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Explained: Why IMD has predicted a cold wave in Northwest India

The India Meteorological Department has predicted a cold wave in parts of Northwest India. What is a cold wave, and why will parts of India witness one now?

Temperature dips on a winter morning in New Delhi on Friday. (Express Photo: Praveen Khanna)

The India Meteorological Department (IMD) has predicted a cold wave in parts of Punjab, Haryana, Chandigarh, Gujarat, Rajasthan and Uttar Pradesh over the next few days.

What is the cause of this cold wave?

According to IMD, Punjab, Haryana, Chandigarh, Saurashtra and Kutch could witness a cold wave or a severe cold wave from December 17 to 21. A cold wave is likely over north Rajasthan from December 18 to 21, while one is predicted for west Uttar Pradesh from December 19 to 21, and Gujarat from December 19 to 20. Dense morning fog is likely over isolated parts of Punjab and Haryana on December 17 and 18. Ground frost is also on the forecast for parts of Punjab, Haryana and Gujarat over the next four days.

A western disturbance as a cyclonic circulation lies over north Pakistan and adjoining Jammu and Kashmir, according to an IMD bulletin on Thursday. Western disturbances are storms that originate in the Mediterranean region and bring winter rainfall to northwest India. “After this western disturbance moves, there is a brief gap. The next western disturbance will set in only on December 22 night,” said R. K. Jenamani, senior scientist, IMD. In this gap of five to six days, the IMD expects strong northwesterly and westerly cold winds over north India. “We did not have this wind pattern earlier. Winds were calm and light all through November. We need consistent winds to bring a fall in temperature,” he said.

DELHI, delhi news, delhi covid news, dehi live updates, delhi news today A cold winter day in New Delhi. (Express Photo: Praveen Khanna)

What is a cold wave?

The minimum temperature is likely to fall by 2 to 4 degree Celsius in these places over the next few days, going by the forecast.

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The IMD records a cold wave when the minimum temperature is equal to or less than 10 degree Celsius at a weather station in the plains, and is 4.5 degrees to 6.4 degrees below the normal temperature for that period. A cold wave may also be recorded at a station in the plains when the minimum temperature is less than or equal to 4 degree Celsius.

For hilly regions, a cold wave is declared when the minimum temperature is less than or equal to 0 degree Celsius and the minimum temperature is 4.5 degrees to 6.4 degrees below the normal. The ‘normal’ temperature is calculated for every five days by taking the average temperature for these days over the past 30 years.

The IMD defines a cold wave qualitatively as “a condition of air temperature which becomes fatal to the human body when exposed.” The IMD also has an ‘impact matrix’ for cold waves – when cold wave conditions persist in isolated areas for more than two days, the impact matrix indicates that the cold is tolerable but “a mild health concern for vulnerable people (infants, pregnant women, elderly, people with chronic diseases etc.” It suggests avoiding prolonged exposure to the cold, and covering the head, neck, hands and toes as “majority of heat loss occurs through these body parts.”

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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 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.

A cyclist walks amidst morning smog in New Delhi. (AP Photo: Manish Swarup)

Why do cold waves occur?

Cold waves usually occur from mid-December to the end of February. Sometimes, a cold wave may set in before mid-December, said Jenamani. He said that the cold waves depend on weather systems and wind patterns from the middle latitudes, that is from Europe or West Asia, since the winds from these regions bring cold weather.

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. They can be triggered by strong westerly winds approaching northwest India and transporting cold air towards the southeast direction. Build up of an extended area of relatively high pressure over northwest Asia can also bring cold waves.

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“Like heat waves, cold waves are hazardous to those who are exposed to the cold,” Jenamani said.

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First published on: 17-12-2021 at 11:26:10 am
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