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THE MYSTERIOUS DUST DEVIL

                                                     A local whirl of dust, usually not more than a few yards in diameter, in which the particles are swept round and round the centre, and are lifted to considerable heights –sometimes to 2,000 or 3,000 feet above the earth’s surface. It is caused by excessive local heating by the sun in an arid region, a strong convection current being formed. Dust devils usually move over the desert at from 5 to 15 miles per hour, though speeds of over 30 miles per hour are sometimes reached, and as many as five or six of them may often be counted by the observer. Being so limited in size, they are harmless even to aviation.       A storm in which a thick mass of dust obscures the atmosphere, and reduces visibility very considerably –sometimes practically to zero. It causes acute discomfort on the surface, and is a danger to aviation. It is caused by a turbulent wind blowing over an arid, dusty surface, and its approach is usually heralded by