Atmosphere And Weather

How Dust Devils Form



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Dust devils, or tornadoes as they are sometimes mistaken to be are well formed, strong whirlling winds that have relatively long life spans. Their sizes range from as small as 0.5 metres wide with a height of about 10 metres, to as large as 1000 metres high, thus giving it a true tornadoe appearance. dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from half a meter wide and a few meters tall to over 10 meters wide and over 1000 meters tall. Dust devils appear all over the world and therefore, have many different names. For example in Australia they are knon as willy-willys, while in the middle east they are referred to as djin (devil).

FORMATION occurs in in relatively flat areas that experience a lot of surface heating. The flat topography provide an endless supply of heat and dust. The later being the key ingredient that allows the viewer to actually see the rotations. This is why flat deserts are an ideal place, as the surface heating is high and there is plenty of dust to create a visible vortex. However with enough surface heating, dust devils are not only confined to deserts. They have been known to form on bitumen, carparks, farm lands, irrigated fields and even firey remains. Little wind shear, clear skies are great aids in forming dust devils as heat is not dispersed but becomes concentrated on the surface. Thus creating a large temperature gradient between the surface and the rest of the atmopshere. By laws of physics this creates rotation.

Usually the dust devils are relatively weak, with average winds clocking at about 70 km/h. However, at such intensities their fuel of warm air runs out and they break down. There have been reports of dust debils exceeding the diemtere of 90 metres and reaching speeds upwards of 100 kmh. Such large systems can last for more than 20 minutes at a time, as was recorded on a few occasions.

FORMATION PROCESS is such, that as the surface warms, the density of the air at the surface decreases. This forces the air to rise, as it rises the air encounters much cooler air in the atmosphere above. Since the cooler air has a much higher density, it automatically resists against the warmer, lighter air. This creates a very fragile stability which can easily be broken, by a small gust of wind, a body passing by, such as a person or a vehicle. Once this occurs the parcel of warm air accelerates upwards, thus causing a whirlwind. The rush of warm air also causes this air to move horizontally inward and into the bottom of the freshly formed funnel. Thus the circular motion of air inward is highly accelerated. As the dust devil moves further up, the rotational speed is increased to due the conservation of angular momentum. Eventually the effect becomes sustaining, as the hot air that rises is quickly replenished by more hot air that rushes in from the surroundings.

The formation of a dust devil becomes complete, when dust is sucked into the body and rotates along the dust devils structure, thus making it visible. An equilibrium between the outside cooler air and the rising hot air within the funnel is achieved. As this occurs the dust devil experiences horizontal movement and automatically moves to areas of warmer surface conditions. Once the warm air supplies are exhausted, cooler air begins to enter the vortex, once this process is underway, the dust devil breaks down almost immediately and dissipates.

INTENSITY of tornadoes are caused by supercell conditions, while dust devils are craeted from rotatory updrafts due to immense surface heating. Although not as powerful as tornadoes, dust devils can grow to a significant size, that allows them to cause much damage.

IMPACT, that dust devils have on surrounding is often minimal. However, the large twisters have caused destruction such as the tearing of house roofs and turning over of cars, as well as uprooting of crop. Interestingly, dust devils are not just localised to planet Earth. Various Martian surveyors have reported dust devil activity on the surface of planet Mars. However, due to more violent atmospheric conditions on Mars, dust devils have been known to grow to almost full size tornado like entitys.

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