A heat wave is declared when the max temperature is over 40 degrees Celsius in plains and at least 4.5 notches above normal, according to the IMD.
On Monday, Delhi experienced a severe heat wave & saw its max temperature which was 40.1 degrees Celsius, which made it the hottest day in March in a total of 76 years, the India Meteorological Department said, on Monday.
Kuldeep Srivastava, who’s the head of the IMD's regional forecasting centre, said that the Safdarjung observatory, which provides the Capital’s representative, recorded a max temperature of 40.1 degrees Celsius, 8 notches more than normal. According to the news agency PTI, he said that "It is the hottest day in March since March 31, 1945, when the national capital recorded a maximum of 40.5 degrees Celsius."
Delhi’s minimum temperature was seen at 20.6 degrees Celsius, around three notches more than normal, as was shown in the data. According to the Met department, the weather stations located at Najafgarh, Narela, Pitampura & Pusa recorded, saw max of 41.8 degrees Celsius, 41.7 degrees Celsius, 41.6 degrees Celsius, & 41.5 degrees Celsius all respectively.
According to IMD - when the maximum temperature is more than 40 degrees Celsius in plains & at least 4.5 notches which is above normal, that is when a heat wave is declared. A severe heat wave is stated if departure from normal temperature above 6.5 degrees Celsius.