the terms "heatwave" and "heatstroke" are widely used. This explainer sets out what a heatwave actually is and how to recognise and prevent heatstroke. Despite the name, a heatwave is not a wave or pulse of heat that travels through the air; the word probably comes from the way the heat feels as it spreads, surges and overwhelms before easing. Some meteorologists prefer the term "heat event."

A heatwave usually develops when several conditions combine. In April and May, the Sun is almost directly overhead over most of peninsular India, and the ground heats up for weeks. When a stable, high-pressure weather system settles over a region, air slowly sinks from higher in the atmosphere, compressing and warming. This sinking air suppresses cloud formation, so sunlight keeps beating down. At the same time, the region may receive hot, dry continental winds from India's interior rather than cooler sea breezes, and dry soil means more of the Sun's energy heats the air instead of evaporating water. When these conditions last for a few days, a heatwave can occur. Large undulations in the jet stream, called Rossby waves, can sometimes create stagnant high-pressure "heat domes" over an area.

Heatstroke is a serious medical condition that occurs when the body overheats and its cooling system fails. It is more severe than heat exhaustion, which usually involves heavy sweating and feeling faint. In heatstroke, the body's temperature regulation breaks down entirely, which can damage vital organs and can be fatal if not treated quickly. Warning signs include hot and dry skin, confusion, a rapid heartbeat, dizziness and nausea. These appear when the body can no longer cool itself effectively through sweating, leading to dehydration and a rising core temperature.

To prevent heat-related illness, the basic steps are to stay hydrated, wear light clothing and use sun protection. The Sun is strongest between 10 a.m. and 4 p.m., so outdoor activity is best planned for early mornings or evenings. Children are more vulnerable than adults because they have a larger skin surface area relative to body size, sweat less, and have a less efficient heat-loss system. If heatstroke is suspected, the person should be moved to a cool place, given cool water to drink if conscious, and helped to lower their body temperature, with medical help sought promptly. The India Meteorological Department (IMD) issues heatwave warnings to help the public take precautions.