CLASSIC DUST DEVILS OF MARICOPA COUNTY, ARIZONA
Nothing says Arizona more that heat and dust. These are the ingredients for one the desert's wonderful meteorological phenomena - the DUST DEVIL.
Dust devils are created when
air near the surface becomes much warmer than the air above. This creates
an instability in which the warm air rises. The affect is pronounced over
the southwest deserts. With intense insolation, temperatures well above
100oF (up to 140oF) are not uncommon on the desert floors
during the summer months. Dust devils typically occur in open areas where
large quantities of air are heated. An isolated area begins to rise due to
its instability and soon much of the surrounding air rushes in and intensifies
this upward movement. The Coriolis effect gives the dust devil its
characteristic spin (counterclockwise in the northern hemisphere). The
open, flat desert floor also minimizes frictional drag so the rush of hot air
towards the rising vortex is maintained. The abundance of dust enhances
our view of the air movement.
Typically dust devils
persist for no more than a few minutes, occasionally lasting 30 minutes or more.
They may be only a few feet in diameter to a few hundred feet across and stretch
hundreds of feet above the surface. Wind speeds can be as much as 60 miles
per hour or more. The largest of the dust devils can do damage to
unsecured structures. Here is a link to dust devil damage done on
September 14, 2000 at the Coconino County Fairground near Flagstaff, Arizona:
http://www.wrh.noaa.gov/Flagstaff/science/dustdvl.htm
This pair of pictures (of the same dust devil) show the
counterclockwise flow of dust around and up the walls of the dust devil.
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This isolated dust devil nicely shows the cylindrical shape created by the ascending air. Commonly a dust devil will begin as a large, slowly rotating mass of air. There may even be multiple small dust devils within the mass, but as the rush of air towards the uplifting vortex increases, a single dust devil emerges. The edges of this dust devil appear more heavily laden with debris due to the fact that the central region of the rising column is relatively clear of dust. |
This is a close up of the base of one of the frequent
dust devils that moved across this ploughed field.
This field, just west of Luke Air Force Base in western Maricopa County Arizona,
proved to be an ideal setting for the study of dust devils. Early July in
this area is marked by some of the highest temperatures reached all summer long
in Arizona. The large, open, and flat patch of land allowed for easy
airflow into the developing parcels of rising air. AND, the abundance of
fine grained dust from the field made the dust devils easily visible.