Delhi's primary weather station, the Safdarjung Observatory, has not recorded any heat wave in the pre-monsoon season for the first time since 2014, officials said on Tuesday.

A few isolated areas, however, witnessed heatwave conditions for a brief period in April and May, they said.

May, historically the hottest month in Delhi with a mean maximum temperature of 39.5 degrees Celsius, has recorded below-normal temperatures and excess rain this time.

Meteorologists attributed the phenomenon to higher-than-usual western disturbances -- weather systems that originate in the Mediterranean region and bring unseasonal rainfall to northwest India -- this pre-monsoon season (March to May).

"Usually, five to six western disturbances are recorded in the northern plains in April and May. We saw 10 western disturbances, mostly strong ones, this time," said Kuldeep Srivastava, head of the Regional Forecasting Centre of the India Meteorological Department (IMD).

Delhi recorded maximum temperatures above the 40-degree mark for just nine days in May with heatwave conditions affecting some parts for two days.

According to IMD data, the Safdarjung Observatory has so far recorded 86.7 mm of rainfall in May. On average, the national capital logs 19.7 mm of rainfall in the whole month.

The city logged more than 20 mm of rainfall in April, the highest in the month since 2017, and heatwave conditions at isolated pockets.

Overall, Delhi has gauged 158 per cent higher rainfall -- 161.2 mm against a normal of 62.6 mm -- during the pre-monsoon period this year.

"The Safdarjung Observatory, which is representative of Delhi, has not recorded any heat wave in the pre-monsoon season this year. This has happened for the first time since 2014," Srivastava said.

The weather station recorded 13 heatwave days in the pre-monsoon season last year -- nine in April and four in May.

It saw just one heatwave day during this period in 2021, four in 2020 and one in 2019.

The threshold for a heat wave is met when the maximum temperature of a 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 degrees.

According to the IMD, a fresh western disturbance will bring gusty winds and rain to the plains, including Delhi, starting June 1.

The maximum temperature is predicted to remain below the 40-degree mark until June 5.

Earlier this month, the weather office predicted below-normal maximum temperatures and fewer heatwave days in northwest India in May.

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