precipitation–evaporation ratio
(Abbreviated P–E ratio; also called precipitation-effectiveness ratio.) For a given locality and month, an empirical expression devised for the purpose of classifying climates numerically on the basis of precipitation and evaporation:
where P is the normal monthly precipitation in inches and T the normal monthly temperature in degrees Fahrenheit. All temperatures below 28.4°F are given the value of 28.4°, and P–E ratios greater than 40 are counted as 40.