One of the hottest deserts in the world, the Sonoran differs from other arid regions in North America in that it has mild winters. There are only a few nights throughout the year when newscasters come on the air and breathlessly implore Tucsonans to bring their plants inside for fear of an overnight frost. According to the Center for Sonoran Desert Studies, nearly half the flora and fauna in the region are “tropical in origin.”
One of the defining climate characteristics of Tucson and the surrounding deserts and mountains is the unique bi-seasonal rain pattern. While Tucson typically receives less than 12 inches of rainfall annually—often much less; the region has been gripped by an ongoing drought for at least a decade—nearly all of the region’s precipitation comes during just two brief rainy seasons.
The most important of these is the summer rainy season, locally referred to as the monsoon, when, from July through to (hopefully) about mid-September, surges of wet, tropical air move up from the south and build over the mountain, creating often violent, localized thunderstorms, mostly in the late afternoon and early evening after a long, hot summer day. The most violent monsoon storms, sometimes called “gully washers” or flash floods, can swell the region’s usually dry river beds and washes in a matter of minutes.
To say that these storms are “localized” is an understatement. It’s a common joke in Tucson—though no less true for being a joke—that, during the monsoon, one can often observe rain falling in one’s front yard but not in the back. One side of town can be awash in a flash flood, while a few miles away the sun is beating down clear and unbroken, without a cloud in the sky.
The strength and, so to speak, success of the monsoon can vary greatly from year to year. For example, according to data collected by the National Weather Service in Tucson, in 1964 the monsoon season yielded an amazing 13.84 inches of rain in Tucson; in 2004, however, the area received just 2.42 inches of rain during the same season.
The winter rainy season lasts from roughly October to March, and is not as localized, nor as closely watched, as the monsoon. If winter brings a fair amount of precipitation to the desert, however, then the whole landscape explodes with dormant annual wildflowers, and the normally monotone landscape is transformed into a patchwork of colors.
© Tim Hull from Moon Tucson, 1st Edition