the air moves from areas of high pressure to areas of low pressure. This creates areas
of differential temperature, and where such cooling occurs precipitation also occurs.
The third scenario for precipitation is caused by external cooling, usually the
presence of cold air masses. When cold air comes down out of the Arctic (from the
Antarctic below the Equator), it comes in along the surface of the earth, forcing the
warm moist air upward. This creates cooling that results in precipitation.
In addition to the variation of precipitation with location and means of formation,
precipitation within a storm also varies with time. As a general rule, the longer the
duration of precipitation, the less is the rate of precipitation. Figure 6.4 depicts the
intensity duration curves for New York City. The figure also shows the recurrence
interval or the number of years that a given precipitation would not be exceeded. For
design parameters, the cumulative worst possible storm is also plotted. It is clearly
Fig. 6.4 Rainfall intensity-duration curves for New York City
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