Earlier this Spring on two successive days, I was driving along our gravel road and noticed a ‘dust devil’ at the same location each time. I am sure that everyone has seen such things at some point in their lives. Seeing one is not at all unusual but noting two in the manner that they occurred prompted me to prepare this little note.
The conditions that must exist to form the dust devils will likely occur many times but they remain invisible. We only see them when they cause fine dust, leaves, paper and similar light materials, rarely snow, to be swept up into a circular funnel. Without the telltale visual evidence, we pay no attention for we simply cannot detect them. Similarly, we seldom have the opportunity to photograph them. They typically only last a few seconds so unless we have a camera already at hand, we don’t get a fair chance to document their existence.
In many respects, they are like miniature tornados with a spinning vortex. ‘Whirlwind’ is a fairly good description. They may be as small as meter wide and ten meters high to rare ones that might be ten times as wide and one hundred times as tall. The motion is nearly always upwards in the form of a swirling updraft under sunny conditions during fair weather. A basic requirement is that there are areas with a differential in heating of different air pockets. The devils form when hot air near the ground surface rises quickly through a small pocket of overlying cooler, low- pressure air. Under the right conditions, the air may begin to rotate. As the air rapidly rises, the column of hot air is stretched vertically, causing intensification of the spinning effect. A fully-formed dust devil is a funnel-like chimney through which hot air moves, both upwards and in a circle. Additional hot air rushes in toward the developing vortex to replace the air that is rising As the hot air rises, it cools, loses its buoyancy and eventually ceases to rise. As it rises, it displaces air which descends outside the core of the vortex. The spinning motion cause the dust devil to move forward. The upward speed of the air within the vortex has been reported to be at least 40 km/hr. On occasion, wind speed in the rotating air can reach as high as 120 km/hr.
Dust devils are most evident in flat terrain that lacks vegetation, or in fields, deserts, or tarmac. Such conditions favor the build-up of heat near the ground surface as well as the increase the chance that dust or sand may be swept up. Clear, or relatively clear, skies enable solar energy to easily reach the ground to warm the surface yet have cooler air above. Ambient wind needs to be very low or absent or else there would be too much mixing of the cooler upper air with the heated air at ground level. If two distinctly different air temperature regimes cannot be established, then a dust devil cannot form.
Somewhat similar vortices form in the center of large fires like forest fires where there is a rather obvious source of heated air and smoke. Vortices also occur in the lea of buildings that are exposed to winds, depending upon the size and shape of building and other characteristics of the surrounding environment. The latter are notably different in origin, arising from mostly lateral winds. The swirling motion can cause snow to form drifts and other debris to form circular piles on the downwind side of the structures. These materials fall to the ground in the centre of the vortex which lacks the air speed to keep the materials suspended.
Dust devils seldom cause significant damage or injury, simply because they are limited in size and do not last. On occasion, they might do limited damage if the particular conditions allow the wind speed to be unusually high. The wind movement could potentially move materials from one place to another and this may be undesirable from a human perspective. Dust might become entrained in the wind and this might be quite undesirable. They could cause the components of the dust or creatures nearthe ground surface (e.g. insects, plant pathogens, weed seeds) to become airborne and moved to new locations. Occasionally small rodents could be forcibly moved from one place to another. In general though, dust devils are mostly just fleeting visible indicators of the fact that our physical environment is constantly in flux.
by W.D. McIlveen
Halton/North Peel Naturalist Club