Farmers' Bulletin 1096. 



as water, as the vertical movement of the air takes place so slowly 

 that the heating effect due to decrease in altitude is more than offset 

 by the cooling due to contact with the ground which has been cooled 

 by radiation. When we are dealing with a steep hillside, however, 

 movements of the surface air are more complex and have little simi- 

 larity to the flow of water down the same slope. 



Like the more nearly level lower ground, the slopes and summits of 

 hills and ridges lose their heat rapidly after sundown through ra- 

 diation, and their temperature falls. The air in immediate contact 

 with them also cools through conduction so that it is soon cooler 



64 



G0 



55' 



SO' 



45' 



40' 



30' 



5' 





\ 



y 



Temperature 225 feet above base station. 



Temperatt/rf feet above base 3 tat/on . 



Temperature 25 feef above base stat/on. 



Temperature af base sfof/on 



FIG. 1. Continuous records of the temperature from 4 p. m. to 9 a. m. at the base and at 

 different heights above the base of a steep hillside, showing the great differences in 

 temperature that sometimes develop on a clear, still night. Although the temperature 

 at the base was low enough to cause considerable damage to -fruit, the lowest tempera- 

 ture 225 feet above on the slope was only 51. Note that the duration of the lowest 

 temperature was much shorter on the hillside than at the base. 



than- the air at some distance out over the valley, but at the same 

 elevation. 



If this cooler air in contact with the hillside begins to flow down- 

 ward directly along the surface of the ground, its altitude will be 

 decreasing more or less rapidly, according to the steepness of the 

 slope, and its density will be increasing. If no further cooling takes 

 place, it will be surrounded by air increasingly colder as it nears 

 the valley floor, while its own temperature tends to increase because 

 of the compression it suffers. As soon as a position is reached where 

 it is warmer than the air surrounding it, its downward movement 

 will be checked and it will tend to rise again until its temperature 

 is the same as that of the air surrounding it. (See fig. 1.) 



