The amount of water that would evaporate from the surface and be transpired by plants were the supply of water unlimited. It is calculated from the mean monthly temperature, with corrections for day length, and was devised by C. W. Thornthwaite as part of his system of climate classification (see thornthwaite climate classification). From PE minus precipitation an approximate index can be calculated of the extent to which the water available for plants falls short of the amount they are capable of transpiring. Compare actual evapotranspiration.