Weibull Formula for Recurrence Interval

The Weibull formula is used in hydrology and statistics to estimate the recurrence interval ($T$) of an extreme event, such as a specific rainfall amount.

The formula relates the recurrence interval ($T$) to the number of years of record ($n$) and the rank ($m$) of the event when the data is sorted in descending order.

The Weibull formula is expressed as:

$ T = \frac{n+1}{m} $

• $T$ represents the recurrence interval in years.
• $n$ is the total number of years in the data record.
• $m$ is the rank of the rainfall event (1 for the most extreme, 2 for the second most extreme, and so on).

This formula provides an estimate of how often, on average, an event of a certain magnitude is likely to occur or be exceeded.