How do dust devils form?

Answer 1

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.