UNFAVORABLE TEMPERATURE 125 



ering half a continent, the atmospheric pressure at the 

 center is high, whence they are also called " highs," the 

 weather is clear, and the wind is calm or light in the center, 

 and flows slowly outward in a clockwise spiral in the outer 

 zones of the anticyclone. The temperature of the clear 

 central area is lower than that of surrounding districts, 

 even lower than those to the north, because of the free 

 radiation of heat from the earth through the clear air, 

 while the cloudiness of the surrounding districts has dimin- 

 ished the night fall of temperature. 



Anticyclones move eastward or southeastward from 

 western Canada, sometimes slowly, sometimes rapidly, 

 at a rate averaging 26 miles an hour. They are shown, 

 with their attendant temperatures, on the weather maps 

 issued by the offices of the United States Weather Bureau. 

 ' Their approach can be observed and their central tem- 

 peratures studied a day or two before the psychrometric 

 method described above can be applied at all. The 

 weather maps and forecasts are sent free only for public 

 display or distribution, but every farmer should arrange 

 to have access to them at the postoffice, store, warehouse, 

 creamery or on the rural mail carrier's wagon where they 

 are posted every day during seasons when frosts are likely 

 to occur. 



210. Cold-air drainage. — Warm air, being lighter 

 than cold air, tends to rise, while the colder air tends to 

 fall. In a still atmosphere, therefore, the cold air accu- 

 mulates in the lowest places. This explains the familiar 

 fact that hollows and valleys are colder in still weather 

 than ridges and mountains. In a falling temperature and 

 in the absence of wind, gentle currents of the colder 

 air tend to follow the water courses, which explains in 

 part why frost so often " goes in streaks." 



