A common phenomenon in arid regions of the world is the whirling vortex called the “dust devil.” Although they resemble tornadoes, dust devils are generally much smaller and less intense than their more destructive cousins. Most dust devils are only a few meters in diameter and reach heights no greater than about 100 meters. Further, these whirlwinds are usually short-lived microscale phenomena: most form and die out within minutes. Unlike tornadoes, which are associated with clouds, dust devils form on days when clear skies prevail. Further, these whirlwinds form from the ground upward, exactly the opposite of tornadoes. Because surface heating is critical to their formation, dust devils occur most frequently in the afternoon, when surface temperatures are highest.

Dust devils form when the hot surface of the earth warms the air above it. When the air near the surface is considerably warmer than the air a few tens of meters overhead, the layer of air near Earth’s surface becomes unstable. In this situation, warm surface air begins to rise, causing air near the ground to be drawn into the developing whirlwind. A light wind will start this rising air to rotate, and once the rotation has begun, it becomes faster due to the same physical principle that causes ice-skaters to spin faster as they pull their arms closer to their body. As the inwardly spiraling air rises, it carries sand, dust, and other loose debris tens of meters into the air. It is this material that makes a dust devil visible. Occasionally, dust devils form above ground covered with vegetation. Under these conditions, the vortices may go undetected unless they interact with objects at the surface.

Most dust devils are small and short-lived; consequently, they are not generally destructive. Occasionally, however, these whirlwinds grow to be 100 meters or more in diameter and over a kilometer high. Larger and more vigorous dust devils have a longer lifetime than smaller ones. One large dust devil, with a height of about 750 meters, lasted for seven hours as it traveled 64 kilometers in western Utah. With winds that whirl at speeds that may reach 100 kilometers per hour, such large dust devils can do considerable damage.