Abstract

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 other 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) Fz, the effective rate of moisture increase caused by factors other than evaporation. Values for Fz 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.

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