A brutal heat wave swept swathes of northwest India on Friday, with the maximum temperature soaring to a scorching 47.4 degrees Celsius in southwest Delhi's Najafgarh, the highest in the country so far this season so far.

The mercury breached 45 degrees Celsius at 19 places in Rajasthan, 18 in Haryana, eight in Delhi and two in Punjab.

Conditions are expected to turn worse with the severe heat wave over the northwest Indian plains predicted to continue during the next five days.

Researchers at Climate Central, a US-based group of climate scientists, said 543 million (54.3 crore) people in India will experience at least one day of extreme heat during this period.

"Human-caused climate change has made this intense heat much more likely. The high overnight temperatures make this event particularly alarming," said Andrew Pershing, vice-president for science at Climate Central.

On Friday, maximum temperatures surged to 47.4 degrees Celsius in Najafgarh and 47.1 in Haryana's Sirsa.

According to India Meteorological Data (IMD) data till 7:30 pm, Najafgarh was the warmest place in the country.

Before this, Kalaikunda in the Gangetic West Bengal region had recorded a high of 47.2 degrees Celsius on April 30.

In Delhi, the mercury settled at 46.5 degrees Celsius in Mungeshpur, 46.2 degrees in Aya Nagar, 45.9 degrees in Pusa and Jafarpur, 45.8 degrees in Pitampura and 45.1 degrees in Palam.

Severe heatwave conditions are very likely in some parts of Rajasthan, Punjab, Haryana and Delhi during May 18-21, the IMD warned.

A fresh heatwave spell will commence over east and central India from Saturday.

The Met office issued a red alert for west Rajasthan, stressing the need for "extreme care for vulnerable people".

It issued an orange alert for Haryana, Punjab, east Rajasthan, Delhi, Uttar Pradesh, Bihar and Gujarat and emphasised "high health concern" for vulnerable people, including infants, the elderly and those with chronic diseases.

It warned of an increased likelihood of heat-related illnesses in people exposed to the sun for prolonged periods or engaging in heavy work.

The threshold for a heat wave is met when the maximum temperature of a weather station reaches at least 40 degrees Celsius in the plains, 37 degrees in the coastal areas, and 30 degrees in the hilly regions, and the departure from normal is at least 4.5 notches.

A severe heat wave is declared if the departure from normal exceeds 6.4 notches.

The Met office had earlier predicted a higher-than-normal number of heatwave days in the northern plains and central India in May.

Normally, the northern plains, central India and adjoining areas of peninsular India experience around three heatwave days in May.

April witnessed record-smashing maximum temperatures in east, northeast, and southern peninsular India, prompting health warnings from government agencies and a few states to suspend in-person classes in schools.

Several places recorded their highest-ever April day temperatures, with the mercury soaring to 47 degrees Celsius.

At least two people died in Kerala due to suspected heatstroke during this period.

According to IMD data, heatwave days in April were the highest in 15 years in Gangetic West Bengal and nine years in Odisha.

At 16 days, Odisha also experienced the longest heatwave spell in April since 2016.

On Wednesday, a group of leading climate scientists said similar heat waves could occur once every 30 years, and these have already become about 45 times more likely due to climate change.

The scientists from the World Weather Attribution group emphasised that heat waves intensified by climate change are making life much tougher for people living in poverty across Asia.

The IMD had earlier warned of extreme heat in India during the April-June period, coinciding with the seven-phase Lok Sabha elections that end on June 1.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.