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 a wall of dust which extends upwards for several thousand feet –sometimes to over 3,000m.
     In advance of the storm, the wind is light and variably, but increases in strength as the storm approaches. Duststorms are frequent in such arid desert regions as the plains of Iraq, North Africa, and Northwest India, and are especially liable to occur in thundery weather; the atmosphere is so dry that the rain associated with the thunderstorm often evaporates before reaching the ground, but if it does not, the dust is washed down with it, and the duststorm is thus much shortened in duration and restricted in scope. It should be distinguished from a sandstorm, a storm in which a mass of relatively coarse sand is blown along through the air by a strong wind. The sand particles are rarely raised more than 15 to 30 m. above the surface, and are not carried far from their source.  
     Dust-Well is a hollow, containing dust, in the surface of a glacier; it is formed when a patch of dust, blown on to the ice, absorbs heat from the sun more rapidly than does the ice itself, and causes the latter to melt, a depression being thus created.