=163. Elements of Annual and Monthly Rainfall.=—In consequence of the great variations in the rate of rainfall, not only for different portions of the country, but at different times during the same storm, it becomes necessary to determine various quantities such as the maximum, minimum, and mean annual rainfall, the actual monthly rainfall for different months of the year, and the maximum and minimum monthly rainfall for as long a period as possible. The minimum monthly rainfall and the minimum annual rainfall are of special importance in connection with public water-supply and water-power questions, since those minima will, in connection with the area of a given watershed, determine the greatest amount of water which can be made available for use. In entering upon the consideration of such questions, therefore, civil engineers must inform themselves with the greatest detail as to the characteristics of the monthly and the annual rainfall of the locality in which their works are to be located. [Illustration: MONTHLY VARIATIONS IN RAINFALL. FIG. 1.] TABLE V. GENERAL RAINFALL STATISTICS FOR THE UNITED STATES. ----------------------------+---------+---------+--------- | | Per Cent| | | of | Per Cent | Mean | Summer | Driest Station. | Yearly | and | Year to |Rainfall.| Autumn | Mean | | Rain to| Year. | | Mean | | | Yearly. | ----------------------------+---------+---------+--------- North Atlantic: | | | Boston | 45.4 | 50 | 60 New York | 44.7 | 52 | 62 Philadelphia | 42.3 | 52 | 70 Washington | 42.9 | 51 | 69 South Atlantic: | | | Wilmington | 53.7 | 61 | 75 Charleston | 49.1 | 61 | 48 Augusta | 48.0 | 50 | 81 Jacksonville | 54.1 | 65 | 74 Key West | 38.2 | 70 | 54 Gulf and Lower Mississippi: | | | Montgomery | 52.5 | 42 | 76 Mobile | 62.6 | 51 | 68 New Orleans | 60.3 | 52 | 64 Galveston | 47.7 | 58 | 50 Nashville | 50.2 | 46 | 67 Ohio Valley: | | | Pittsburg | 36.6 | 53 | 70 Cincinnati | 42.1 | 50 | 60 Indianapolis | 42.2 | 51 | 59