In the wake of cold waves affecting large parts of India, reports of frost are emerging from multiple states. The frost may pose an additional risk to rabi crops, which already face an uncertain future due to the deficient northeast monsoon and feeble Western Disturbances (critical precipitation-bearing systems) in December.

Air frost occurs when ground-level air temperature is below zero degrees Celsius. Ground frost, meanwhile, refers to icy deposits on the ground formed when soil moisture freezes. Recently, in Kerala's Munnar, minimum (night-time) temperatures plunged to -4°C, blanketing the hills in white frost. Pretty though the sight was, it was bad news for tea plantation owners in the hill station, who say frost has resulted in huge losses for a second consecutive year.

The same situation was reported in Ooty in neighbouring Tamil Nadu, where 50,000 acres under cultivation have been destroyed, according to a local source quoted by The Times of India.

Over the last three weeks, severe cold wave conditions have affected the northern states of Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Punjab, Haryana, Delhi, Chandigarh and Rajasthan. Parts of central, east and peninsular India also experienced similar conditions. In many cases, the cold has been accompanied by frost.

Frost, both on the ground and on plants, has put a number of crops at risk, from wheat, gram and seasonal vegetables in Maharashtra, to brinjal, cumin and mustard in Rajasthan, and coriander, potato and gram in Madhya Pradesh. Punjab, Haryana, Himachal Pradesh and Uttar Pradesh are also reportedly reeling under frost-related crop damage.

Cold wave, dry weather form dangerous combination

Frost can affect crops in various ways. It can form a layer on stems, leaves and fruits, causing the plant cells to rupture. Similarly, ground frost limits the availability of soil oxygen and moisture to plants. "When plants cannot access soil nutrients in this way, it is called a physiological drought," says Dr. Kripan Ghosh, Head of the Agricultural Meteorology Division that provides weather updates to farmers under the Gramin Krishi Mausam Seva initiative of the Ministry of Earth Sciences.

The current frost conditions have seemingly resulted from a combination of factors. One is the cold wave that has been sweeping India since mid-December. The lack of moisture in the winter air was compounded by poor rainfall since December 20, as well as the lack of active Western Disturbances (WDs). Strong WDs are typically characterised by rain, clouds and winds, all of which have been missing in the last three weeks. This combination of low moisture and clear skies, along with calm winds, creates ideal conditions for the ground temperature to fall sharply overnight, increasing chances of ground frost, says Leon Brown, Head of Global Meteorological Operations at The Weather Company.

Temporary relief expected

In its Thursday bulletin, the India Meteorology Department (IMD) has referred to the arrival of a fresh western disturbance, and said that rain or snow expected in Punjab, Jammu & Kashmir and Himachal Pradesh up to the weekend. This could provide farmers a welcome, if temporary, respite.

However, as the WD's effects subside, drier weather could be back, says Brown, who predicts, "Next week, the ground frost risk will return to the north of India, with lower night temperatures resulting from calm winds, clear skies and lower dew points."

So what can farmers do to tackle the challenges caused by frost? Ghosh says, "There are various measures to protect crops from frost—from watering the fields in the evening, to shielding the plants with straw, polythene or gunny bags, or even using heaters or certain chemicals that lower the freezing point. Then there are long-term measures such as better site selection, choosing more weather-hardy plants, and changing the sowing date to avoid frost risk." Using these techniques can help reduce the risk of frost-related damage to a certain extent, Ghosh says.

At present, there is no official word yet on the damage to agriculture caused by the extreme cold and frost, so only time will reveal the full picture. However, as the rabi harvest season approaches, understanding the real scale of the problem will be the first step to managing the risk better in subsequent years.