268 THE SMALL GRAINS 



consideration. Evaporation increases so much from north 

 to south, that even within the limits of the United States 

 it must be seriously considered as a third factor with 

 temperature and rainfall in the establishment of new 

 crops. From southwest Kansas southward, this factor 

 is so important comparatively, that the line of greatest 

 climatic resistance turns actually to southward in spite of 

 the winter temperature factor, which would tend to turn 

 it to the north. The amount of evaporation is affected, 

 locally, of course, by several conditions, including the 

 degree of clearness of the atmosphere. An example 

 of the effect of atmospheric haze is given by Briggs and 

 Belz (1913). In their observations, they found that 

 during the haze of four months following the eruption 

 of Mt. Katmai, the average evaporation, as meas- 

 ured at 15 stations, was reduced about 10 per cent from 

 normal. 



283. Winds. Evaporation is greatly increased by 

 high winds. Results of recent experiments seem to show 

 a much less effect upon plant transpiration, but the 

 increase of the latter from this cause is, without question, 

 considerable. Constant winds, amounting to a gale dur- 

 ing the day and continuing for several days at a time, which 

 often occur on the high plains, also cause much damage to 

 crops in a mechanical way, through soil blowing, and 

 whipping of plants against each other. On the south- 

 western plains high winds are probably the chief obstruc- 

 tion to successful " dry-land farming," and therefore the 

 great help to the latter will lie in the planting of drought- 

 resistant shrubs and trees in a form to best protect the 

 crops, such as hedges or narrow belts running at right 

 angles to the direction of prevailing winds. At certain 

 points there are known to be over 60 days of gales during 



