December 30, 2019 6:19:12 pm
The India Meteorological Department (IMD) on Monday (December 30) confirmed that it is the coldest December day in Delhi since 1901, with the maximum temperature recorded at just 9.4 degrees Celsius.
The lowest December day (maximum) temperature at Delhi Safdarjung before this was 11.3 degrees Celsius, recorded on December 28, 1997, the IMD said.
How has December 2019 been different?
Every year, in the second half of December and the first half of January, temperatures routinely drop to 2-4°C at some point of the day in many places in the north and northwest India.
In December, the maximum daily temperature does not rise beyond 16-18°C in most of Punjab, Haryana, Himachal Pradesh and western UP. In Delhi and northern Rajasthan, daily maximum temperatures are usually not over 20-22°C for most of December.
However, this winter, in many parts of the region, maximum temperatures on some days have been nearly 10°C below normal.
In Delhi, the average maximum temperature for December was less than 20°C until December 27. This has happened only four times in the last 118 years, and the IMD has said this month would most likely become the second coldest December for Delhi since 1901.
Maximum temperatures had averaged 17.3°C in December 1901.
Delhi has been clocking consecutive “cold days” since December 14. In December 1997, consecutive “cold days” lasted for 13 days, out of a total of 17 such days during the month.
In technical terms, when is a “cold day” declared?
A cold-day condition is said to prevail when the maximum temperature during the day is at least 4.5°C below normal. If the maximum temperature is at least 6.5°C below normal, it is classified as a “severe” cold day.
Cold-day conditions prevailed in the north since December 15 and intensified after December 21. The most intense cold day so far — when maximum temperatures fell 7° to over 12°C below normal — was on December 25 over Punjab, Haryana, Chandigarh, Delhi, Uttar Pradesh, north Rajasthan and some isolated areas in Bihar and Madhya Pradesh.
So why is it so cold this year?
There is nothing unusual in the climatic conditions that influence temperatures in this region at this time of the year, scientists say.
The cold wave usually arrives from the west, through the Western Disturbance wind system. This system is also responsible for causing rain in northern and northwestern India, having picked up moisture on its way from the Mediterranean Sea.
The intensity of the cold also depends on the amount of snowfall that happens in Jammu and Kashmir, Ladakh, Himachal Pradesh, and nearby areas.
“All these factors have their annual variabilities. They combine in different ways to produce different kinds of winter conditions. If you look at the climatic conditions this year, no special set of circumstances is visible at the macro level that can be held responsible for causing extreme cold. That can imply that the extreme cold being witnessed is just one of the outlying cases of natural variability that we see from year to year,” a former IMD scientist told The Indian Express.
A cold-wave condition is currently prevailing in areas north of Jammu and Kashmir, in northern Afghanistan, Tajikistan, Uzbekistan, Turkmenistan, and parts of northern Iran.
In all these areas, average temperatures have been 1° to 5°C below normal over the last several days. This is adding to the chill bring brought about by the Western Disturbances.
North-central China and Mongolia are also experiencing similar conditions.
But there must be some specific explanation?
Well, some things must be noted.
* Low clouds: This extended cold spell has been triggered by Stratus clouds hanging low over a large geographical area stretching all the way from Pakistan to Bangladesh. The north-south width of the Stratus blanket is 500-800 km, affecting all of North India.
R K Jenamani, senior scientist at IMD’s National Weather Forecasting Centre (NWFC), New Delhi, said that formation of such clouds are unique over the Indo-Gangetic Plains, and that these clouds have been observed only since 1997.
” As these clouds are formed at a height of 300 metres to 400 metres from the surface, they largely block the day’s sunlight, resulting in cold days,” he said.
* Western Disturbances: Moderate to intense western disturbances, occurring frequently, have contributed to the severe cold over all of North India.
Also, the flow of northwesterly winds over Northwest India, at a low height, has increased the chill factor, making days much colder than normal during December.
Again, this December has witnessed haze, fog, and rainfall after the passing of each western disturbance, triggering cold weather conditions over large swathes of North India.
* Climate Change: This cannot be discounted. The unusually cold December this year could just be another instance of extreme climates becoming more frequent, a result of climate change.
Across the world, the frequency and intensity of both heatwaves and cold waves have increased over the last several years, and are predicted to increase further.
The same is the case with extreme rainfall and drought. This year, India experienced an unusually wet August and September. The amount of rainfall in September was a once-in-a-century event. Scientists say climate change is bringing in greater uncertainty in weather patterns, making them more difficult to predict.
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