606 



Winchell and Miller 



Various formulas, and tables for obtaining the gradient 

 velocity have been given by Shaw, Gold, Patterson, and 

 Humphreys. The revised nomogram of Humphreys 3 

 has been used in obtaining the trajectories marked A and 

 B in fig. 3, for the dust-bearing upper currents that 

 arrived at Madison at the beginning and end of the 

 observed time of the dustfall. 



Fig. 3. 



Fig. 3. Curve C. shows path of center of storm of March 7-10, 1918. 



Curve B. shows trajectory of upper air current that arrived at Madison, 

 Wis., at 11 : 30 A. M., March 9, when the dustfall began. 



Curve A. shows trajectory of upper air current that arrived at Madison, 

 Wis., at 3 : 00 p. m., at end of dustfall. 



Curve D. shows conjectured trajectory of dust-bearing lower current 

 ascending to upper stratum. 



Horizontal hatehures show snow-cover 7 P. M., March 4, 1918. 



NE-SW hatehures show area of rainfall during 24 hours preceding 

 7 a. m., March 8, 1918. 



NE-SE hatehures show area of rainfall during 24 hours preceding 

 7 a. m., March 9, 1918. 



The storm of March 7-10, 1918, was characterized by 

 strong winds at the surface throughout its passage from 

 Utah eastward, so that the mechanism for eroding the 

 surface and carrying the dust up into the atmosphere was 



3 Journal Franklin Inst., November, 1917, page 673, revised. 



