206 MINNESOTA BOTANICAL STUDIES 



the time that the anemometers were operated there were days when 

 the instrument placed on the blow-out rim indicated an hourly 

 velocity much higher than this figure. Single hours during espe- 

 cially windy days showed a velocity varying from 35 to 60 miles per 

 hour at this station. 



1 20 



Fig. 5. Wind velocity. The graphs represent averages of 12-hour periods 

 for 21 days. 



A phase of wind action that must not be overlooked is related 

 to prairie fires. The region is subject to thunderstorms during 

 the spring and early summer months. These storms are usually 

 accompanied by much lightning which is often responsible for the 

 kindling of prairie fires. Fires are also started near railways by 

 "live" cinders from passing locomotives. Occasionally numerous 

 other sources of origin result in the initiation of these conditions. 

 Irrespective of the origin of a prairie fire, if the wind is high at the 

 time and fire-fighters few, as is usually the case, the fire frequently 

 becomes of uncontrollable proportions. Driven by a powerful wind 

 such a fire may sweep for miles across the hills and may sometimes 

 burn over thousands of acres. The immediate destruction of the 

 range is not the only damage done by such a fire. It may happen 

 after the fire has swept the uplands free of vegetation, that the wind 

 striking now upon the exposed sandy slopes results in the rejuvena- 

 tion of blow-outs and of general maximum sand movement so that 

 the grazing value of the area burned is reduced to an extremely low 

 figure for many years. 



The main sandhill region of the state may be said to lie within 



