I'll be heading back to southern Utah for some fall colors in a couple of weeks, so I thought I'd touch on a subject related to that area. While we typically think of the U.S. southwest as largely dry and arid, which it typically is, it is actually impacted by an atmospheric cycle that operates on the same mechanism as the monsoon that sweeps into India every year.
For most of the year, the southwest is dominated by dry conditions and prevailing winds are from the west. These winds carry little moisture after having to climb up over a number of mountain ranges, dumping rain as they rise. As a result, very little moisture makes it into the Great Basin and Four Corners areas of southwest North America.
Referred to variously as the North American monsoon, the Southwest United States monsoon, the Mexican monsoon, or the Arizona monsoon, the conditions that create it begin to develop over southwest Mexico in late May or early June. Just as on the Indian sub-continent, seasonal movements of high and low pressure systems redirect flows of air in the atmosphere, steering moist air drawn from several sources into the area.
A persistent ridge of high atmospheric pressure moves northward into the United States from Mexico, while intense heating of the land results in a thermal low to the west over northwestern Mexico and the southwestern United States. The development and positioning of these two systems cause a shift in atmospheric flow patterns from the predominantly westerly flow, to a pattern dominated by flow from the southeast to east.
This new circulation pattern draws moisture from the Gulf of California, the eastern Pacific Ocean, the Gulf of Mexico, and even the Great Plains of the U.S., sending it from the southwest to the northeast and bringing intense thunderstorms to Utah, Arizona, New Mexico and Texas. These thunderstorms typically develop as temperatures rise during the day, resulting in afternoon downpours that roll across the landscape from July through September.
This week's photo was taken on the Utah/Arizona border on a late afternoon in July. The setting sun is lighting up the higher ridge to the east, while dark clouds roll past beyond. The foreground is in diffused, almost shadowed light from clouds to the west, blocking the sun.
This photo was taken with a Canon EF 28-135 mm f/3.5 – 5.6 IS USM lens zoomed to 35 mm on a Canon EOS 5D Mk III. This is an HDR composite of three exposures. Exposure was set to f/16 and ISO 1600, with shutter speeds set at 1/90 sec., 1/180 sec., and 1/350 sec.
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