During steady rain, the ceiling lowers in a discontinuous fashion. The ceiling heights may be predicted with sufficient accuracy by using a set of empirically determined rules. To obtain a relation for the time of occurrence of these ceilings, the factors which influence cloud formation are considered. An expression is derived for the rate of moisture increase due to evaporation from falling raindrops. The rate of moisture change, given by this expression, is combined with the effect of the

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... r factors in order to obtain a formula which may be applied to find the time a ceiling of given height will occur. The variables in the forecast formula are (1) the wet-bulb temperature depression measured before the start of rain and (2) F" the effective rate of moisture increase caused by factors other than evaporation. Values for F, are found empirically. An approximate method, based on the surface value of the depression, is used for finding the time of occurrence of the 800-, 500-, and 300-foot ceilings. This approximate method appears to be best suited for forecasting the 500-foot ceiling. formula for the rate of downward growth of the fractocumuli forming underneath the rain cloud during steady precipitation. His derivation is based on the assumption that all rain forms by the melting of snow falling out of the nimbostratus, accompanied by cooling of the air just beneath the cloud. This cooling causes a steepening of the temperature gradient below the zero isothermal, consequently increasing convection wbich results in the formation of the fracto-cumuli. Assuming that the heat required to melt all the precipitation comes from the surrounding air, Findeisen shows that the rate of downward growth of the low cloud is given by: = 6 . N dt where N is the rate of rainfall in mm.hr." and defat is the rate of lowering in cm.sec.". According to equation (1) clouds formed in rain lower at a rate directly proportional to the intensity of rainfall. An attempt to apply the result to forecast ceilings during 1 The definition of the term "ceiling" is subject to change. In order that no confusion shall result, it will he employed here to mean the height, above the ground, of the bsse of the lowest cloud layer covering more than half the sky. This deflnition approximates no specification as to the amount of cloudiness is intended, terms such as "base height of the official meaning in effect at the time of the ceiling data used in this report. When cloud (or cloud layer)" or "height of base of cloud" will be employed. 133