snow devils or “snownadoes” are extremely rare weather phenomena. So rare in fact, that only six have ever been captured on camera — with four of those photos coming from Ontario, Canada. These events are so uncommon because they require very specific meteorological conditions to form.
Since snownadoes are so rare, very little is known about them. However, they are closely related to waterspouts — a rotating column of air that forms over water — since they form over either frozen lakes or snow-covered areas.
For snow devils to occur, the necessary conditions include a colder air mass passing over a relatively warmer surface heated by sunlight, and a low-level wind shear (change of wind speed or direction with height) or colliding air currents to get the rising air to spin.
A warmer surface causes the snow or ice to form fog or steam, and if there is a column of colder, low-pressure air above this fog, it will begin to rise, and the wind shear or currents will cause it to rotate and begin to pick up loose snow forming the recognizable funnel shape.
The combination of these conditions is what makes snownadoes so rare and less intense than tornadoes — which are extremely dangerous. However, snownadoes have been reported as large as 30 feet wide, 45 feet high, and capable of lifting objects over 1,500 pounds.
Snownadoes usually occur under or before a snow squall (a short, but intense period of heavy snowfall, often accompanied by strong winds), meaning they are often indicators that more snow is on the way.