There was an unusual formation seen over the weekend in the burn scar of the Horse Gulch Fire east of Helena: what looked to some like a skinny dark tornado was actually a dust devil.
These dust-filled vortices are created by strong surface heating and are generally smaller and weaker than a tornado, but sometimes not by much.
Dust devils can have internal wind speeds of up to 100 miles per her.
Typical diameters of dust devils can be just a few feet to as wide as 300 feet. The height can be small but top out at nearly 500 to 1,000 feet, appearing like a tornado. A dust devil can last an hour or more.
Even though they are generally smaller than tornadoes, dust devils can be destructive as they lift dust and other debris into the air. Small buildings can be damaged and even destroyed by a strong dust devil.
Dust devils form with strong surface heating, typically with clear skies and light wind when the ground can warm the air to temperatures well above the temperatures just about the ground. The hot air at the ground will create a sudden uprush as heat rises, and the rising air forms a vacuum of air inward at the bottom, producing a spin.
This dust devil formed over the hot black ash on the ground from the horse gulch fire. It appears to lift soot a few hundred feet into the air. Wind speeds could have reached 40 to 50mph.
