Criteria for Cold Wave

Wind chill factor plays an important role and brings down the actual minimum temperature depending upon the wind speed. The actual minimum temperature of a station should be reduced to “wind chill effective minimum temperature (WCTn)” based on wind chill factor.

For declaring “Cold Wave” and “Cold Day” WCTn should only be used.

If WCTn is 10°C or less, then only the conditions for cold wave should be considered.
a) When normal minimum temperature is equal to 10°C or more.
Cold Wave Departure from normal is -5°C to -6°C.
Severe Cold Wave Departure from normal is -7°C or less
b) When normal minimum temperature is less than 10°C.
Cold Wave Departure from normal is -4°C to -5°C.
Severe Cold Wave Departure from normal is -6°C or less.

When WCTn is 0°C or less, Cold Wave should be declared irrespective of normal minimum temperature of the station. However, this criteria is not applicable for those stations whose normal minimum temperature is below 0°C.

Cold Wave conditions for coastal stations
For coastal stations the threshold value of minimum temperature of 10ºC is rarely reached. However, the local people feel discomfort due to wind chill factor which reduces the minimum temperature by a few degrees depending upon the wind speed. The “Cold Day” concept may be used following the criteria given below:
Criteria for describing “Cold Day” for coastal stations
i) Actual minimum temperature of a station be reduced to WCTn.
ii) This WCTn should be used to declare “Cold Wave” or “Cold Day”.
iii) When minimum temperature departure is -5ºC or less over a station, “Cold Day” may be described irrespective of threshold value of 10ºC
iv) However, when a threshold of 10ºC is reached “Cold Wave” be declared.
v) When a station satisfies both the Cold Wave and Cold Day criteria, then Cold Wave has a higher priority and has to be declared.
Heat wave/ Cold Wave and hot Day/ Cold Day are area specific phenomena. Therefore they may be described for a Met. Sub-division or a part thereof, when at least two stations satisfy the criteria.