Drivers marvel at dust devil over Hwy. 401 in Mississauga

Drivers cruising on Hwy. 401 near Mississauga last weekend encountered a bewildering sight towering over the highway.

"It looked like a tornado," said Qasim Swalah, who captured the phenomenon on video near Hurontario Street last Saturday.

Fortunately, it was not and instead a harmless dust devil — a rotating column of air made visible by the dust and debris it gathers.

"You could see all the debris from below just turning round and round," Swalah told CBC News. "It was something exciting for me. I've never seen something like that."

At least one person praised Swalah for stellar video editing but the weather conditions that afternoon were perfect for dust devils to form, according to Environment Canada senior climatologist Dave Phillips.

The weather agency does not consider dust devils severe weather so sightings are not recorded in its database but Phillips said "I have no doubt that the dust devil is real."

The dust devil was mistaken for a tornado by others who saw it last weekend.

But Phillips said it "formed from the ground upwards, not like tornadoes which spin from rotating clouds."

He added that the size of the dust devil was impressive in its diameter and height and stayed intact longer than most dust devils. 

How do dust devils form?

Unlike a tornado, which is spawned by a strong thunderstorm, dust devils form on clear and calm days where the sun is able to heat up the surface, according to CBC Toronto meteorologist Jay Scotland.

This heated surface air begins to rise rapidly through the much cooler air around it just like a hot air balloon.

The other ingredient needed to make a dust devil is wind shear. Wind shear is wind blowing from either different directions or at different speeds with height through the atmosphere. This is what causes the rising column of air to spin.

If conditions are right, dust devils can become very big like the one seen in the video but are rarely harmful.