From AMS Glossary
One of the simplest measures of precipitation effectiveness, given by Lang as
where P (cm) is precipitation and T (°C) mean temperature for the period in question.
This index recognizes only that as temperature increases, the effective moisture decreases due to greater evaporation. A number of greater refinements of this concept exist: De Martonne's index of aridity; Angström's humidity coefficient; Gorczyński's aridity coefficient; Thornthwaite's precipitation-effectiveness index and moisture index; and Köppen's formulas for outlining steppe climate and desert climate.