Saturday, July 30, 2011

July in the Sonoran Desert

It is this time of year that the desert waits for the violent monsoon rains.  So far the storms have been hit or miss and much of the desert has not received any significant rainfall.  As is typical, higher elevations are more likely to receive rain than the lower and hotter elevations.  And eastern portions of the Sonoran Desert, such as around Tucson, are far more likely to receive rain than western portions, such as around Yuma. The end of June through mid-July are unbearably hot, highs over 110 degrees are quite common.  Rain has been absent from the desert for months on end, and the desert life hides from the relentless heat and sun.  Life simply dries up or congregates around waterholes this time of year.  Everything becomes sluggish except for the Saguaro fruit ripening which livens things up for a few weeks.  But as Saguaro harvest wains so does activity of nearly all desert life.  The desert simply waits and longs for cooling life giving rain.
An Anvil Cloud that often proceeds a Monsoonal thunderstorm.  An Anvil Cloud is a type of cumulus cloud.
Even though the rains have not come that does not mean the violence of these storms has not come to the desert.  As is typical of the desert, the rain comes with a lot of hype.  Often these monsoonal storms are proceeded by humidity, gigantic anvil clouds, dark stormy clouds, wind, clouds that are raining but rain never hitting the ground, dust storms, and sometimes if we're lucky rain.  This whole process can be quite disappointing to desert dwellers to say the least.  Too often these storms simply end in just a lot of dust, wind, and no rain.  This disappointing end does provide some relief however.  Even in the absence of rain clouds shade some of the suns scorching rays and the rain that never hits the ground still cools the air considerably in some cases.  Humidity that proceeds monsoons can also drop the temperatures five to ten degrees for weeks ahead of the first rains.  In the Sonoran Desert if its not the 110 degree dry heat, its the 105 degree humid heat, you decide what is worse...
Haboob dust storm in Phoenix, Arizona.  Picture from Wikipedia.
Dust storms in the Sonoran Desert are quite the interesting phenomena.  These are not sand storms like in the Sahara Desert, they are actually clay particles picked up and blown around by the wind.  The reason for this is that clay is the smallest dust particle and is easiest to be picked up off a dry landscape and blown around.  The technical name for these "clay storms" are haboobs.  Yes, this is an awkward weird word but it is Arabic, so of course it won't seem normal to English speakers.

Haboobs are very important soil formation events.  All of the dust and clay within these dust storms has to fall on soil somewhere.  Dusts from these storms is deposited on soils and enriches them with magnesium, calcium, iron, and of course clay.  After these things are deposited on soil surfaces, rain water will carry them deeper into the soil.  After thousands of years of this happening iron stains the soil red, magnesium stains rocks on the surface black, calcium forms a rock-like layer called caliche about 2-3 feet deep, and clay forms a dense clay-rich horizon just above the caliche called an argillic (Click here for more info from an prior blog entry).  Caliche and argillic soils are extremely important in determining what types of plants live in a particular locations.  Some plants prefer soils with caliche or argillics while others do not.  More on that another time though.

The rains should come to nearly the whole Sonoran Desert within the next few weeks or so.  That is, at least the eastern portion.  Normally Tucson receives about six inches of rain, Phoenix three, and Yuma less than one during Monsoon season.  But actually amounts vary from year to year.  In Phoenix I have seen years with only a half inch or so, and other years with nine inches.  It has been a few years since we have had a really good amount of rain during Monsoons season, maybe that means this will be a good one!

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