Delhi mercury below 5°C in earliest dip since 1987

Delhi's Safdarjung observatory recorded a minimum temperature of 4.9°C, marking the earliest cold wave day since 1987, with more cold expected.

Cold winds and clear skies brought the minimum temperature at Delhi’s Safdarjung observatory to 4.9°C early in the morning on Wednesday, 4.7°C below normal, the earliest the weather station (the oldest in the city) has recorded a minimum below 5°C since 1987 and as early as it did in 1996. India Meteorological Department (IMD) officials said the last time the station recorded a minimum temperature below 5°C earlier than this was on December 6 in 1987, when it was 4.1°C.

Delhi mercury below 5°C in earliest dip since 1987

The minimum temperature recorded at the Safdarjung station on Wednesday also qualified it as the first cold wave day of the season. IMD considers a departure of 4.5°C or more below normal as a cold wave if the recorded minimum is 10°C or less. The weather department has issued a yellow alert for the city, forecasting cold wave conditions to persist on Thursday and Friday, with the minimum likely to dip further and hover around 4°C.

But Wednesday was not the earliest cold wave day recorded at the Safdarjung station. In 2020, a cold wave day was recorded on November 15. In 2014, one was recorded at the station on November 14.

Meteorologists attributed the cold wave conditions to clear skies in Delhi and cold winds blowing from snow-clad mountains in the north. Western disturbances – storms from the west of India, usually originating in the Mediterranean region -- led to snowfall in the mountains earlier in the month and kept the capital somewhat cloudy. Since clouds prevent heat accumulated during the day from radiating back, minimum temperatures were higher than normal earlier in the month. After the storm passed, clouds disappeared in Delhi, causing the minimum to drop sharply.

Not all parts of the capital experienced a cold wave or a minimum temperature below 5°C on December 11. Data on minimum temperature for the day was available from 12 of 15 stations in the city. Only three stations – those at Ayanagar, Lodhi Road, and Safdarjung -- recorded a minimum of 5°C or less. Similarly, only four stations – those at Ayanagar, Narela, Najafgarh, and Safdarjung – met cold wave conditions.

How cold was the capital on average? IMD does not publish an average minimum for Delhi, but its gridded data can be used to answer this question approximately. Delhi’s average minimum on December 11 was 6.72°C on December 11 according to the gridded data, which covers an area slightly bigger than Delhi’s political boundaries. This made December 11 this year the coldest since 2010 by minimum temperature, when a minimum of 6.52°C was recorded on the day; the 17th coldest since 1951, the earliest year for which gridded data on temperature is available; and 2.11°C colder than normal.

Does this mean that this is a colder than normal December in Delhi? Not so far by minimum temperatures. According to the gridded data, the average minimum in Delhi up to December 11 was 9.79°C, 1.09°C warmer than normal and the 15th warmest since 1951. This is because the month started warmer than normal and has cooled down below normal only in the past two days.

To be sure, maximum temperatures – which represent day time temperatures -- are indeed trending cooler than normal this December in Delhi. The average maximum for the month up to December 10 (gridded data for December 11 will be available only on December 12) in the gridded data is 23.9°C, 1.2°C cooler than normal and the 14th coolest since 1951. This trend may carry forward when gridded data for December 11 becomes available. The Safdarjung station recorded a maximum of 23.0°C on the Wednesday, 1.8°C cooler than the normal for the station.

Mahesh Palawat, vice president at Skymet, a private weather forecasting service, said snowfall was recorded during the western disturbance earlier in the month, the impact of which is slowly being felt across the plains. “Forecasts were that cold wave conditions would hit parts of northwest India from December 11 onwards. Delhi-NCR in particular has been recording consistent winds of 10-15 km/hr, with clear skies also leading to a drop in temperature,” he said, adding winds will continue to remain strong, leading to a further dip in temperature over the next two to three days.

“We are not likely to see another western disturbance in the next few days, which generally slows down wind speeds and brings cloudiness. This means wind speeds will remain consistent (10-20 km/hr) and remain northwesterly during the daytime, causing minimum temperature to hover between 4-6°C in Delhi till the weekend,” said an IMD official, adding clear skies, particularly at night-time were also leading to a sharp dip in temperature. “When skies are clear, surface heating during the day is lost equally quickly at night-time.”