Swirl of dust

A swirl of dust is formed by good weather , when dry and unstable air enters in rotation and raises the dust or the sand of the ground. The swirls of dust are known like willy-willies in Australia, according to an indigenous word , and dust devils with the the United States of America. Arabic thought that these phenomena were related to the Djinn S.

These swirls have a Diamètre energy of some Centimètre S with more than 10 Mètre S and have a vertical extension of a few meters to more than 1.000 Mr. the majority of the swirls of dust are not dangerous but some can be enough powerful to raise heavy objects. For example the May 7th 2006, Trenton (North Dakota) a 4 year old little girl played on a trampoline. The latter was raised of 8 meters in the airs what caused minor wounds with the young girl.

Cause

In an area where one has a important Insolation, the air is heated close to surface. Heat close to the ground is transmitted vertically by radiation to a more or less thick layer of air. If the air is dry in this layer, the variation in temperature follows the dry adiabatic and is very unstable.

As the ground is not heated uniformly, there will be differences between two points close to the ground. The hotter air moving towards a fresher zone thus will undergo a Poussée of Archimedes upwards and will enter in Convection. The air going up will be replaced by air coming from another point on the ground and according to the configuration of the winds, this will be able to generate a rotation of the air.

The favorable conditions with the production of a swirl of dust are:

dry Air in the layer between surface and 700 hPa
Difference between the Temperature and the Dewpoint of surface of at least 16°C
Weak winds in the layer between surface and 850 hPa
Zone in surface where the warming can vary (borders between a parking asphalted and the grass for example).

The height and the diameter of the swirl depend on the instability and the dryness of the air. By looking at a thermodynamic Diagram, like a Téphigramme or a émagramme, one can evaluate the Potential Énergie of Convection Available (EPCD) and thus the potential intensity of the swirl.

The formation of the swirls of dust is similar to that of very weak the Tornade S but as the air is dry, no condensation will be formed and thus not cloud. Contrary to the tornadoes, the swirls of dust cannot go to seek the winds of the higher levels for their rotation what in limit their power.

Places favorable to the development

The swirls of dust can occur a little anywhere since one obtains the requirements are dry and unstable air with a difference in warming on the ground. In the moderated climates, they generally occur in spring whereas the air is still dry but that the days lengthen. One generally sees them in the parkings or another place which absorbs more energy than the surrounding zones. In the deserts, they are very current in any season because the air is dry and hot there by definition. Downtown, they will be of short duration because the houses and other buildings prevent them from circulating with complete freedom, these swirls often dying by striking an obstacle.

Electric activity

The particles of dust while moving will produce static electricity and when the fields is rather intense in the swirl (more than 10.000 volts per meter), that can generate a weak discharge and a radio signal.

Swirl of fire

The favorable conditions with a swirl of dust can be found in the cases of an intense fire. Indeed, the differences of temperatures around a Forest fire or of a broad blazing inferno will generate a circulation of surface of the colder zones towards the source of heat. If the mass of air is unstable, a swirl will be formed and the remains of the fire will be aspired there. One can apply the same techniques of forecast in this case.

Swirls over Mars

The Ground is not the only place where swirls of dust were seen. As of the missions Viking of the years 1970, one could notice some on red planet and in 1997, the Mars Pathfinder detected a swirl passing to him above.

The black trail in the photograph of right-hand side is the result of such a phenomenon over Mars. The blackest point is the swirl itself which goes up the wall of a crater.

One observes even broad a dry Cyclone on this planet which comes from such an origin.

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