46 ELEMENTARY FORESTRY. 



much space to give here, the best authorities unite in attributing 

 them to local origin. 



Mr. George C. Curtiss describes the process of the produc- 

 tion of a typical hot wind as follows: "The necessary condi- 

 tions are those of the 'warm wave,' namely, a diminishing pres- 

 sure to the northward, producing southerly winds which initially 

 elevate the temperature above the normal. A cloudless sky 

 favors an intense insolation, as a result of which the dry ground 

 is soon raised to an extreme temperature, and the air is heated 

 from it by radiation, reflection and conduction. The resulting 

 diminution of density due to the rise of temperature furnishes 

 impetus to previously existing horizontal currents, and by 10 

 o'clock in the morning the hot wind is fully developed. Hun- 

 dreds of miles of hot dry earth contribute to maintain and feed 

 the current, and, gathering strength as the sun mounts higher, 

 the hot wind sweeps over the defenseless prairie. Neither hills 

 nor forests rise in its path to break its power or dispute its sway, 

 and, with no enemy save the tardy raincloud, the fetid blast sucks 

 out the life-sap of the growing grain. It will be readily seen 

 then that each of the states, Kansas, Nebraska and North and 

 South Dakota, develops its own hot winds and cannot charge 

 them to the account of its neighbors." 



The local origin of these winds at once suggests the desira- 

 bility of frequent windbreaks on the prairie farms, as offering 

 the most practical way of breaking them up. Irrigation of 

 large areas will also undoubtedly do much to prevent them. 



