Photo Jul 25, 7 21 26 AM
Early morning clouds coming up from the south and southeast in late July
(Click for bigger image)

“Monsoon” means a seasonal change in the prevailing winds. In Phoenix, that accounts for the first of our two rainy seasons, the one that occurs now in these summer months.

In summer, temperatures in the Arizona desert soar and, as you know, hot air rises. That rising air pushes the jet stream off to the north and, with it, the normal flow of air from the Pacific that is raked over and squeezed dry by the Rocky Mountains also moves north and away from Arizona.

The hot rising air has to be replaced and so the convection draws in new air from the south and southeast. Those air masses carry moisture from the Gulf of California and the Gulf of Mexico but since no mountains intrude on their route to Arizona, their moisture makes up to to where it encounters the Rockies and, pushed upward, the air dumps its load on Arizona.

It rains.

Hence, the summer monsoon season in Phoenix and the southwest.

Most intense from mid-July through August and then on into September, the sometimes daily dust storm — “Haboob” is the more accurate meteorological term — and thunderstorm with sometimes intense but brief periods of rainfall are the most familiar consequences. Approximately one-third of Phoenix’s annual 6″ rainfall occurs during this period.

Weeks later, desert grasses, cactus and wild flowers will bloom, occasionally in astonishing glory. And while it is typical to see all the mountains around Phoenix turn green, we all know that in a few weeks they will again turn yellow and then brown and be highly vulnerable to fire.

Phoenix also has a second, less dramatic, rainy season. It occurs during the winter and is again related to the jet stream. But unlike the summer when rising hot air pushes the jet stream away, in the winter and with generally cooler air that retains its moisture over the Rockies, the jet stream proceeds with its load of Pacific Ocean moisture in full strength across the southern United States.

Winter storms are typically less intense in the Phoenix area than their summer counterparts and day-long winter showers may occur. (Because the winds continue as normal from the west during the winter rainy season, it is technically not a “monsoon” period.)

Regardless, desert grasses will again appear along with a few flowers weeks after the winter rains but usually not with the intensity as seen in the summer.

Winter rain, 2010 Desert Midwinter Competition

The Phoenix Rod and Gun Club has its annual “Desert Midwinter” competition in mid-February and while most days are sunny and mild, the winter rains do occasionally add a Camp Perry weather “feature” to this winter competition.

Additional reading is available many places but one of the best is AT THIS LINK in a PDF by Michael A. Crimmins, a student at the College of Agriculture and Life Sciences at the University of Arizona in Tucson.

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