Ecologists have long attempted to single out critical environmental factors for use in distributional, successional, and other ecological studies. Temperature and relative humidity have been foremost in consideration. In recent years the importance of relative humidity, and even of temperature, has been minimized. The evaporating power of the air is looked upon as a far more critical factor for plants. The paucity of evaporation records and the lack of widely acceptable methods for measuring evaporation have prompted the consideration of factors which can be used in lieu of atmospheric evaporating power.