The first cold wave of the season, sweeping swathes of north India and finally bringing in the much-awaited winter chill, is quite normal for this time of the year. In fact, it was a little late this time in coming after a comparatively warmer first half of December.
The biting cold is the result of dry northerly and north-westerly winds from the Himalayas, which are bringing in frigid conditions to the northwestern plains.
Though the nights have been getting colder steadily over the past few weeks, the days have been quite warm as the day temperatures have been above normal until Christmas, when the maximum temperature fell to 16.5 degrees Celsius and the minimum to 5.3 degrees in Delhi.
A day later, on December 26, the maximum temperature in Delhi was 15.6 degrees while the minimum was 5 degrees. Mungeshpur in north Delhi had a maximum of 11.4 degrees, Ayanagar 14.2 and Palam 12.6 degrees Celsius. Gurgaon recorded a maximum and minimum of 13.7 and 3.9 degrees and Noida 13.2 and 7.4. Elsewhere in north India, Fatehpur Shekhawati recorded a minimum of -1.5°c, Churu and Pilani froze at zero degrees, and Hisar, Narnaul and Alwar recorded 2 degrees.
An India Meteorological Department (IMD) official said similar conditions will prevail for the next couple of days after which the intensity of fog may decrease, leading to a marginal increase in day temperatures and abatement of the cold wave.
Cold wave and cold day
The chill is biting more because both cold wave and cold day conditions are prevailing at the same time, which in turn is being triggered by icy cold winds from the Himalayas and a dense layer of fog that is blocking sunshine, experts explained.
In the plains, the IMD declares a cold day when the maximum temperature of a region falls 4.5 °C to 6.4 °C below normal. A severe cold day is declared when the departure is more than 6.4 °C.
A cold wave is declared when the minimum temperature of a region falls 4.5 °C to 6.4 °C below normal while being below the 10 °C threshold.
“Northwest India is reeling under moderate to severe cold wave conditions. The cold has become more intense as the maximum temperatures have fallen substantially and are hovering around 4.5 to 5 °C below normal, resulting in cold day and cold wave conditions,” explained SN Mishra, an independent climate change and weather consultant.
Mishra explained that the chill factor gets more intense due to a reduction in the mean maximum temperature rather than a fall in the minimum temperature.
“Essentially, a reduction in the range of day temperature intensifies the cold,” Mishra, former director of Weather and Climate services in the Indian Air Force, said.
The present cold wave conditions are due to uninterrupted cold and dry north-westerlies sweeping northwest and central India. The icy cold winds coupled with scattered cloud cover during the day and clear skies at night are reducing the temperatures, Mishra said.
“In Kashmir, ‘Chillai-Kalan’ is a 40-day period of severe winter and ‘Chilla Jade Din Chalis’ (biting cold for 49 days) in northwest India, generally from the last part of December to the first week of February (December 20 to February 10),” Mishra said.
Last winter too, northwest India was reeling under cold conditions around the same time. “At least one cold wave is considered normal before December 31,” Mishra said.
Warm start to December
In fact, the first half of December this year was comparatively warmer as clear skies, coupled with a lack of western disturbances and the absence of rain, ensured day temperatures were above normal.
Mahesh Palwat, vice-president of meteorology and climate change at private weather forecaster Skymet, said the chill was delayed this year because of the absence of major western disturbances over the western Himalayas that would have led to heavy snowfall.
“Though western disturbances were present, they were feeble. Only light snowfall was seen until now and that is why the chill was largely absent.”
The weather in the northern plains in winter is generally influenced by climatic activity in the Himalayas. When it snows heavily, the icy winds bring down the mercury.
“So unless there is heavy snowfall in Gilgit-Balistan, Muzafarrabad, Ladakh, J&K, Himachal, Uttarakhand, the winter chill won’t be severe as the winds from the Himalayas won’t be cold enough to drop the mercury in the plains,” he said.
When there is a strong western disturbance in the hills, it induces a cyclonic circulation over the northern plains, which generates rainfall. Palawat said: “This winter, we have not seen a single rainfall activity over north-west India. So the humidity content was not enough for dense fog to persist throughout,” he said.
And when the day temperatures did drop, the fog was not consistent and occasional sunshine increased the mercury, he said.
Raghu Murtugudde, a professor at the department of atmospheric and oceanic science, University of Maryland, said a large cyclonic circulation moving slowly westward is pulling the winds more towards the southwest and cooling every state from Haryana to even Maharashtra.
“Once the cyclonic circulation moves into the Arabian Sea, we may revert to a warmer winter over the northwest. The truant western disturbances added to the warmer temperatures, which may return soon.”
The IMD had this month forecast a warmer winter over parts of the northwest, primarily due to subdued western disturbances.
Discover the latest Business News, Sensex, and Nifty updates. Obtain Personal Finance insights, tax queries, and expert opinions on Moneycontrol or download the Moneycontrol App to stay updated!
