In general, climatology as applied to the effect of climate on crops. It includes especially the length of the growing season, the relation of growth rate and crop yields to the various climatic factors and hence the optimal and limiting climates for any given crop, the value of irrigation, and the effect of climatic and weather conditions on the development and spread of crop diseases. This discipline is primarily concerned with the space occupied by crops, namely, the soil and the layer of air up to the tops of the plants, in which conditions are governed largely by the microclimate.