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MICHIGAN ACADEMY OF SCIENCE, 



4. Map showing wind directions and velocities, also extent of snow covering, in relation to 



loess soil regions, January 27, 1902. 

 Meteorological data taken from reports of Weather Bureau. 

 The loess is shaded. 



On January 20, just one week before the dustfall, the loess region, 

 except in the Ohio valley was mostly under a thin cover of snow. On 

 the date of the shower the area of snow had been greatly extended, 

 especially westward, and the depth considerably increased. Much of 

 the loess was then under six inches and more of snow, but the tract 

 nearest to us in southern Wisconsin was outside the three inch limit and 

 some of it may have been practically bare. In middle Nebraska, South 

 Dakota and in southern and western Illinois the snow covering was also 

 light and in these regions there may have been some exposed soil. These- 

 conditions had remained essentially unchanged during 48 hours or more 

 preceding the dustfall. It remained then to choose whether the dust had 

 probably been raised in remote parts and its long transit made longer in 

 time and space by shifting winds or whether it had been transported from 

 the nearest locality in the course of a few hours. 



On January 25, when the storm center was far west, the winds were 

 light and mostly from eastward over the Mississippi and lower Missouri 

 valleys. The snow cover had not been materially increased since the 

 twentieth but there was not sufficient energy in the winds to disturb an 

 exposed surface of the lightest soil. On the following morning, after 

 most of the new snow had fallen, the winds were decidedly stronger and 

 mostly from westward. The highest velocity reported by the Weather 

 Bureau was 36 miles per hour at Duluth. Other significant records were: 

 St. Paul, 20; Milwaukee, 14; Des Moines, 12; Davenport, 12; Sioux City,. 

 20; Omaha, 18; Chicago, 18; Kansas City, 18. These velocities are not 

 high but they indicate a general movement of air currents of considerable 

 force. 



On the twenty-seventh the general direction of the winds was from the- 

 w^est but velocities had become much lower west of the Mississippi. In 

 the lake region velocities remained about as on the previous date. a» 



