Weather plays the most prominent role in the basic inputs to crop production system. Weather elements also appear as vital components in the control complex of the system with less significant contributions at other points in the system.
In a survey of climate, weather and human food systems, climate, soils, terrain, natural vegetation, pests and diseases are the major biophysical, and, in part, social and economic constraints on human food systems.
There is also the close interrelation between climate, weather uncertainty, indirect climatic influences on soils, natural vegetation and pests and diseases, and human food systems.
It is clear that the various constraints on human food systems limit the species which can be grown and can be used for human food. However, the constraints limiting the crop production can be mitigated through the use of available resources and of inputs such as irrigation, mechanization and fertilisers.
The most essential climatic elements for the proper growth of crops are solar radiation, temperature and moisture. The availability of suitable radiation, assuming unlimited carbon dioxide, water, and soil nutrients ensure the photosynthesis of the maximum amount of plant tissue within a crop.
There are a large number of plants which respond to the length of the day and the total amount of solar radiation available to them. Remember that there are certain plants which are very sensitive to day length, while others are indifferent to it.
Temperature is the most critical factor in the crop growth. It affects the growth of crops in several ways. There are certain crops like coffee, bananas and sugar cane which are very sensitive to frosts.
On the contrary, apples require very low temperatures for the development of flowers. There are other crops like coconuts and pineapples which need temperatures above 21° C for their best growth.
Maize is another crop which thrives best when the mean temperatures during part of the growing season are above 21° C. Temperatures below 15° C are harmful for the growth of crops such as citrus fruits, cotton, sugar cane and rice.
Most of the vegetables need at least 8° C while threshold for wheat is about 3° C. The length of the growing season is also an important factor in the growth of crops. Increasing elevation also increases the time for the maturity of a crop.
For certain crop such as potatoes soil temperature is more important than the air temperature. Potatoes require the minimum of 8°C and the maximum of less than 28°C. Ideal temperature is about 18°C for the successful growth of potatoes crop. For the best germination of cotton seeds the soil temperature should be at least 10° C.
According to Robertson and Coulter, the concept of degree-days allows estimation of the suitability of a crop for a particular climate assuming ample water availability. According to them if the threshold temperature for a crop to grow is 6°C, a day of 16°C should count as 10 degree- days.
It is to be remembered that the total number of degree-days over a certain period of time is called the accumulated temperature and gives an approximate index for comparing the thermal requirements of plants. It also provides an indication of the suitability of an area for a particular crop.
Besides temperature, sufficient soil moisture and a suitable temperature are both important factors affecting the crop growth. In fact, rainfall and the rate of evaporation determine the soil moisture. Evaporation rate is influenced by such weather elements as temperature, wind speed and humidity.
So far as usefulness of rainfall for the crop growth is concerned, moderate moisture conditions are more useful than alternation of wet and very dry conditions.
The yield of a crop is also influenced by weather conditions. For example, the optimum temperature for wheat, the most widely grown agricultural crop, is 25-31°C. Weather changes cause variations in wheat production from season to season and region to region.
A harsh winter results in decrease in production of wheat. For better yields of this crop higher than normal temperatures in the early periods of growth and lower temperatures than normal in the later stages are needed.
It is a well-known fact that different crops need different temperature, moisture and other weather elements.
Applications of meteorological information to the management of weather sensitive agricultural crops may include the use of climatology to adjust planting practices for new genetic characteristics of plants; management of animal production; crop yield forecasting and the choice of correct size and type of farm machinery etc.
Thus, weather elements exercise control on all the stages of agricultural production chain. They do not only influence the growth of crops but also their sowing, harvesting transport and marketing as well.