The worst type of damage to the standing crops is done by frost. There are two types of frost: (i) advection frost or airmass frost. This type of frost occurs when the temperature of an airmass at the surface is below freezing; and (ii) radiation frost, which occurs when the night skies are clear and there is a temperature inversion.
Under these atmospheric conditions ice crystals form on cold objects. Airmass frost is also called black frost or a freeze. Such frosts are common in winter in temperate and frigid zones. It is particularly harmful to the winter-wheat.
If this type of freeze occurs in early autumn or late spring it proves to be more disastrous. The only possible solution to this problem is to avoid planting before such frosts occur in spring.
Another solution is the selection of early maturing varieties of crops which will mature before the occurrence of autumn frost. As a matter of fact, in the subtropical regions frost occurring in any part of the year is damaging to the standing crops to a greater or lesser degree.
It may be mentioned that air mass frost is more harmful to citrus fruits. It is worth remembering that only costly crops can justify the use of various costly frost preventing measures.
The damage caused to various crops by radiation frost is not as serious as in case of an airmass frost. Besides, it occurs at certain places, while certain other places do not experience it. This type of frost may kill fruits on the trees, but it does not kill plants and trees. However, the financial loss due to crop damage may be very great.
There are certain sensitive crops like potatoes, tomatoes and melons which are more vulnerable to radiation frost. In the northern plains of India, during the winter season, the radiation frosts bring disaster to potatoes, tomato and many other sensitive crops.
In India the farmers irrigate their fields to ward off the dangers from the expected frosts. In hilly areas the citrus and other sensitive crops are planted on the slopes exposed to the sun avoiding the valleys which are subject to winter frosts at night.
In developed countries like the U.S.A., France and Switzerland the high value crops are protected from the frosts by the use of covers and mulches which reduce the loss of soil heat. Direct frost prevention measures aim at breaking up the inversion commonly associated with intense nighttime radiation.
The most common practices comprise stirring the air, heating it or providing a protective blanket of smoke. Besides, giant-size burners are used which create convection currents and cause the mixing of air in the inversion layer. Another method of mixing surface air is by means of huge fans operated by electric or gas-driven motors.
Sometimes aeroplanes and helicopters are also used for mixing of the air above crops. However, it is a very expensive method. In California and Florida, practically every grove is provided with thermometers and heaters.
An advance warning of the occurrence of frost makes the frost – prevention methods more effective. Forecasting of air mass frost depends on the air mass analysis in detail. However, the detailed forecast of radiation frost is all the more difficult. But warnings of frost can be made.
The farmers must know the soil conditions, the heat storage during the previous period, the stage of crop growth, the dew point, the wind factor and the expected degree and duration of minimum temperatures below freezing.
Because radiation frost occurs in scattered localities, so every farmer must be in possession of instruments that could register local temperature, humidity and wind, so that he can have an idea of the danger for his orchard or cropland.
It is important to bear in mind that the unexpected frost causes the maximum damage to his crops. Thus the knowledge of the frost-tree season is essential in agricultural planning.