Page:Popular Science Monthly Volume 49.djvu/679

Rh 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:

Supplementing these direct observations, estimates have been obtained 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 continued 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, thirty-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, tornadoliketornado like [sic] winds. A mean of the averages is not far from twenty-four hours, and this is perhaps a safer estimate of the average time of a single dust storm. These atmospheric 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.