The return period and frequency are statistical descriptors of the severity of a storm event. The return period is the expected length of time between two rainfall events that exceed a specific magnitude. Frequency, or exceedance probability, is the inverse of the return period. As you might expect, the higher the return period, the more infrequent the storm event, and the higher the intensity of the rainfall.

For example, a storm with a 5-year return period represents an event that is expected to be exceeded once every five years (on average). The frequency is 1/5, which means that there is a 20% probability of a storm exceeding that magnitude occurring in any given year. Note that the return period does not mean that two storm events exceeding a given magnitude will not occur in the same year, nor does it guarantee that a storm event exceeding this magnitude will occur within any given five year span. It just means that these storms will occur at an average rate of once every five years.