The Seasons of the Sonoran Desert
Most days are sunny with approximately 360 days of sunshine per year. The amount and seasonality of rainfall are defining characteristics of the Sonoran Desert. Because of the 2300 foot elevation in Tucson, there is usually a year-round 30 degree differential between daytime and nighttime temperatures.
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Springtime is a perfect time for outdoor activities. It brings warm temperatures and mostly dry, windy weather to the desert. Triple digits are usually hit for the first time by the second half of May. Depending upon the winter rains, the desert comes alive with color as most desert plants bloom throughout the springtime season.
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From July to mid-September, the summer monsoon brings surges of wet tropical air and frequent, but localized, thunderstorms. Beware of flash flood warnings, lightning / thunderstorms storms, and micro-bursts of extreme localized winds. The UV index is dangerous during the mid-day, but early mornings and evenings are usually comfortable and a great time to be outside.
As the summer temperatures starts to drop again and the monsoon weather pattern changes, Tucson residents breathe a sigh of relief. It is amazing how 90 in October does not seem nearly as hot as 90 in May. It is probably some of the best autumn weather in the country and brings visitors from the north back to the Sonoran Desert.
Winters are relatively mild; most areas rarely experience frost. From December to March frontal storms from the Pacific Ocean occasionally bring widespread, gentle rain to the area. The Tucson Valley may experience 1 or 2 light snowfalls per year, just enough to make it exciting. Traveling from the desert floor to Arizona's "sky islands" can take you from comfortable a swimming temperature to a snowy peak in just a few hours. Tucson boasts of the southernmost snow skiing in the U.S.