Dust Devils

Dust-devils are silent, miniature tornadoes, due to extreme instability near hot ground. They differ from tornadoes in being less powerful, and in developing up from the ground instead of down from a cloud. Also, dust-devils usually occur under clear skies. They are triggered by wind gusts, which may be very strong when the lapse rate is superadiabatic. The intensity is enough to pick up dust, so they become visible, though there is a strong wind only close to the column. The column may be 1-50 m in diameter and reach several hundred metres high. A small dust-devil lasts only a few seconds, whilst a large one endures for half an hour.

They are common in arid parts of Australia (where they are called 'willy-willies'), as well as in Arizona and Egypt. They occur especially when dry, cleared ground is heated by strong sunshine in summer, typically between 11.30 a.m. and 2.30 p.m.

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