Phoenix has an arid climate, and its average annual maximum temperature is the highest of any major US city. In fact, out of the world's large urban areas, only some cities around the Persian Gulf, such as Riyadh, Saudi Arabia, Baghdad, Iraq, and cities in India such as Kanpur and Jaipur have higher average summer temperatures. 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). Overnight lows greater than 80°F occur frequently each summer, with the average July low being 81°F and the average August low being 80°F. The all-time highest low temperature was 96°F (36°C), which occurred on July 15, 2003.

The dry desert air makes the hot temperatures more tolerable early in the season, but the influx of monsoonal moisture, which generally begins in early July and lasts until mid-September, raises humidity levels and discomfort due to mugginess. For the most part, the winter months are mild.

Phoenix averages 85% of possible sunshine and receives scant rainfall, the average annual total at Phoenix Sky Harbor International Airport being 8.3 inches (210 mm). March is the wettest month of the year (1.07 inches or 27 mm) with June being the driest (0.09 inches or 2 mm). Although thunderstorms are possible at any time of the year, they are most common during the monsoon from July to mid-September as humid air surges in from the Gulf of California. These can bring strong winds, large hail, or rarely, 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.

On average, Phoenix has only 5 days per year where the temperature drops to or below freezing. The long-term mean date of the first frost is December 15 and the last is February 1; however, these dates do not represent the city as a whole because the frequency of freezes increases the further one moves outward from the urban heat island. Frequently, outlying areas of Phoenix see frost, but the airport does not. The earliest frost on record occurred on November 3, 1946, and the latest occurred on April 4, 1945. 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. Snowfall was first officially recorded in 1896, and since then, accumulations of 0.1 inches (0.25 cm) or greater have occurred only seven times. The heaviest snowstorm on record dates to 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. Prior to that, 1 inch (2.5 cm) had fallen on January 20, 1933. On February 2, 1939, 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. On January 21, 2007, snowfall occurred throughout much of the city and covered the ground completely in the East Valley cities of Mesa and Tempe. This followed a very rare cold spell which caused temperatures to tumble into the 20s.

Source: U.S. National Climatic Data Center, Asheville, N.C.; a division of the National Oceanic and Atmospheric Administration [1]