208 THE WEATHER 



236. Frost Fighting ; by Checking Radiation. Smudge pots, 

 or fires of any fuel causing dense smoke, also various types 

 of overhead screens (Fig. 142), are often used in orchards to 

 prevent frost. The smoke or screens retard radiation and 

 sometimes keep the temperatures near the ground 4 to 6 

 warmer. 



237. Frost Fighting; by Warming the Ground Air. The 

 radiation that chills the night air takes place chiefly from the 

 ground and vegetation. Therefore, the air cools in a com- 

 paratively thin layer next to the ground. Cooling makes 



FIG. 143. Newly planted cranberry bog. Two or three inches of sand 

 were spread over the bog before the plants were set. The ditch at the side of 

 the bog drains the water off quickly when danger of frost is passed. 



it denser and heavier so that it remains on the ground and 

 continues to cool, while the air a few feet or a few yards 

 above remains considerably warmer throughout the night. 

 Orchardists often prevent, frost by warming this shallow 

 bottom layer of cold air with fires of oil, coal, wood, etc., placed 

 on the ground, 30 to 50, or more, per acre. 



Cranberry marshes, in Wisconsin and elsewhere, are often 

 flooded with water (often completely submerging the vines) 

 to protect them from frost. Water holds its heat better 



