During the Monsoon, funnel clouds sometimes greet southern Arizona creating a unique spectacle in the sky.
When the Monsoon flow from the tropics is moving very rapidly, wind aloft will be quite strong. In contrast, wind at the surface will be much slower.
Figure 1: Wind aloft is fast.
Figure 2: Wind at the surface moves relatively slowly.
This difference can, and sometimes does, create a pinwheel effect in between the strong wind aloft and the weaker wind below.
Figure 3: This difference creates a rotation, much like a pin-wheel.
When a gust of wind comes along, it can flip the rotation on its side, resulting in a swirling funnel cloud forming from the cloud.
Figure 4: A funnel cloud during Monsoon 2012 taken by "Tucson Glider".
Add to this any area of low pressure, like an inverted trough, and the rotation characteristics of the low pressure system itself can be magnified in a single storm, helping to increase the chance for funnel development.
Figure 5: An inverted trough (an area of low pressure) is embedded in the red highlighted area during Monsoon 2012, July 4.
If a funnel cloud makes contact with the ground, it then becomes a tornado.
Most tornadoes in the Monsoon are quite weak. More damage comes from straight-line wind generated in a microburst than in the typical Arizona tornado.
Read about Inverted Troughs here.
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