DUST AND SAND STORMS IN THE WEST. 657 



This gives an average diameter of two hundred and sixteen 

 miles. 



Information has also been sought with regard to the length of 

 time that these storms continue to blow at any one place. In 

 several instances the reports state this time, and in these cases it 

 varies from one to forty-two hours, and averages a little over 

 thirteen hours, as may be seen below : 



Duration of Dust Storms. 



2 storms lasted for 1 hour. 



2 " " " 2 hours. 



1 Btorm " " 4 " 



2 storms " " 6 " 



2 " " " 8 " 



2 " " " 12 " 



1 storm " " 16 " 



1 " " " 18 " 



2 storms " " 24 " 



2 " " " 30 " 



1 storm " " 42 " 



Supplementing these direct observations, estimates have been ob- 

 tained from a few parties, and these make the time considerably 

 longer. These estimates are as follows : " Sometimes an hour, 

 sometimes three days, coming with great violence at intervals.^' 

 (One hour, seventy-two hours.) — " We have known one that con- 

 tinued a week, with one day for a recess." — " The sand storms last 

 from one day to three days, but sometimes only a few hours." 

 (Four hours, twenty-four hours, seventy- two hours.) — "The sand 

 storms last about one day." (Twenty-four hours.) 



Leaving out the storm that lasted six days, thirtj^-two hours 

 would appear to be a fair average for the other estimates. But 

 these estimates perhaps apply more particularly to the greatest 

 storms, while the previous table no doubt includes a rather large 

 proportion of short, tornadolike winds. A mean of the averages 

 is not far from twenty-four hours, and this is perhaps a safer esti- 

 mate of the average time of a single dust storm. These atmos- 

 pheric disturbances occur on the steeper gradients of areas of 

 low barometric pressure, and they partake of the progressive 

 eastward motion of these. Their duration at any particular 

 point will hence depend upon their areal extent and upon the 

 velocity of the low area. If the diameter of the dust storm be 

 divided by the time, we will have this velocity, which in this case 

 will be less than ten miles an hour. This is less than half the 

 usual rate of eastward progress of a low area in the eastern part 

 of the United States, but it corresponds more nearly with the 

 rate observed in the Great Basin, where most of these storms 

 occurred. 



VOU XLIX. — 52 



