Dust devils–atmospheric whirlwinds that resemble small tornadoes

Dear Tom,
Last month, on a clear, calm afternoon, I looked outside and saw a tornadic effect. Leaves were airborne, blowing in a circular motion, and this huge swirl crossed my neighbor’s driveway, filling it with dust. What was that?
— Chris Robbins, Villa Park
Dear Chris,
You were eyewitness to a dust devil, an atmospheric whirlwind that resembles a small tornado but is formed by completely different mechanisms not associated with thunderstorms. Rotating either clockwise or counterclockwise, dust devils form on sunny, warm and relatively windless days, usually over expanses of dirt or pavement. Hot air at the surface rises into cooler air above, forming a swirling column often extending 50 feet or more into the air. Wind speeds can surpass 50 mph, though they’re usually in the 20-30 mph range. Dust devils are rather rare here, but they frequently occur in desert areas.