For a continuous flow porometer, stomatal resistance can be calculated from a measurement of the relative humidity of the chamber air if the leaf and chamber are isothermal. This calculation will be in error because evaporation will cool the leaf, and, for a translucent chamber, incoming radiation will heat it. For an opaque chamber, the difference between the calculated and the true resistance is independent of the stomatal resistance measured, and the magnitude of the difference can be found through a simple experiment. Knowing the relative humidity of the chamber air and the approximate temperature of the leaf, the true stomatal resistance can be determined.