Strong swirling winds, but over a small area. Spinning dust and debris. Brief, often a handful of minutes. Almost out of nowhere on a warm, clear day.
The literature that studies them calls them dust devils, but to the people that live with them they are dust whirls, whirlwinds, sand trumpets, sand hoses, devils and spirits. In Australia, one might be called a Willy-Willy. In Ireland, a “sí gaoithe“, which translates to something along the lines of “fairy wind“.
They don’t actually require dust or sand. Dust and sand do serve to make these sudden little whirlwinds visible, but they aren’t a requirement. In fact, the whirlwind need not be visible at all.
And they are whirlwinds. Small and brief whirlwinds, to be sure, but whirlwinds none the less. While they appear somewhat akin to a tornado, the formation is somewhat different. A dust devil is generally the product of a warm day with clear skies.
We had another one through recently, resulting in some impressive destruction in a small area. It happens on a warm day, when the sun heats up an area on the ground (asphalt and other dark surfaces heat up more) and that hot air rises. Then, cooler air from nearby rushes in to fill it. A spinning motion forms, and a dust devil is born.
So where are these likely to happen? In areas where heating occurs beside places which will be cooler. Perhaps because they are shaded, perhaps because they are near water, perhaps simply because a nearby lawn has recently been watered and is cool.
Where do they go once formed? In the direction a light breeze pushes them, up hills but not down them. They will cross water and travel through forests (Benjamin Franklin wrote an account of one doing that). Sometimes, though this is less common, they stay in place.
The general gist is that dust devils are going to be more common in some areas than others, based on the local conditions and landscape. They’re generally going to follow similar routes and stay moving. Landscaping may present some potential for shielding buildings that are routinely hit by them. While these little whirlwinds could theoretically last most of a day, they’ll typically disperse within 5 minutes. Dust devils are found across the world and usually do not cause major damage to people or structures.
Also, if anyone thought the reading was boring, it turns out that the early studies to determine maximum wind speed in dust devils involving dropping kangaroo rats from two and three story buildings. The rats were apparently angry, but unharmed. If you’re interested in the particulars, you can find them towards the end of the fifth page in the pdf. Enjoy!
Trego's Mountain Ear 