The Dustfall of March 9, 1918. 605 



grams per square meter, or 4800 kilograms per square 

 kilometer ; in more familiar units, this amounts to more 

 than 13-5 tons per square mile. Observers of the U. S. 

 "Weather Bureau, quoted above, report that this colored 

 snow fell at least from Dubuque, Iowa, to Chelsea, Ver- 

 mont, in an east-west direction, and from Madison, Wis- 

 consin, to Newberry, Michigan, in a north-south direction. 

 This is about 900 miles east and west, and 300 miles north 

 and south as shown by the map, fig. 1, on which the 

 localities are indicated at which the discolored snow was 

 observed. It covered an area of at least one hundred 

 thousand square miles and probably much more. There- 

 fore, the total quantity of dust may be estimated as at 

 least a million tons, and probably considerably more. In 

 fact, it seems likely that the material was brought down 

 throughout the area covered by this snow storm, and in 

 that case, the quantity deposited would run into the tens 

 or hundreds of millions of tons. 



Origin of the dust. — While the meteorological data do 

 not afford evidence as to the exact locality from which the 

 dust came that was deposited at Madison, yet the possible 

 field may be limited very materially by appealing to them. 



The winds near the ground can be eliminated at once, 

 first, because the dust was brought down by sleet, which 

 is known to be frozen rain, that is to say, rain formed in 

 an upper, warmer, stratum, falls through a cold lower 

 stratum and is frozen in it; and second, because the 

 lower wind, traced back along its course, is found to have 

 come from the northeast, blowing only over snow-covered 

 ground, and the waters of Lake Michigan, during the 

 time that it was under the influence of the storm, so that 

 it could not have blown up soil or sand. 



In dealing with upper air currents, say from 500 to 

 2000 meters above the ground, it is usually assumed by 

 meteorologists, that the velocity is determined by the dis- 

 tribution of pressure as observed with barometers at the 

 surface, and that the direction is along the momentary 

 direction'of the isobar. The velocity of the wind has been 

 shown by Shaw 2 to be a resultant of the gradient velocity 

 and the storm movement in only certain types of storms, 

 but for the sake of simplicity in obtaining a first approxi- 

 mation, these conditions have been assumed in this case. 



2 Eevolving fluid in the atmosphere, Proc. Koy. Soc, Lond., ser. A, 94, 

 p. 34-52. 



