This article is both an extension and revision of a paper in which a large number of 24 hour rainfall data was analyzed for determining an enveloping rainfall. A series of annual maximum rainfalls for seven durations ranging from 5 min to 24 hrs. were selected for the following reasons: they are closer to the required results than any other statistic series; they are amenable to probability analysis because the items in the series are independent of each other and occur in a random fashion; and, they can be extracted from publications that are readily available.

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In recent years, the terms, "once-in-a-century" and 100year, have been attached to various rare and catastrophic events; recent examples are the rains associated with Hurricane Agnes and the 1972 Rapid City storm. The background for the use of these terms is examined from the points of view of both the professional and nonprofessional user. A brief review is presented of the methods of analysis of extreme values with applications to rainfalls. The main unsolved problem in rainfall frequency analysis is the lack of a satisfactory procedure for determining the frequency of various rainfall patterns over a fixed geographical area.