A heatwave refers to a prolonged period of abnormally hot weather, usually lasting for two or more days. However, the precise definition of a heatwave is not universally agreed upon. During a heatwave, temperatures rise to levels that are significantly higher than the average temperatures for a particular region or time of year. According to the World Bank, India could soon experience heatwaves that break the human survivability limit.

The World Meteorological Organisation (WMO) has defined a heatwave as a period of at least five consecutive days during which the daily maximum temperature exceeds the average maximum temperature by at least five degrees Celsius.

The US National Weather Service (NWS) defines a heatwave as a spell of "abnormally and uncomfortably hot and unusually humid weather" spanning two days or more.

The India Meteorological Department (IMD) defines it as a period of time when the temperature rises to 5-6 degrees Celsius or above the normal temperature for a particular region.

What causes a heatwave? Heatwaves are typically caused by a buildup of hot air that becomes trapped in a specific area. This phenomenon happens when a high atmospheric pressure system moves into a region and lingers for at least two consecutive days.

Heatwaves form when high pressure aloft, typically between 3,000-7,600 meters in altitude, strengthens and persists over a region for several days or even up to several weeks.

High-pressure systems cause air to move in a downward direction, which creates a force that inhibits the upward movement of air near the earth's surface. As a result, the descending air forms a sort of "cap" that confines warm ground air within the system. The increased concentration of pressure in the area makes it challenging for other weather patterns to enter, leading to the persistence of a heatwave for several days or even weeks.

In addition to the inhibiting wind, high-pressure systems also have the effect of barring clouds from entering the region. This combined effect leads to a sustained accumulation of heat at the earth's surface, resulting in a heat wave that people experience.

Notable episodes of modern heatwaves: The Canada and US heatwave of 2021 resulted in the deaths of approximately 1,000 people. In Europe in 2003, a heatwave claimed the lives of over 30,000 people. Russia experienced a deadly heatwave in 2010 that caused the deaths of 55,000 people. The US heatwave in 1988 claimed over 4,000 lives, and in India in 2015, a heatwave resulted in the deaths of 2,500 people.