Meteorological measurements, at the Earth's surface and in the atmosphere, and satellite photographs are exchanged internationally. They can be used to follow the daily and monthly behaviour of the climatic variables that affect agriculture. The consequences of variation can be assessed to some extent by using heuristic models of crop growth and to a lesser extent from statistical correlations with yields. The movement of air masses within the tropics has a systematic but irregular pattern, which is still not well understood. However, the statistical correlation of some weather variables, such as geopotentials, with the rainfall to come, can be used to indicate the character of the coming season in some parts of the tropics.