Dust Whirls. 821 



.one mile, that the column was approximately vertical, and that 

 the angle of elevation was 80°, the dust Avhirl would be not far 

 short of six miles in height; and if it were only half a mile away 

 (which, however, seemed altogether too short a distance), the 

 height would approximate towards three miles. 



These figures are probably far too high, due, perhaps, to the 

 assumption that tli/e dust column was approximately vertical. 

 The column might appear to be vertical, and yet could perhaps be 

 much bent towards the observer. This would materially reduce 

 the figures. 



Another means of checking the height is to ascertain the height of 

 either of the two clouds associated with the dust columns, )>ut no 

 data have been obtained for this. These clouds were two of many 

 similar scattered over, all apparently of moderate height. Again, 

 if the general average height that these clouds form at were known, 

 an idea of the height of the dust whirl would be obtained, but 



(there appears to be hardly any information available as to such 

 clouds in Australia in this connection. Records of other countries 

 show that the upper surfaces of cumulus clouds may be over 3000 

 feet high. 16 lliere can be no doubt, however, that this particular 

 dust whirl wa» of great height, reaching probably to several thou- 

 ,. sands of feet above the earth's surface, and the quantity of dust 

 raised must have been enormous. When first viewed close to the 

 .sun's rays, the dust whirls presented a majestic spectacle. 



The writer is indebted to Dr. Griffith Taylor for a reference to 

 Mr. Quayle's Memoir on Clouds, and for some information con- 

 ■ cerning clouds, which he kindly obtained from Mr. Quayle. 



Dii.<if Whirls in Relation to Erosion. 

 Dust whirls must play an important part in the erosion of sub- 



.arid Western Australia. From the preceding table it will be 

 recognised that they are fairly numerous, even in a small area, 



.and that they include columns of dust of great height. It must 

 also be remembered that, unless high, numerous whirls, even com- 

 paratively close to an observer, are not seen by him. If. the whole 



• of the sub-arid portions^ of the State be considered, a vast amount 

 of fine sand, and dust must be displaced even in the course of one 

 favourable day. The material is either lifted well into the air, or 

 is dragged along or kept cloee to the surface of the ground. In 

 ;the former case the material is chiefly fine dust, and in the latter 



16. Davis. Op. cit., pp. 179 and 180. 



