These maps show the percentage of average precipitation that can be expected for a specified length of time and specified return interval. You may choose 1- to 5-year lengths, and return periods from 25 to 200 years. These return periods actually refer to probability of occurrence in a single year. A 100-year return means that there is a 1 in 100 (1%) chance of occurrence in a single year. Thus, a contour labeled "85" on a map showing "60-month drought, 50-year return period" means that during a five year stretch, there is a 2% chance of having only 85% of the average five-year precipitation total. Such maps help to show the likelihood of receiving only 85% of the average precipitation for some specified period.