The base observatory at Safdarjung recorded a minimum temperature of 10.5°C today, about 3.6°C above the normal. The lowest minimum so far during the week was 7°C as recorded on 03rd January, close to normal. The mean minimum temperature during the first week at the record observatory Safdarjung was 8.5°C, above the average by 1.5°C. Similar observations were recorded at the two main other observatories, Palam and Lodhi Road. The Palam minimum temperature is generally less than the other two observatories, which mostly have similar records with minor variations.
As per the pentad normal, Safdarjung has an average of 7°C in the first week. The minimum temperatures mostly go about 8°-9°C during last days of the month. In Jan 2023, the minimum temperatures had dropped to under 5°C during the first week and the lowest was 2.2°C on 07thJan 2023. Last year also, the mercury levels had plunged below normal on several occasions.
Normally, the drop in temperature in the plains follows snowfall in the hills. Cold wave conditions intensify due to the cold snap of frigid winds blowing down the slopes of mountains. There have been two rounds of moderate to heavy snowfall across the mountains. More so, the state of Himachal Pradesh has been battered more than the other two, Jammu & Kashmir and Uttarakhand. Then, why are the temperatures warmer than normal for the capital city?
The only plausible cause which could explain this anomaly is ‘dense fog’. Dense to very dense fog has prevailed on a regular basis during the first week. The depth of the layer of fog and its duration restricts the drop in night temperatures. The other day, dense to very dense fog persisted for nearly 10 hours, at a stretch troubling rail, road and air traffic. More than the fog, there have been frequent ‘sky obscured’ conditions responsible for these warmer temperatures. During sky-obscured conditions, the surface visibility may not appear as bad but the fog particles get suspended in the air, a little above the surface but within the friction layer. Favourable light wind conditions further extend the duration of this dome. Significant fall in temperature invariably happens between midnight and wee hours. The sky-obscured conditions restrict the escape of radiation, which otherwise is the main cause of cooling.
Most places in the plains, including Delhi, today also experience sky-obscured conditions. Such conditions remain conducive to cold day conditions and impede the cold wave, which is yet awaited during January. All the seven days of this month so far, have recorded below-normal day temperatures, both at Safdarjung and Palam. Today there will also be a repeat of the same. The minimum temperatures of Delhi are likely to drop for the next three days and the lowest may plunge to 6°C. However, the approach of a western disturbance, coupled with induced circulation over plains, may trigger a change in the wind pattern. This will lead to a spell of rain and thundershowers over North India between the 10th and 12th Jan. These conditions will suppress any sharp sinking of temperatures during the second week, as well, more so over the weekend.