Winter gradually setting in as mercury dip continues in Delhi

The winter chill in Delhi continued to pick up pace, as the Capital recorded its coldest day of winter for the second day in a row clocking a minimum temperature of 15.6 degrees Celsius (°C) on Friday. Forecasts by the India Meteorological Department (IMD) predicted the minimum temperature to drop to 14°C by Tuesday and 13°C by Wednesday in the next week.
However, the minimum temperature was still two degrees above the normal temperature for this time of the year. According to IMD data, the minimum temperature has remained above the normal temperature since October 17.
Moderate fog was witnessed across the city, with the Palam observatory reporting a minimum visibility of 500 metres at 6.30am. The lowest visibility fell to at Safdarjung was 400 metres, at 7 am. The lowest visibility in the city was 300 metres on Thursday and zero on Wednesday.
The smog over the past two days also kicked off, for the first time this season, the cyclical phenomenon known as “inversion” – where pollutants get trapped close to the surface due to low temperature.
Inversion takes place when the surface air is cooler than the air at higher altitudes – the inverse of what usually happens. For this, a combination of both calm winds and low temperature is needed, leading to the mixing height—an invisible layer where pollutants interact—dipping and coming closer to the surface. The lower the mixing height, lesser the space for pollutants to move freely and escape. This ends up creating “lid-like” layer over the city.
The minimum temperature has gradually been dipping these last three days, aided by the return of cold northwesterly winds. From Wednesday, Delhi has also been recording moderate to dense fog — further preventing sunlight from reaching the surface until post-noon. The minimum temperature also dropped to a season-low of 16.1°C on Thursday, and continued the trend on Friday.
IMD scientists attributed the main cause of the sudden onslaught of fog to the fresh snowfall reported in parts of the Himalayas on Tuesday, which, they said, carried cold moisture into the air across the country’s northern and northwestern plains. These powerful “transport winds,” blowing up to 100km/hr at an altitude of 12 kilometres over the past two days have fuelled fog formation across the Indo-Gangetic Plains, further cooling the air across the region.
An IMD official said: “This brought moisture and subsequent fog. We still have jet stream winds of over 100km/hr blowing at a height over 12 kilometres. These are cold winds from the mountains and thus, are leading to a drop in temperature. The wind direction is expected to remain the same over the weekend,” the official said.
Delhi’s winter has been slow to arrive this year, as temperatures for early November have stayed unusually warm. In the first fortnight, till November 14, the average minimum temperature has been 17.45°C, against the normal—or long period average—of 13°C. The average maximum temperature in the same period has been 32.3°C, against the normal of 28.4°C.
Higher-than-usual day and night temperatures were recorded all through October too, making it the warmest October since 1951, according to IMD. The average monthly maximum and minimum temperature this October was 35.1°C and 21.2°C, respectively. They were 36.2°C and 22.3°C, respectively, in 1951.
The maximum temperature, meanwhile, was 30.5°C, two degrees above the normal, as recorded at the Safdarjung observatory, which is considered representative of Delhi weather. It was 27.8°C a day earlier.
Other weather stations recorded lower minimum and maximum temperatures, with the lowest minimum of 11.1°C clocked at Ridge weather station, which was four degrees below the normal, followed by 14°C recorded at the Ayanagar weather station, which was around the normal. The lowest maximum was 27.7°C at Narela, followed by 28.2°C at Mayur Vihar.