On a really hot day the landscape between Tucson and Phoenix is often dotted with huge dust devils: swirling wind that resembles a tornado, but there is no cloud above it.
Here is a dust devil in Arizona:
Figure 1: A dust devil in Arizona (courtesy NWS Phoenix)
Check out the damage done at the Coconino County Fairgrounds by a dust devil on 9/14/2000.
Figure 2: Damage at the Coconino County Fair Grounds (courtesy of NWS Flagstaff)
Figure 3: Damage at the Coconino County Fair Grounds (courtesy of NWS Flagstaff)
So, how do these interesting hot-weather features form?
It takes two different temperatures existing on the desert floor in close proximity. The following figure shows a bare patch of desert next to a vegetated patch.
Figure 4: Patch of bare ground next to patch of vegetated ground.
Imagine being barefoot on the bare patch. The sun beats down on it all day, heating the desert soil/sand to super hot temperatures. You wouldn't be able to stand on that bare, hot patch of desert for very long without shoes.
But, the vegetated patch will be much cooler. Imagine mesquite and Palo Verde trees casting shade all day.
From previous Weather 101's, you know that hotter air rises and cooler air sinks. The air above the bare, hot patch will be hot, light and buoyant. In essence, the air over the hot, bare patch will RISE.
The opposite will happen over the cooler, vegetated patch. There, the air will be sinking since it is relatively cool, dense and heavy.
Figure 5: Air rises over the hotter, bare patch in relation to the cool, vegetated patch where the air sinks.
This motion above the two patches then starts a micro circulation of desert air wherein air is rising over the hot patch and then sinking over the cool patch. In essence, the rising and sinking air connect to create a single, circular flow.
Figure 6: A circulation pattern develops.
This pattern, though, has a horizontal axis. In other words, it needs to be flipped over on its side before it can resemble a dust devil. That's what a gust of wind can do!
Figure 7: A gust of wind blows the circulation onto its side.
And, voilà! It looks like this! It's now a dust devil!
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