Statistics are often misused and misunderstood. Climatologists occasionally refer to a particular storm or flood as a 100-year or 500-year storm. Hydrologists occasionally make reference to a 100-year or 500-year flood plain.
These references are often misunderstood and even misused.
Some people are led to believe that a 100-year or 500-year flood is something that happens exactly every 100 or 500 years. Others assume that if there has recently been a 100- or 500-year flood, then one is unlikely again soon. Not so.
At face value, a 100-year event has a 1 percent chance of happening in any year and not just once in 100 years. Furthermore, these numbers are usually calculated based on a rather short period of record. Our climate is always changing and so is our land use. Much of the time, when weather frequency statistics are presented, the period of recorded weather is simply not long enough for a good statistical sampling. This leads to unfortunate misunderstandings.