Sonora has a Köppen classification of BWh, which is a subtropical hot desert. During the hottest months (May-June) Pacific currents carry moisture over the Cascades and Sierra Nevada mountains where the atmosphere is cooler and less able to retain moisture. This results in rain on the windward side of these ranges. The Rain Shadow effect leaves the leeward air dry as the current moves towards Sonora. Another major factor is the North Pacific High Pressure Zone. Hot air rises over the equator and moves north to settle over Sonora. This keeps away weaker low pressure systems along with rain.
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Me exploring the Sorora Desert during Spring Break
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Sonora is one of the wettest North American deserts. Average rainfall of 3-15” is split between two rainy seasons; summer and winter. The late summer rains (July-September) are the most dramatic and result from monsoon winds which pull Pacific Ocean moisture over the hot desert floor. The moist hot air rises and expands in Thermal columns. This creates instability in the atmosphere and results in isolated thunderclouds that can quickly poor huge volumes of rain. Here is a great link to the
Anatomy of a Monsoon Thunderstorm. The winter rains (December-January) are more predictable and result from low pressure storm tracks that travel from the Pacific Northwest through California to the Sonora desert. I experienced this during my visit over spring break. The day I arrived it was 80⁰ and sunny. The next day it barely reached 50⁰; it rained and even snowed!
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The forecast called for rain but I did not expect to see snow!
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I want to share a couple of unique forms and processes common in Sonora. The Dust Devil is a special type of Thermal where hot air rushes upward in a column that sucks in the surrounding hot air resulting in a spinning dusty cyclone.
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| Arizona Dept. of Transportation Photo |
The Haboob is a dust storm caused by down drafts at the front of a desert thunderstorm. Check out this video!
References:
http://www.desertmuseum.org/books/nhsd_patternsrain.php
http://geoplan.asu.edu/aztc/monsoon.html
http://www.blueplanetbiomes.org/sonoran_desert_climate.htm
http://tucsoncitizen.com/wryheat/2011/07/08/more-on-the-haboob-dust-storm-that-covered-phoenix/
http://www.azdot.gov/CCPartnerships/Haboob/index.asp
