From Glossary of Meteorology


The chance that a prescribed event will occur, represented as a pure number p in the range 0 ≤ p ≤ 1.

The probability of an impossible event is zero and that of an inevitable event is unity. Probability is estimated empirically by relative frequency, that is, the number of times the particular event occurs divided by the total count of all events in the class considered.
See probability theory.