This article throws light upon the seven forecasts of weather parameters. The forecasts are: 1. Maximum Temperature Forecast 2. Minimum Temperature Forecast 3. Wind Forecast 4. Cloud Forecast/Precipitation 5. Dew Forecast 6. Evaporation 7. Frost Forecast.
1. Maximum Temperature Forecast:
The forecast of maximum temperature is based on sunshine during the day. Cold and warm advection have pronounced effect on the temperature conditions. Air temperature is very important from agriculture point of view. Temperature provides working conditions for the growth of crop plants. For the normal growth, the temperature should be optimum.
There are extreme temperatures beyond this optimum. These extremes are called maximum air temperature and minimum air temperature. The forecast of these extreme temperatures is always helpful for taking precautionary measures to save the crops being subjected to highest and lowest temperature.
2. Minimum Temperature Forecast:
The forecast of minimum temperature is based on the cloudy conditions during night. Cold and warm advection have pronounced effect on the night time temperature. During winter season minimum temperature remains close to freezing point. In order to save the crops from low temperature injury, the forecast of minimum temperature is very important.
Predicted minimum temperature should not be less than expected dew point temperature. During summer season, temperature rises and it becomes maximum during the months of May and June. Temperature increases abruptly when warm advection takes place. As a result, heat wave conditions are intensified which are harmful for the crop plants.
Similarly during winter season, temperature decreases due to cold advection. It leads to cold wave conditions and frosty nights. Such type of weather conditions are quite prevalent in north-west India. The frosty nights have adverse effect on the growth of crop plants. The crop plants, which are susceptible to low temperature conditions, suffer maximum damage when air temperature falls below freezing level.
3. Wind Forecast:
Wind is generated by the movement of air-mass due to the horizontal pressure gradient set up between the two stations. The forecast of wind is made based on the spacing of the isobars. Closer the isobars, higher the strength of the winds and vice-versa. Generally strong surface winds are associated with low pressure areas or cyclonic circulation.
If the low pressure area is intensified and becomes cyclone, then the strength of the winds becomes very strong and at times may exceed even 64 kmph. On the other hand, light winds are associated with high pressure area or anticyclone circulation.
Wind forecast depends upon the pressure distribution on the surface chart. Closer the isobars, stronger would be the wind speed. The maximum wind speed at the surface can not exceed the highest wind speed at 850 mb. Wind forecast is important for spraying chemicals and fertilizer application.
Harvesting efficiency increases under calm conditions. In view of the anticipated windy condition, irrigation is avoided, otherwise lodging of the crop plants takes place. Generally stable weather conditions prevail in the morning and in the evening hours. Therefore, calm conditions associated with low temperature are favourable for spraying insecticides, fungicides and herbicides.
4. Cloud Forecast/Precipitation:
The information on these weather parameters is useful for farmers in taking decision on agricultural operations like spraying, irrigation and harvesting. Sunshine is also required for agricultural operations like threshing and drying the agricultural produce.
The forecast of the cloud cover depends upon the synoptic situation. If the area is under the influence of low pressure associated with upper air cyclonic circulation or upper air trough, then cloudiness or precipitation may be expected depending upon the available water vapours.
Rainfall is described in the forecast as isolated, where less than one-third of the recording stations are expected to receive rain. It is described scattered, when one-third to two-third stations are expected to record rain and wide spread, when more than two third stations are expected to experience rainfall.
Conditions for clear weather:
(a) There must be anticyclone i.e. dry and cold winds must blow in clockwise direction.
(b) There must be negative vertical motion. Descending air mass becomes hot due to adiabatic warming.
(c) Relative humidity should be less than 50 per cent.
5. Dew Forecast:
Dew appears in the morning hours on vegetation near the earth’s surface, when the air temperature falls to dew point temperature. The condensation of water vapours from the air upon the vegetation surface cooled by radiation losses is called ‘Dew’. The amount of dew is very small as compared to precipitation.
There are large variations in the quantity of dew deposited on the surface. Dew is a form of precipitation. It is very important for agricultural plants in those areas where scanty rainfall occurs. In the desert areas, dew provides moisture to the vegetation for its survival.
Crops grown under rainfed conditions are greatly benefited from the moisture provided by dew in the morning hours. But dew is favourable for the incidence of plant diseases in the crops grown under irrigated conditions.
Therefore, dew forecast can be made in different categories. According to the pictorial guide devised by Dudevani (1947), dew can be divided into six categories. These are very light, light, moderate, heavy, very heavy and wet.
Conditions favourable for dew:
i. Clear sky and little surface moisture.
ii. Light winds or wind speed should be less than 8 kmph.
iii. Rapid outgoing radiational cooling at night.
iv. Presence of high pressure area or anticyclone winds.
v. Absence of positive vertical motion.
vi. Absence of turbulence.
If these conditions are fulfilled then dew is predicted.
Conditions not favourable for dew:
i. Cloudy conditions are favourable for the reflection of long wave radiation back to the earth.
ii. Presence of low pressure area or cyclonic circulation.
iii. Strong surface winds i.e. wind speed should be greater than 16 kmph.
iv. Presence of negative vertical motion. Descending air does not allow the air temperature to reach dew point.
The rate of evaporation depends upon the prevailing weather conditions and the anticipated weather conditions. Forecasting of evaporation is required to apply irrigation to the crops. Application of irrigation depends upon the type of soil and the crop grown. It is simply based on the amount of accumulated evaporation since the date of application of irrigation. The loss of water depends upon the weather conditions.
The atmospheric demand increases under hot and dry weather conditions and decreases under humid weather conditions. The prediction of evaporation is helpful in planning irrigation in advance so that water can be used judiciously.
7. Frost Forecast:
Frost is the form of precipitation. Like dew, frost also plays an important role in agriculture. The formation of frost depends upon the dew point. When the dew point decreases below freezing temperature, then the water vapours directly change to ice crystals. This process is called sublimation. The layer of the ice crystals is called frost. The frost causes low temperature injury to the crop plants.
The vegetable plants are sensitive to frost injury. Frost generally occurs during winter season. In north India, it is a common feature during December, January and February months, when the temperature falls below freezing level.
Conditions favourable for the formation of frost:
i. Sky should be clear.
ii. There must be a high pressure area or anticyclone wind.
iii. Calm conditions.
iv. Excessive radiational cooling.
v. Low water content in the air.
vi. Absence of turbulence.
vii. Absence of positive vertical motion.
These type of conditions are generally found when the dry weather continues for a few days. Therefore, the forecast of frost is very important. The farmers can adopt protection measures, if the forecast of frost is made well in advance. The prevention of frost and protection of plants from damaging frost can be achieved by using various methods.