Tucson has hot summers and temperate winters and is almost always cooler and wetter than Phoenix due to its higher elevation. This desert climate has two major seasons, summer and winter; plus three minor seasons: fall, spring, and the monsoon.
Tucson averages 11.8 inches (299.7 mm) of precipitation per year, more than most other locations with desert climates. Summer is characterized by daytime temperatures that exceed 100 °F (38 °C) and overnight temperatures between 66 and 85 °F (19 and 29 °C). Early summer is characterized by low humidity and clear skies; mid-summer and late summer are characterized by higher humidity, cloudy skies and frequent, but often warm, rain.
The monsoon season can begin any time from mid-June to late July, with an average start date around July 3. It typically continues through August and sometimes into September. A monsoon usually begins with clouds building up from the south in the early afternoon followed by intense thunderstorms and rainfall. The evening sky at this time of year is often pierced with dramatic and beautiful lightning displays.
Winters in Tucson are mild relative to other parts of the United States. Daytime highs in the winter range between 64 and 75 °F (18 and 24 °C), with overnight lows between 30 and 44 °F (−1 and 7 °C).