The plains of North India experience Cold Wave and Cold Day conditions during winter season (December-February) every year. It is a severe weather phenomenon which causes loss to life and public property. As per India Meteorological Department’s criteria, Cold Wave and Cold Day conditions are defined as:
(A) Cold Wave:
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
i) Cold Wave: Negative Departure from normal is 4.5°C to 6.4°C
ii) 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.
(B) Cold Day:
It should be considered when minimum temperature is 100C or less for plains and 0°C or less for Hilly regions.
Cold day: Maximum Temperature Departure is -4.5°C to -6.4°C
Severe Cold day: Maximum Temperature Departure is < -6.4°C
Factors causing Cold Wave Conditions:
- Cold Wave conditions are associated with fall in minimum temperatures during the Winter season. In this season prevailing winds over the Indo Gangetic Plains (IGP) are northwesterlies. As northwesterlies are the winds from colder regions of Central Asia/Hindukush region, they bring temperature fall over the IGP and hence the Cold Wave Conditions.
- Whenever a Western Disturbance (WD) approaches clouding develops over IGP bringing fall in Maximum temperatures and rise in minimum temperatures. Thus, Cold Wave conditions over IGP get abated at the approach of a WD.
- When a WD moves away from the Indian region, clear skies start appearing over the IGP leading to rise in maximum and fall in minimum temperatures.
- Whenever a WD 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 anticyclone in lower and mid tropospheric levels is also a driver of Cold Waves. Such an anti-cyclone gives rise to sinking motion over the IGP leading to fall in minimum temperatures.
Factors causing Cold Day Conditions:
- Cold Day conditions are associated with fall in maximum temperatures during the Winter season. Cold Day conditions occur when there is persistent low clouding over North Indian plains/IGP in the winter season.
- Low clouds prevent solar insolation entering into earth’s surface which causes fall in maximum temperatures.
- Left entrance and right exit of a Jet core belong to upper level convergence which in turn causes sinking motion over the surface hence causing Cold Wave and Cold Day conditions.
- If an active WD affects North India in the winter season and causes widespread rain over the region along with isolated hailstorms it leaves behind abundant moisture over the IGP. Now if after the passage of the WD, lower level westerlies (cold and dry) are not strong enough to absorb this abundant moisture it will cause persistent low cloud cover and hence the Severe Cold Day conditions over the region. Similar phenomenon was witnessed during second half of December, 2019. (Figure below)
- Development of anomalous high over the region causes low cloud cover which in turn leads to preventing the solar heating of the earth surface and hence the severe cold day conditions. (Figure below)
Monitoring and forecasting process:
IMD utilizes all its resources to monitor round the clock forecast with a lead period of 05 days and warn against the adverse weather to the general public, disaster managers, media and other stake holders. For monitoring the weather systems, IMD uses all type of Synoptic charts, INSAT-3D Rapid half hourly imagery, every 10 minutes DWR products for Srinagar, Patiala, Delhi, Lucknow, Jaipur and Bhopal. Various Numerical Prediction Models like IMD GFS, WRF, ECMWF, NCMRWF NCUM, GEFS and various international models are utilized for this purpose. The digitized decision support system known as SYNERGIE was utilized for decision making and for development of consensus forecast.
Compiled by: Krishna Mishra & Naresh Kumar.