Mean Temperature from 1000 MB to 500 MB as a Predictor of Precipitation Type

The frequency of occurrence of frozen precipitation forms relative to the frequency of occurrence of non-frozen forms is determined as a function of geographical location and of the mean temperature (or thickness) of the layer from 1000 mb to 500 mb. The results are derived from the data for a large number of stations in the United States during two winter seasons. A map of the thickness values for equal probability of occurrence of frozen and non-frozen forms illustrates the effects of altitude and continentality. It is found that the form of the precipitation can be specified with a confidence of 75 percent or more when the thickness value at a station is 100 feet or more from its “equal probability” value. Some refinements of the technique are discussed.