Delhi’s winter on Friday entered its business end, suggesting it was time to pull out the thick quilts, dust off the mittens, and crank up the heater. The minimum temperature plunged below 5°C, chillier than Shimla, Dalhousie or Dharamshala, as icy winds wrapped the Capital in a frosty embrace.

The mercury dropped sharply to 4.9°C, four degrees below normal and nearly a degree-and-a-half lower than Thursday. This is only the second time the temperature has dropped below the 5°C threshold this early in the winter in at least a decade. In 2020, the minimum was 4.1°C on December 15. Friday was also Delhi’s coldest December day since New Year’s Eve on 2021, when the mercury was 3.8°C. The temperature did not dip below 5°C at all last December, plateauing at 6°C on December 26.

Friday’s temperatures bucked forecasts by the India Meteorological Department (IMD), which on Thursday said the day’s low would remain around 6°C.

The mercury has plunged sharply over the past week, dipping from 9.4°C on December 8 to 8.5°C the next day, then 8.3°C on December 10 and 6.5°C the day after that. It was 6.8°Con December 12 and 7.4°C on December 13.

IMD officials said the absence of an active western disturbance has allowed winds from the northwest an unfettered path into the city. Western disturbances bring in warmer winds from the southeast and southwest and briefly disrupt winter temperature patterns.

However, the maximum temperature jumped to 25.4°C, two degrees above normal and higher than 24.1°C on Friday, as warm winds gave Delhi a balmy December afternoon.

Clear skies have helped keep the nights cool and days warm, said IMD officials.

“Clear skies allow in ample sunlight during the day. However, in the absence of any clouds at night, the heat given out by the surface is lost fairly quickly and night-time temperature plummets,” said Kuldeep Srivastava, scientist at IMD.

To be sure, dipping day-time temperatures and, consequently, a narrower gap between the maximum and minimum temperature are a more accurate bellwether of the winter’s spread than the night-time value alone.

A shallow fog also draped the city on Friday morning, with the visibility dipping to 500m, according to IMD. The agency classifies a fog as shallow when the visibility is between 500m and 1,000m, as moderate fog when it is between 200m and 500m, and as dense when the visibility plunges below 200 metres.

According to the weather department, Delhi’s minimum temperature will pick up on the back of an approaching western disturbance.

Read Here | This winter will be warmer than usual: IMD forecast

“A fresh western disturbance is approaching Jammu and Kashmir and adjoining Himachal Pradesh. Though this will not directly impact Delhi, there will be a change in wind direction to south-westerly. Some cloudiness will also return, so the minimum should rise by a degree or two,” Srivastava said.

The cold winter usually gathers pace till end-December, before stabilising for large parts of January, which is the city’s coolest month. The chill then starts giving way marginally in February before the winter exits in March.

Mahesh Palawat, vice-president at Skymet, said Delhi was cooler than most of northwest India largely due to local factors.

“We saw clear skies and calm winds, both of which lead to heat being lost quickly at night,” Palawat said, adding that Delhi’s minimum will fall even further after Christmas.

Meanwhile, pollution levels remained largely unchanged, with the air quality index (AQI) remaining “very poor” for the 10th straight day, as calm surface winds and local pollutants kept the Capital breathing hazardous air.

Delhi on Friday clocked an average 24-hour AQI of 323, as against 326 a day ago and 378 the day before that. With this, the Capital’s average AQI so far this month has been 327.This is the highest average AQI for Delhi in the first half of December since 2020, when it was 335. Last year, the first 15 days of the month had an average AQI of 300 and in 2021, it was 322.

With this, Delhi enters its second phase of winter pollution – the final fortnight of the year.

Central Pollution Control Board (CPCB) data since 2018 shows that Delhi records two 15-day spells where the air quality typically deteriorates — in the first half of November and the second half of December, the latter being the second most polluted time of the year in the city based on the average AQI.