Another important aspect of rain is how often a storm of a specified intensity and duration may be expected to occur. The recurrence interval is defined as the number of years (on the average) before a storm of a given intensity and duration can be expected to recur. A storm that would be expected on an average of once in 25 years is said to have a 25-year recurrence interval, or is called a 25-year storm. It is important to remember that this is all based on the laws of probability, or chance, and that these estimates are based on averages only. There is nothing to prevent a 25-year storm from happening in two successive years, or even more than twice in one year, although the odds against such frequent occurrence are great.
The National Weather Service has studied the rainfall records of major storms for many years. The results of these studies have been published in the form of Rainfall Intensity Recurrence Interval Charts. Such a chart would look like Figure 16.7.
Thus, a storm is considered to have a recurrence interval of 2, 5, 10, 25, 50, or 100 year, or longer, depending on the average number of years expected to pass before a storm of similar intensity and duration occurs again. For a given duration we would expect the intensity of a 100-year recurrence interval storm to be greater than that of a 10-year storm. A curve can be plotted showing the relationship between rainfall intensity and the expected duration for each recurrence interval.
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