The minimum temperature in Delhi dropped to 6.9 degrees Celsius on Sunday – the lowest in the month of November in 17 years, as a cold wave swept through the city, according to the India Meteorological Department, reported PTI.

The last time it was this cold in November was in 2003, when the city recorded a minimum of 6.1 degrees Celsius. With this, Delhi stumped its own record on Friday, when the Capital recorded a minimum of 7.5 degrees Celsius, the lowest in the month in 14 years. The all-time record for the lowest minimum temperature in November is 3.9 degrees Celsius, recorded on November 28, 1938

“The Safdarjung Observatory, which provides representative data for the city, recorded a minimum of 6.9 degrees Celsius,” said Kuldeep Srivastava, the head of the IMD’s regional forecasting centre. “The mercury dropped to 6.1 degrees Celsius at the Palam weather station.”

For the plains, the IMD declares a cold wave when the minimum temperature is 10 degrees Celsius or below and is 4.5 notch below the normal temperature for two consecutive days. However, for small areas such as Delhi, a cold wave can be declared if the criteria is fulfilled even for a day, Srivastava said.

He added that the icy cold winds blowing from snow-laden western Himalayas have led to a dip in the mercury. However, the minimum temperature would rise by two to three degrees Celsius in the next four to five days under the influence of a fresh Western Disturbance, the IMD official said.

But why is it so cold?

Winter in Delhi this year has broken several records. The month of October was the coldest in 58 years, with the mean minimum temperature settling at 17.2 degrees Celsius, the lowest since 1962, when it was 16.9 degrees Celsius.

Srivastava told the Hindustan Times that both maximum and minimum temperatures are likely to remain around two degrees to 2.5 degrees Celsius below normal this entire season.

“Also, since winter is setting in early with temperatures already low, the minimum is likely to fall to five to six degrees Celsius as early as December 10,” he added. To put things into perspective, this usually happens after December 20, he said.

Experts said the Capital is experiencing a harsher and a longer spell of winter this year because of a Pacific Ocean weather phenomenon known as La Nina, which leads to a cascading impact on the global weather in winter months.

During La Nina, temperatures in central Pacific Ocean drops below normal levels, triggering wind patterns that can influence weather in far away regions. “When global conditions like La Nina are prevailing, there is a tendency for regions under northwest India to get colder,” VK Soni of IMD’s environment monitoring research centre, told the newspaper.

Soni added that at present, the dip in mercury is accompanied by strong winds that help in blowing away pollutants. But after November 23, the wind speed is likely to reduce. This would lead to a deterioration in the air quality, he said.

Delhi’s air quality index at 12 am on Sunday was recorded at 264 in the “poor” category, data from the Central Pollution Control Board showed. According to SAFAR, which provides real time updates. the overall AQI was recorded at 259.

Local factors, too, have contributed to making days in Delhi colder. “Delhi has had a cloudless streak, with only one Western Disturbance so far bringing in any moisture,” Srivastava explained. “When there are no clouds, the ground cools faster and the minimum temperatures stay low.”