Tucson - Climate

Climate

Tucson has hot summers and temperate winters. Tucson is almost always cooler and wetter than Phoenix because of its higher elevation.

Tucson has a desert climate (Köppen Bwh), with two major seasons, summer and winter; plus three minor seasons: fall, spring, and the monsoon. Though Tucson receives more precipitation than most other locations with desert climates, it still qualifies due to its high evapotranspiration in spite of receiving 11.8 inches (299.7 mm) of precipitation per year; in other words, it experiences a high net loss of water. A similar scenario is seen in Alice Springs, Australia which averages 11 inches (279.4 mm) a year, but has a desert climate.

The most obvious difference of climate from most other inhabited regions is the hot and sunny climate. This difference is a major contributing factor to a rate of skin cancer that is at least 3 times higher than in more northerly regions. Media reports heat related deaths increasing among illegal immigrants in and around Tucson. Heatstroke related deaths have been recorded since 1999 in the Pima County Area.

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 rain.

The monsoon 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. 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 that during spring: dry, with 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 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). 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. The last notable hard freeze occurred on February 3–4, 2011, when the temperature dipped to 18 °F (−8 °C) on two consecutive nights. Although rare, snow has been known to fall in Tucson, usually a light dusting that melts within a day. The most recent snowfall was on February 20, 2013 when 2.0 inches of snow blanketed the city, the largest snowfall in several years.

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.