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Dust devils and dust storms

Dust devils

On Earth, watch out for spirals of dust starting to whirl unexpectedly upwards into dry air. They are likely to disappear as suddenly as they appeared, with another one whipping up nearby a few minutes later.

Dust devils are the result of hot summer sun beating down on desert sands. In local hotspots, surface temperatures can reach as much as 55°C. The air above a hotspot is warmed more than the air over the surrounding land, and rises as a strong updraught, pulling in cooler air to replace it. The inflow of cooler air from every direction helps the rising air to twist and lift fine soil into the spiral.

At the base of the dust devil, wind speeds can be as much as 45mph (75kph). Dust devils are usually less than 1 metre in diameter, and last a minute or less. The biggest can be up to 100 metres across and hundreds of metres high, and can last for up to half an hour.

A typical Nevada desert valley experiences several dozen sizeable dust devils every summer day. Each of them pumps more than 100kg of fine soil many hundreds of metres into the air.

On Mars, dust devils play a major role in maintaining the dust-laden atmosphere. Because the Martian atmosphere is much thinner than the Earth's atmosphere, Martian dust devils can reach 0.6 miles (1km) in diameter and lift huge amounts of dust 5 miles (8km) above the surface of the planet.

Dust storms

You'll know a dust storm is on its way by the appearance of a huge wall of dust moving inexorably towards you across the landscape. Dust storms form when there are huge amounts of fine soil available to be carried into the air by surface winds and dust devils.

On Earth, giant dust storms enveloped the American mid-west for several years in the 1930s. During the long drought of 1983, a 1,000ft (300m) high dust storm enveloped the city of Melbourne in Australia.

Much of the surface of Mars is covered in fine dust. Clusters of powerful dust devils and 100mph surface winds create vast dust storms which can envelop the surface of the entire planet with dust for several months at a time. The worst storms are likely to occur in the southern summer when Mars is nearest the Sun and the planet receives its maximum amount of solar heat.

Dust storms play an important role in the Martian climate, redistributing heat and controlling local weather patterns.

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Watch geologist Steve Metzger chase a dust devil

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