Watch | What is a heat wave?

A video on the yearly phenomenon that is one of the most dangerous of natural hazards

Several parts of north India have been reeling under an intense heat wave. Many districts in Rajasthan, Haryana, Uttar Pradesh, Madhya Pradesh have been posting temperatures over 45 degrees Celsius or five degrees above what is normal.

The maximum temperature in Delhi settled six degrees Celsius above the normal at 46, making May 26, 2020 the hottest day the national Capital has seen in May in 18 years.

The heat wave has been caused by the dry north-westerly winds that started blowing on May 21 causing a gradual rise in temperature.

Heat waves are among the most dangerous of natural hazards, the frequency and intensity of which will rise in the 21st century due to climate change.

The World Meteorological Organization defines it as five or more consecutive days during which the daily maximum temperature surpasses the average maximum temperature by 5 °C or more. 

But some countries have adopted their own standards, like India. A heat wave, according to the India Meteorological Department, requires that temperatures increase 5–6 °C or more above the normal temperature. 

Also read: Heatwave likely to continue during next 24 hours, says IMD

Heat waves may span several days to several weeks.  It may aggravate the health effects of heat-related stress, causing heat exhaustion, dehydration, and heatstroke.

But heat wave-related death tolls and destruction are not always immediately obvious. Heat waves are a significant cause of weather-related mortality.

According to the World Health Organisation, more than 1,66,000 people died due to heat waves from 1998-2017.

The Indian heat wave of 2015 killed more than 2,500 people. According to EM-DAT, the International Disaster Database, it was the fifth deadliest heat wave in world history.