210 



THE WEATHER 



ables the lighting of the fires to be left until they are actually 

 needed, and shows the attendants where the fires are needed 

 first. All this saves expense. 



Cold air 

 level vplapds 



in valley 



FIG. 145. How frosts sometimes occur both on the uplands and in the 

 valley bottom, while the slopes escape. Solid arrows, cold air; broken arrows, 

 warm air. 



238. Air Drainage and Frost. On hilly or rolling land, 

 the valleys have frost later in spring and earlier in autumn than 

 the surrounding slopes and low hills. On a slope the cooled, 

 heavier air, close to the ground, settles downward and out- 



FIG. 146. One type of landscape where orchards are successful on inter- 

 mediate slopes, while frosts prevent them both above and below. Lemon 

 groves beyond the rounded oak-forested hills across the lake. 

 (F. A. Carpenter, in Monthly Weather Review.) 



ward and mixes with the warmer air above the lower slope; 

 while at the same time its place next the ground is taken by 

 warmer air moving in horizontally from the uncooled mass of 

 air above the valley (Fig. 145). These mixing movements 

 keep the temperature warmer on sloping ground, and often 



