7/24/11
A monsoon in Arizona? Yes, indeed. The term monsoon was originally defined for the Indian subcontinent. It is actually a wind shift which brings precipitation. The Arizona Monsoon (some call it the summer thunderstorm season) is part of the larger Mexican Monsoon which occurs throughout the southwest area of the continent from Mexico to as far north as Wyoming.
Two things happen to bring on our monsoon. Heat builds in this area in April, May and June (with temps well over 100 F), and an upper level high pressure system (Bermuda High) moves north from winter to summer.
These two events cause the wind to shift from a west or northwest direction to a south or southwest direction bringing tropical moisture from the Gulf of Mexico and the Gulf of California and with it humidity, clouds and rain.
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| Monsoon Thunderstorm |
The surface heating and strong moisture influx create powerful thunderstorms. In Arizona we generally get the thunderstorms from July to September. These are brief downpours, but they flow on dry, hard-packed ground which causes flash flooding. We get an average of 2.65 inches of rain for all of July, August and September. Not much considering other areas of the country. But this is the desert. We get about 8 inches of rain a year.
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| Arizona Dust Storm July 5, 2011 |
Sometimes, with these powerful thunderstorms, come dust storms. You might have seen the photos of the two monster dust storms (or haboobs as meteorologists call them) that we have had in Arizona this month. These dust storms are caused by strong winds flowing downward and outward from thunderstorms, kicking up dust in the dry desert areas. The dust storm we had on July 5 was 5,000 feet high (1500m) and 100 miles (160 km) wide. Anything outdoors - furniture, cars, uncovered pools - was covered in dust. We only got a brief shower out of that storm, the first rain we've seen since March. That's why xeriscaping ( dry or water efficient landscaping) is common and popular here.
