|An example of a dust devil|
(Copyright: John Roenfeldt-Inflow Images)
What is a Dust Devil?
A common wind phenomenon that occurs throughout much of the world, including Arizona,
are dust devils . An example of a dust devil can be seen to the
left. These dust-filled vortices, created by strong surface heating, are generally
smaller and less intense than a tornado. Typical diameters of dust devils range from 10
to 300 feet, with an average height of approximately 500 to 1000 feet. Dust devils
typically last only a few minutes before dissipating, however they sometimes
for tens of minutes, under optimal conditions. Over the deserts or other locations
with very dry soil, dust devils can reach heights of several thousand feet and last on the
order of an hour or more. Wind speeds in larger dust devils can reach 60 mph or greater.
Even though they are generally smaller than tornadoes, dust devils can still be destructive
as they lift dust and other debris into the air. Small structures
can be damaged, and even destroyed, if in the path of a strong dust devil.
How do Dust Devils Form?
Dust devils form in areas of strong surface heating, usually at the interface between
different surface types, such as asphalt and dirt, or even irrigated fields and dirt roads.
Typically, they occur under clear skies and light winds, when the ground can warm the air to
temperatures well above the temperatures just above the ground. This is a very unstable
condition, since the heated air is less dense and lighter than the cooler air above it. If the temperature
of the ground becomes much warmer than the air above it, vertical mixing will take place
to release this unstable configuration. Once the ground heats up enough, a localized
pocket of air will quickly rise through the cooler air above it. The sudden uprush of hot
air causes air to speed horizontally inward to the bottom of the newly-forming vortex. This
rapidly rising pocket of air may begin to rotate, and if it continues to be stretched
in the vertical direction, will increase in rotation speed. This increase in rotation speed
from vertical stretching is similar to the increased spinning of an ice skater as they bring
their arms in toward their bodies. Meteorologists call this principle "conservation of angular momentum".
As more hot air rushes in toward the developing vortex to replace the air that is rising,
this spinning effect is intensified. As the air rises, it cools and eventually will descend back
through the center of the vortex. Under optimal conditions, a balance between the hot air rising
along the outer wall of the vortex and the cooler air sinking in the vortex occurs. The dust devil
then begins to move across the ground, picking up more and more dust, which highlights the vortex
making it visible to the eye. The dust devil,
once formed, is a funnel-like chimney through which hot air moves both upward and circularly.
If a steady supply of warm unstable air is available for the dust devil, it will continue to move
across the ground. However,
once the warm unstable air is depleted or the balance is broken in some other way, the dust
devil will break down and dissipate.
Do Dust Devils Form in Northern Arizona?
Dust devils can form anywhere in Arizona, due to a combination of our southern
latitude and increased solar heating, our dry sandy soil types, and large areas of lightly vegetated terrain. Because
of the necessity of warm clear days and light winds, dust devils occur most frequently over
Northern Arizona in the months of May and June, but can occur at anytime during the year
under the right conditions. Dust devils are most prevalent in the desert regions, but can
occur even in the forests of the Mogollon Rim. A good place to look for dust devils
is along the boundary between irrigated fields or forest meadows and dry, sunbaked ground
or dirt roadways, usually between 11 a.m. and 2 p.m. when the sun is most intense.
On September 14, 2000, a large well-defined dust devil hit the Coconino County Fairgrounds
causing property damage and personal injuries. Tents were blown down and a ticket booth was knocked
over. A large wooden beam in one of the pavailions was split due to the strong winds. Due to this
amount of damage, it is estimated that wind speeds may have reached up to 75 mph. Dust devils are not only common
to our planet, but have also been found on Mars as well.
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