An unmanned weather station on the outskirts of Delhi recorded a blazing all-time high of 52.9C but experts say the reading can’t be counted for the record-books.

The temperature was recorded in Delhi’s Mungeshpur automatic weather station Wednesday and it could be the highest-ever temperature the country has ever witnessed. But the reading was an “outlier compared to other stations” and could be “due to an error in the sensor,” said the Indian Meteorological Department.

The highest-ever confirmed temperature in India was 51C, recorded in Phalodi in Jodhpur on the edge of Rajsthan’s Thar Desert in May 2016. Mercury levels above 50C are usually own seen in western parts of Rajasthan in India.

The weather office said it’s investigating whether the temperature sensor was working correctly. "It’s not official yet,” said Union Minister of Earth Sciences Kiren Rijiju, calling the 52.9C reading “very unlikely.” In any event, the reading at Delhi’s primary weather station Safdarjung was 46.8 degrees, the highest level in 79 years. That was 6C above normal and the continuation of a heatwave that has raged through May, which is normally Delhi’s hottest month as it is in the rest of India.

Temperatures in Delhi have crossed 45C for the last three days and have been over 40C for a lengthy spell of 15 days. Brief rain in Delhi on Wednesday evening cooled temperatures slightly but the heatwave conditions are expected to last at least a couple more days.

“This time is that it has been a very prolonged heat spell. There will be no respite likely for the next 48 hours,” says Air Vice Marshal (retd) Gurprasad Sharma, president of leading private forecaster Skymet Weather.

“This is a prolonged spell and covering a large area Rajasthan, Punjab, Haryana. Delhi, West Uttar Pradesh and parts of Madhya Pradesh also,” adds Sharma. Night-time temperatures, too, have remained stubbornly high, hovering around 30C. The IMD has issued a "red alert”, telling people to hydrate and warning of a “very high likelihood of developing heat illness and heat stroke in (people) of all ages.

Sharma points out that “The Club of 50”, as he calls the places where the temperature has breached 50C, has expanded enormously this year.

“The new stations with readings in excess of 50C include Churu and Phalodi in Rajasthan and Sirsa in Haryana,” he says, adding others which could be on the verge of joining the Club of 50 include Ganganagar (49.4C) and Barmer (49.3C).

“Normally you don’t have temperatures crossing 50C in more than one place,” says Sharma. Pakistan, which is often hotter than India, has seen temperatures soaring past 50C in several places. An intense heatwave in Jacobabad and Nawabshah, which are both in the Sindh region, has sent thermometers climbing to over 52C. In other places like Rohri, Khanpur and Hyderabad, also in Sindh, temperatures have crossed 50C.

“Temperatures in Pakistan have even crossed 52C for days together in the recent past,” says Sharma. The highest temperature recorded in Pakistan was 54C in the city of Turbat in southwestern Balochistan in 2017, according to the Pakistan Meteorological Department.

The Pakistan heatwave has sent hot westerly winds gusting over the border into India. Sharma points out that occasional heatwaves or rainy spells cannot be attributed to climate change. But he adds that climate patterns are altering. Last year was second warmest year for India since 1901. Causing concern is the fact the five warmest years in recent Indian weather history were recorded within the last 14 years, suggesting a rapid increase in surface temperature and change in weather pattern.

“What indicates climate change is not an individual event, but the pattern,” he said. “This year it was a very comfortable April. In north india there was not a single day over 40C. Then you have a hot May. “What climate change does is what we saw. April nothing happens. May has the longest spell of frequent heat.”

Similarly, south India was very hot earlier this year while it was relatively cool in the north. In Kerala and in Telangana, temperatures were very hot and the heatwave stretched all the way to Indonesia. Now, there has been rain which has brought temperatures down quite sharply.