Explainer: What is a cold wave & what triggers them in north-west India?

North-west India is bracing itself for a possible cold wave over the coming weekend, as predicted by the India Meteorological Department (IMD). “Cold wave in isolated pockets very likely over Punjab till December 20 and over Haryana, Chandigarh and north Rajasthan from December 18 to 20,” said the IMD on its Twitter handle.

The first cold wave this season

The IMD forecast said that the minimum temperature is likely to dip by 2-3 degrees Celsius over most parts of north-west and adjoining central India, and Gujarat, over the next two to three days. Punjab, Haryana, Chandigarh, Saurashtra and Kutch could witness a cold wave or severe cold wave from December 17 to 21.

A cold wave has been predicted over north Rajasthan from December 18 to 21, the IMD forecast read, while similar conditions are predicted for west Uttar Pradesh from December 19 to 21, and Gujarat from December 19 to 20.

This is the first cold wave predicted for this season. India witnessed 99 cold wave days in 2020, which was however lower than 2019, according to Statista report. Cold waves first hit Punjab and Haryana before Delhi, since they come from the western direction.

Cold wave alerts have been issued by the IMD only since the winter of 2016. But IMD has mentioned the occurrence of cold waves and the deaths due to such events in its annual climate summaries since 2009.

Also read: North India braces for cold wave as temperatures go south

What triggers a cold wave?

On December 15, the IMD spotted a western disturbance as a cyclonic circulation over north Pakistan and in Jammu and Kashmir. Western disturbances are storms that originate in the Mediterranean region and bring winter rainfall to north-west India. But after this western disturbance moves away, there is a brief gap and the next western disturbance will set in only on December 22 night, according to media reports quoting IMD.

It is in this gap of five to six days, the IMD expects strong northwesterly and westerly cold winds from colder regions of Central Asia/Hindukush region lead to a dip in temperature over north India.

These cold waves are set off by weather systems from the middle latitude areas such as Europe or west Asia that bring cold winds to north-west India, said reports.

According to the IMD, the factors that bring cold waves to India include the movement of cold air masses brought about by upper-level winds. Strong westerly winds approaching northwest India can transport cold air towards the southeast direction. Consistent winds are required to trigger a fall in temperature.

Build up of an extended area of relatively high pressure over north-west Asia can also bring cold waves. Further, the IMD website stated that whenever a western disurbance affects north India, winds in lower levels over the region are either from Arabian Sea or from both Bay of Bengal and Arabian Sea. Given that both types of winds are the moist ones, minimum temperatures rise. At the same time, clouding over the region leads to lesser penetration of solar insolation into the earth and hence fall in maximum temperatures.

Formation of an anti-cyclone in lower and mid-tropospheric levels can also be a driver of cold waves. Such an anti-cyclone gives rise to sinking motion over the Indo-Gangetic plain leading to fall in minimum temperatures.

Also read: Himachal receives fresh snowfall as cold wave intensifies

What caused the cold wave last winter?

In January 2021, due to the La Nina condition which is a weather phenomenon caused due to the cooling of the Pacific Ocean combined with western disturbances, brought in rains and snowfall resulting in a sharp drop in temperature and long spells of cold in northern and central India, including Delhi.

So, what is a cold wave?

Cold waves quite commonly occur from mid-December to the end of February. A cold wave is described as one when the minimum temperature is likely to fall by 2 to 4 degrees Celsius.

According to the IMD, cold wave conditions occur when the mercury falls below 10 degrees Celsius in the plains, and the minimum temperature stays at least 4.5 degrees to 6.4 degrees Celsius below the normal level.  A cold wave may also be recorded at a station in the plains when the minimum temperature is less than or equal to 4 degrees Celsius.

A cold wave is described qualitatively by the IMD as “a condition of air temperature which becomes fatal to the human body when exposed.”

IMD’s impact matrix for cold waves

IMD also uses an ‘impact matrix’ for cold waves and provides suggestions on how to stay safe depending on the severity of the cold wave. For example, when the bitter cold wave conditions continues to prevail in isolated areas for more than two days, the impact matrix shows that though the cold is tolerable and it raises a flag of a “mild health concern for vulnerable people” (infants, pregnant women, elderly, people with chronic diseases etc). At this time, people should avoid prolonged exposure to the cold, and cover up their head, neck, hands and toes since it is largely through these body parts that a majority of heat loss happens.

In ‘severe’ cold wave conditions, where the minimum temperature is less than or equal to 10 degree Celsius, and departs from the normal by 6.5 degrees Celsius or more, or if cold wave conditions persist for four days or more, the IMD’s impact matrix indicates an increased likelihood of illnesses like flu, due to prolonged exposure to the cold.

The fallouts of a cold wave

In 2018, a cold wave in January in north India had claimed 100 lives. Till 2018, cold waves had killed more than 2,000 people, mostly in the severe winter months of December and January, said reports. The most severe of these cold waves were observed in 2011 and 2010 when 650 and 600 had died in the northern plains region. Uttar Pradesh is the most affected state when it comes to cold waves.

Besides harming crops, trends show that the colder and drier it gets, the threat of H1N1 influenza virus spreading is even more, said media reports.

National Disaster Management Authority’s list of precautionary measures during a cold wave

Have adequate winter clothing

Stay indoors as much as possible

Prefer mittens over gloves

Drink hot drinks regularly

Store adequate water as pipes may freeze

Have emergency supplies ready

Check weather reports regularly