Context
With temperatures steadily dropping in North India, the India Meteorological Department (IMD) has forecast ‘cold wave’ to severe cold wave conditions over Punjab, Haryana, western Uttar Pradesh, parts of Gujarat and north Rajasthan over the next five days.
What is cold wave?
- The IMD defines a cold wave qualitatively as “a condition of air temperature which becomes fatal to the human body when exposed.”
- Cold waves usually occur from mid-December to the end of February.
- Sometimes, a cold wave may set in before mid-December.
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It is suggested to avoid 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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Impact matrix of cold wave
- 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.”
Declaration of cold wave
- Plain regions
- In the plains, a cold wave is declared if the minimum temperature dips to 4 degrees Celsius.
- A cold wave is also declared when the minimum temperature is 10 degrees Celsius or lower and is 4.5 notches below normal.
- The ‘normal’ temperature is calculated for every five days by taking the average temperature for these days over the past 30 years.
- Hilly regions
- 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.
Severe cold wave:
- A severe cold wave is declared when the minimum temperature dips to two degrees Celsius or the departure from normal is more than 6.4 degrees Celsius.
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Factors responsible for occurrence of cold wave
- 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.
- Cold air mass: The factors that bring cold waves to India include the movement of cold air masses brought about by upper-level winds.
- Westerly wind: They can be triggered by strong westerly winds approaching northwest India and transporting cold air towards the southeast direction.
- High pressure area: Build up of an extended area of relatively high pressure over northwest Asia can also bring cold waves.
