absolute drought

In British climatology, a drought period of at least 15 days during which no measurable daily precipitation has fallen.

In the United States similar criteria have been used to define a dry spell (which, in turn, has a different definition in Great Britain). Ordinarily, these criteria are applied regardless of the season of the year.

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.