A cold wave is declared when there’s a significant drop in minimum or night time temperature.
It should be based on the actual minimum temperature of a station. Cold Wave is considered when minimum temperature of a station is 100C or less for plains and 0°C or less for Hilly regions.
Based on Departure
Cold Wave: Negative Departure from normal is 4.5°C to 6.4°C
Severe Cold Wave: Negative Departure from normal is more than 6.4°C
Based on Actual Minimum Temperature (For plain stations only)
Cold Wave: When minimum temperature is ≤ 04°C
Severe Cold Wave: When minimum temperature is ≤ 02°C
Cold Wave conditions for coastal stations
When minimum temperature departure is -4.5°C or less over a station, “Cold Wave” may be described if the minimum temperature is 150C or less.
What are the reasons behind cold wave?
It prevails under the influence of Western Disturbance over Afghanistan and neighboring areas.
A reason behind this below-normal drop in temperatures is due to the climate pattern La Niña.
One of the main reasons is snowfall in high altitude areas north of Delhi, including places in Jammu and Kashmir, Himachal Pradesh, and Uttarakhand.
Cold winds blowing from these areas lower temperatures across northwest India every winter, including Delhi.
What is La Niña?
It refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall, as per the WMO.