Climate Information for the "Valley of the Sun"
Phoenix has an arid, semitropical climate, with little seasonal change during the year. Clear blue skies are typical on most days, with an average of 300 sunny days per year. The temperature reaches or exceeds 100 °F (38 °C) on an average of 89 days during the year, including most days from early June through early September. On June 26, 1990, the temperature reached an all-time high of 122 °F (50 °C). The dry Arizona air makes the hot temperatures more tolerable early in the season; however, the influx of monsoonal moisture has been known to make August in Phoenix almost as humid as summers in the Southeastern United States.
The normal annual rainfall at Sky Harbor International Airport is 8.29 inches (211 mm). March is the wettest month of the year (1.07 inches or 27 mm). Rain is particularly scarce from April through June. Although thunderstorms occur on occasion during every month of the year, they are most common during the monsoon season from July to mid-September as humid air surges in from the Gulf of California. These can bring strong winds, large hail, or tornadoes. Winter storms moving inland from the Pacific Ocean occasionally produce significant rains but occur infrequently. Fog is observed from time to time during the winter months.
At the airport, the mean date of first frost is December 12 and the last is February 7; however, these dates do not represent the city as a whole because the frequency of freezes varies considerably among terrain types and elevations. Some areas of Phoenix may see frost for a month or more before and after the airport readings. The earliest frost on record occurred on November 3, 1946, and the latest occurred on April 4, 1945. Successive winters without any frosts at the airport have been recorded, and the longest period without a freeze stretched from November 23, 1979 to January 31, 1985. The all-time lowest temperature in Phoenix was recorded at 16 °F (-8.8 °C) on January 7, 1913.
Snow is extremely rare in the area, though still can occur from time to time. Since it was officially recorded in 1896, snowfall has accumulated to 0.1 inch (0.25 cm) only 7 times. The heaviest snowstorm occurred on January 20-21 1937, when 1 to 4 inches fell (2 to 10 cm) in parts of the city and did not melt entirely for four days. Another 1 inch (2.5 cm) fell on January 20, 1933. On February 2, 1935, 0.5 inches (1 cm) fell. Most recently, 0.4 inches (1 cm) fell on December 21-22 1990. Snow also fell on March 12, 1917 November 28, 1919, and December 11, 1985.