212 THE WEATHER 



the drainage of cold air from the lower surrounding slopes. 

 On the level uplands, the chilled air does not drain away, also 

 radiation is freer there because of elevation and so increases 

 upland cooling. Because of these conditions many of the 

 orchards in the fruit-growing regions of America and Europe 

 are found on hillsides (Fig. 146). 



239. Wind and Frost. Occasionally frosts are prevented 

 by wind. The wind by stirring and mixing the air constantly 

 brings fresh supplies of warmer air in contact with the cool- 

 ing ground and vegetation, and so keeps all temperatures 

 above freezing. 



Figures 147 and 148 show the average dates of earliest kill- 

 ing frost in autumn and latest in spring for the United States 

 east of the Rocky Mountains. 



IV. THE WATER VAPOR OF THE AIR 



240. The Moisture of the Air. The presence of invisible 

 water vapor in the air is shown by the formation of clouds, 

 fog, rain, snow, dew, and frost. This vapor is of great im- 

 portance. It supplies directly or indirectly all of the water 

 found on the land areas of the earth. It affects our comfort, 

 health, and business nearly as much as the temperature does. 



Water vapor extends above the loftiest clouds but is 

 mostly in the lower 2 or 3 miles of air. It mixes readily with 

 dry air. When present in large amounts, water vapor makes 

 the atmosphere occupy a little more space than the dry air 

 alone would fill. Water vapor is about %o as dense, or heavy, 

 as dry air, and when much vapor is present the resulting 

 moist air is lighter than dry air at the same temperature. 

 This slightly reduces the total pressure of the atmosphere in 

 that locality and makes the barometer read slightly lower than 

 it would under the same conditions if no water vapor were 

 present. 



241. Moisture of the Air ; Its Source. This vapor of water 

 comes by evaporation, chiefly from water surfaces, vegetation 



