Additionally, if more than ten percent of the total possible observations at a individual station were missing for a particular averaging period, those data were not included. In the calculation of average event durations, we defined an event as the occurrence of freezing rain for one or more consecutive hours. Surface observing stations used for this study. Freezing rain occurrences increase rapidly from October through December and decrease at a slower rate from February through April. National summation of freezing rain observations by month , 1982-1990.
Spatial variability is high, with large gradients surrounding the maxima.
The maximum exhibiting the highest spatial variability exists over New York and Pennsylvania. Number of freezing rain observations September through April, 1982-1990. The large maximum over southeast Washington and northeast Oregon is co-located with the Columbia River Basin. The size of the basin may, in part, explain the size of the maximum. Storm systems entering the Pacific Northwest generate precipitation and a favorable low-level thermal stratification that interact with cold air trapped in the Basin to produce freezing rain.