Tucson has a hot semi-arid climate , with two major seasons, a hot summer and mild winter. Tucson averages 11.8 inches (299.7 mm) of precipitation per year, concentrated between the Pacific storms of winter and the North American Monsoon of summer. Fall and spring months tend to be sunny and dry. Despite being at a more southerly latitude than Phoenix, Tucson is slightly cooler and wetter due to a variety of factors including elevation and orographic lift in surrounding mountains—though Tucson does occasionally see warmer daytime temperatures in the winter.
Summer is characterized by daytime temperatures of over 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 rain. The sun is intense in Tucson during part of the year, and those who spend time outdoors need protection. Recent studies show the rate of skin cancer in Arizona is at least three times higher than in more northerly regions. Additionally, heat stroke is a concern for hikers, mountain bikers, and adventurers who explore canyons, open desert lands, and other exposed areas.
While monsoon season officially begins on June 15, the arrival of the North American Monsoon is unpredictable, and it varies from year to year. On average, Tucson receives it first monsoon storms around July 3. Monsoon activity generally persists through August and often into September. During the monsoon, the humidity is much higher than the rest of the year. It begins with clouds building up from the south in the early afternoon followed by intense thunderstorms and rainfall, which can cause flash floods. The evening sky at this time of year is often pierced with dramatic lightning strikes. Large areas of the city do not have storm sewers, so monsoon rains flood the main thoroughfares, usually for no longer than a few hours. A few underpasses in Tucson have “feet of water” scales painted on their supports to discourage fording by automobiles during a rainstorm. Arizona traffic code Title 28–910, the so-called “Stupid Motorist Law”, was instituted in 1995 to discourage people from entering flooded roadways. If the road is flooded and a barricade is in place, motorists who drive around the barricade can be charged up to $2000 for costs involved in rescuing them. Despite all warnings and precautions, however, three Tucson drivers have drowned between 2004 and 2010.
The weather in the fall is much like spring: dry, with warm/cool nights and warm/hot days. Temperatures above 100 °F (38 °C) are possible into early October. Temperatures decline at the quickest rate in October and November, and are normally the coolest in late December and early January.
Winters in Tucson are mild relative to other parts of the United States. Daytime highs range between 64 and 75 °F (18 and 24 °C), with overnight lows between 30 and 44 °F (−1 and 7 °C). Tucson typically averages one hard freeze per winter season, with temperatures dipping to the mid or low-20s (−7 to −4 °C), but this is typically limited to only a very few nights. Although rare, snow occasionally falls in lower elevations in Tucson and is common in the Santa Catalina Mountains. The most recent snowfall was on January 26, 2021 when a winter storm caused snow to fall throughout most of the southwest. Tucson airport recorded 1 inch of snow, while Summerhaven in the mountains received 25 inches.
Early spring is characterized by gradually rising temperatures and several weeks of vivid wildflower blooms beginning in late February and into March. During this time of year the diurnal temperature variation normally attains its maximum, often surpassing 30 °F (17 °C).
At the University of Arizona, where records have been kept since 1894, the record maximum temperature was 115 °F (46 °C) on June 19, 1960, and July 28, 1995, and the record minimum temperature was 6 °F (−14 °C) on January 7, 1913. There are an average of 150.1 days annually with highs of 90 °F (32 °C) or higher and an average of 26.4 days with lows reaching or below the freezing mark. Average annual precipitation is 11.15 in (283 mm). There is an average of 49 days with measurable precipitation. The wettest year was 1905 with 24.17 in (614 mm) and the driest year was 1924 with 5.07 in (129 mm). The most precipitation in one month was 7.56 in (192 mm) in July 1984. The most precipitation in 24 hours was 4.16 in (106 mm) on October 1, 1983. Annual snowfall averages 0.7 in (1.8 cm). The most snow in one year was 7.2 in (18 cm) in 1987. The most snow in one month was 6.0 in (15 cm) in January 1898 and March 1922.
At the airport, where records have been kept since 1930, the record maximum temperature was 117 °F (47 °C) on June 26, 1990, and the record minimum temperature was 16 °F (−9 °C) on January 4, 1949. There is an average of 145.0 days annually with highs of 90 °F (32 °C) or higher and an average of 16.9 days with lows reaching or below the freezing mark. Measurable precipitation falls on an average of 53 days. The wettest year was 1983 with 21.86 in (555 mm) of precipitation, and the driest year was 1953 with 5.34 in (136 mm). The most rainfall in one month was 7.93 in (201 mm) in August 1955. The most rainfall in 24 hours was 3.93 in (100 mm) on July 29, 1958. Snow at the airport averages only 1.1 in (2.8 cm) annually. The most snow received in one year was 8.3 in (21 cm) and the most snow in one month was 6.8 in (17 cm) in December 1971.