Using the percent of climatological stations reporting rain as a measure of the raininess of a particular day in Arizona, a large increase in rainfall within a few days is found to occur about July 1 in most Arizona summers. By means of flow charts, upper air sequences, mean soundings, and diurnal temperature ranges, this increase is shown to be the result of a rather sharp transition from one dominant air mass to another over the state. The occurrence appears to be related to index, and a hemispherical singularity also appears to be related to the phenomenon.

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* The research reported in this document has been sponsored by the Geophysics Research Directorate of the Air Force Cambridge Research Center, Air Research and Development Command, under Contract No. AF 19(604)-992.

# This paper is extracted from a Master of Science Thesis, Department of Meteorology, University of Wisconsin.

1 Mr. Lowry's present address: Oregon State Board of Forestry, Salem, Oregon.