pluvial index

(Rare.) The amount of precipitation falling in one day, or other specified period, that is likely to be equaled or exceeded at a given place only once in a century, that is, a precipitation amount that has a return period of 100 years.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.