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Teresa M. Schulz and Perry J. Samson


Nonprecipitating low cloud frequencies for 1982 have been extracted from the U.S. Air Force 3-Dimensional Nephanalysis archives for central North America. These data were compiled from satellite, surface and pilot reports and contain, among other parameters, the low cloud type and present weather code for each 3-h time period for grid points in the Northern Hemisphere. The overall average precipitating to nonprecipitating low cloud ratio for central North America was 0.25, indicating that nonpredpitating low clouds were very common. Seasonal frequencies of these nonprecipitating low clouds are presented. The persistence of clear and nonprecipitating cloudy skies from one time period to the next was also calculated on a seasonal basis. Annual average values for transitions between clear and cloudy skies as well as between nonprecipitating and precipitating low cloudy time periods are also presented. A cluster analysis was performed on the seasonal nonprecipitating low cloud frequency data. It was found that the original 4096 grid points could be described well by seven robust clusters. Thew clusters are identified and their distinguishing characteristics are discussed. It was found that not only were these clusters distinct from one another, but they exhibited considerable within-cluster consistency.

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