Guide · 1 min
Return periods, explained
1-in-100 within 10 yrs9.6%
1-in-100 within 30 yrs26%
1-in-100 within 75 yrs53%
A return period restates a probability as a time: an event with a 1% chance in any given year is a "1-in-100-year" event. It does not mean it happens once a century.
The chance it happens in your lifetime
P = 1 − (1 − 1/T)ⁿ
| Event | 10 yrs | 30 yrs | 75 yrs |
|---|---|---|---|
| 1 in 10 | 65% | 96% | >99.9% |
| 1 in 100 | 9.6% | 26% | 53% |
| 1 in 1,000 | 1.0% | 3.0% | 7.2% |
The catch: it describes the baseline climate
A return period is computed from a baseline distribution. If the climate has shifted since that baseline, the real-world odds have shifted too. A value that was "1 in 500 years" under 1951–1980 can be far more common today. Not Normal shows return periods against whichever baseline you choose, so the drift is visible rather than hidden.
More guides
Move the baseline on the field →