A study of radar echo distribution in Arizona during July and August

Arizona's summer thunderstorm regime differs from that of any other section of the country. The mountainous sections of the state, notably the White Mountains northwestward to the San Francisco Peaks, have one of the highest frequencies of afternoon thunderstorms in the United States. The desert valleys of Arizona, particularly around Phoenix, have a diurnal regime of thunderstorm activity which is diametrically opposite to that of the mountainous country located less than 100 mi les to the north and northeast·. Forty-eight percent of the thunderstorms that occur in Phoenix are observed in the period from 1800 to 2400 MST. This is one of the highest percentage fre- quency of occurrences at this time of day in the United States. John E. Hales, Jr. Includes bibliographical references (page 8).