The effects of changes in weather, especially extremes in weather, are an inseparable part of seasonal effects. Without weather, the price of an agricultural commodity would lack the surprises that cause them to jump around during the growing period. Once planted, you would have a very good idea of the expected supply; one half of the supply-demand equation would be constant each year.
Each agricultural product has its own particular sensitivity to weather. Grain planting can be delayed due to rain, causing some farmers to switch from corn to soybeans; hot weather during pollination will significantly reduce yields; a freeze in September can stop the ripening process and damage production. Freezes are of greater concern than droughts and affect more products. In general, patterns for crop sensitivity to weather depend upon their location in the Northern or Southern Hemisphere. Active trade between world markets shows that crops grown primarily in the Northern Hemisphere are allected by weather developments in the Southern Hemisphere.