Tuesday, August 19, 2014

A Day for Dust Devils

As I crossed the Great Plains yesterday, intense heat, bright sunshine and calm conditions set the stage for dust devil formation and I encountered dozens of those mini-tornadoes on my journey.  Dust devils form when hot surface air breaks through the cooler overlying atmosphere and spins as it moves upward and its axis narrows.  The rising column of air creates a vacuum which pulls in more hot air from the surrounding plain, sucking up dust as well; due to variable friction at the surface, the funnel moves across the landscape, fueled by hot surface air over which it moves.  Atop the dust devil, the rising air cools and vents to its rim, falling back toward the surface and reinforcing the shape of the vortex.

Since they most often develop in dry, flat, open country, these small atmospheric cyclones generally cause little damage though larger dust devils can destroy property.  Their structure is dependent upon stable conditions that permit the inflow of hot air at the surface while protecting the integrity of the vortex; surface winds destabilize dust devils by disrupting the column (and injecting cooler air) and pockets of cool surface air (e.g. above or near lakes, vegetation or shaded ground) cut off the inflow from below.  For all of these reasons, dust devils tend to be short-lived.

Those who do not cross deserts or arid plains may encounter dust devils on tilled farm fields or on large parking lots.  While the phenomenon is identical, it is more appealing to watch their movement across vast, open country than to observe vortices of litter swirl across our fields of asphalt.