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<br />CHAPTER 2. GENERAL PROCEDURES <br /> <br />Section 2.01. StOrM magnitude <br /> <br />Most hypothetical floods are computed from storm rainfall. the <br />severity of which is a function of the depth-area-duration relationship <br />and the time sequence and areal distribution of rainfall conforming to <br />that relationship. An illustrative depth-area-duration relationship is <br />shown in figure 2.01. Because all storms are more severe for some area <br />sizes and durations than for others, it is virtually impossible to <br />assign an exceedence probability to any particular storm. Probability <br />estimates are confined to total precipitation amounts for specified <br />durations and areas, and such estimated probabilities do not necessarily <br />apply to resulting runoff, since the time and area distributions of <br />rainfall and the ground conditions will also affect runoff severity. <br /> <br />Section 2.02. Storm types <br /> <br />There is some merit in considering general storms separately from <br />local storms. Precipitation in both types is caused by lifting of <br />moist air to higher levels. causing the air to cool through expansion <br />and thus to reduce its capacity to retain water vapor. <br />General storms are associated with tropical or extra-tropical <br />cyclones. the latter generally associated with distinct frontal systems. <br />Precipitation in general storms is caused by convergence of air over <br />extensive areas through cyclonic or orographic action. Usually large <br />general storms are accompanied by high winds that persist for relatively <br />long durations over large areas. <br />Local storms are those that result from individual convection cells. <br />often accompanied by thunder and lightning. The horizontal extent of the <br />cells is limited by the height of vertical development. In the most <br /> <br />2-01 <br />