Standing Rabi crops toying with cold wave, dense fog, ground frost

A cold wave modified by ground frost and dense to very dense fog are now weighing in on Rabi crops in parts of North-West and Central India in what is the first instance of a ‘combined charge’ this year.

An India Met Department update said that dense to very dense fog prevailed over east Uttar Pradesh during the morning hours of Tuesday.

Moderate to shallow fog drifted out over Himachal Pradesh, Punjab, Haryana, Chandigarh, Delhi, north Rajasthan, Bihar, Odisha and most parts of North-East India.

During the 24 hours ending on Wednesday morning, cold wave conditions prevailed in Haryana, Chandigarh, Delhi, South Rajasthan, East Madhya Pradesh, East Uttar Pradesh, Bihar, Saurashtra and Kutch.

It is on top of these that the Met is predicting ground frost on Wednesday and Thursday over Punjab, Haryana, Chandigarh, Delhi, and North Rajasthan where minimum (night) temperatures would remain 4 deg Celsius or less. Fog and ground frost strike mostly in tandem and have contrasting implications on Rabi crop which has already taken a hit from the warmer than usual winter this year.

Rabi wheat may be less sensitive to frost, but other crops such as mustard, gram and pulses are known to be extremely sensitive.

The vegetation will not necessarily be damaged on a night where the air cools down leaves to below their freezing point.

But once frost forms, it is possible that sharp ice crystals may damage leaf cells. Certain bacteria are known to be particularly effective at triggering frost formation.

Fog is largely caused by advection (wind), a process in which moist air passes over cool ground and gets in turn cooled. Local winds blowing in from the hills (katabatic winds) slow down over the valley to produce convection, with the embedded moisture getting converted into fog.

Katabatic winds are those that flow from high elevations of mountains, plateaus, and hills down their slopes to the valleys or plains below. The outlook of the Met for the next five days has said that moderate to dense fog would occur at isolated place over Punjab, Haryana, Chandigarh, Delhi, Uttar Pradesh and Bihar.

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