Friday, August 21, 2009

Desert Monsoon: The Fifth Season

Unlike much of North America, where each passing year is marked by four seasons, the Sonoran Desert is blessed with five, although some of us might claim that it is all one long summer! The fifth season is our summer monsoon, a stormy season that drenches the desert with torrents and flash floods. Even though it is hotter than heck, this is the most vibrant of Sonoran seasons, the one that defines the region.

Monsoon season generally begins in mid-July and lasts through mid-September. This follows a hot, dry summer that stretches from May through early July, when sane people migrate to more hospitable climates. Monsoon is an Arabic word meaning “shifting wind” and is technically not related to rainstorms. The Sonoran Monsoon marks a time when prevailing winds shift from westerly and northwesterly, to southerly and south easterlies coming from tropical and subtropical regions in Mexico. This is partly generated from the “Bermuda High” that creates hurricanes in the Mid Atlantic Ocean. At the same time, as the desert heats up and hot air rises, a vacuum, or low pressure zone, is created that sucks up moisture from the south. When the air becomes saturated, huge cumulonimbus clouds form, erupting into spectacular downpours and lightning storms that make awe-inspiring entertainment for any sky watcher.

Haboobs, gustanados, macro- and microbursts are some of the wild winds associated with monsoon season. When air from an approaching thunderstorm hits the ground, a massive dust storm rising up to 3000 feet from the desert floor and lasting up to three hours may shroud the landscape. This is a haboob. Gustanados are large dust devils originating from the ground, creating clouds of dust that sometimes look more like smoke. Micro and macro-bursts are powerful downward funnels of air that can create sudden winds up to over 50 mph. The difference is in size, with macro-bursts flowing more than 2.5 miles from their center and lasting 5-20 minutes. Microbursts cover smaller areas for shorter times. When you wake up to a landscape of uprooted trees, it is usually the gustanados or downbursts that are to blame.

Although the winds may be destructive, a monsoon thunderstorm also brings welcome relief from the heat, cooling the air by twenty degrees or more in a few minutes. The rains, however, are spotty, leaving some parts of the desert bone dry, while bringing downpours to other areas. Even if our neighborhood receives no rain at all, a storm over Deem Hills could result in flash floods in small arroyos and nearby Skunk Creek. An average storm will drop 2 inches of rain where it falls, or a quarter of the average annual rainfall in the Phoenix metro area of about 8 inches!

Summer rains also bring new life to the desert. We are gifted with a second season of wildflowers, featuring bright orange Arizona poppies (Kallstroemia grandiflora). These are not poppies at all, but are more closely related to creosote bush. Hornworms, the larvae of sphinx moths, and other caterpillars may also emerge in abundance. Butterflies break from chrysalises. Toads wake from summer hibernation in cool burrows to mate and lay eggs in pools created by rainstorms. People are wakened from summer torpor as well, with hope for a cooler fall season. This is a good time to start planting a winter garden.

For more information about monsoon weather and ecology, check out “A Natural History of the Sonoran Desert,” published by the Arizona Sonoran Desert Museum in Tucson, or pull up the ASU Dept. of Geographical Sciences and Urban Planning meteorology webpage at

No comments:

Post a Comment