134 Principles of Plant Culture. 



is 4 below the freezing point of water. A slight 

 wind, a hazy atmosphere, or a few fleecy clouds would 

 render frost doubtful. With a dry-bulb reading of 45 

 and a dew point of 25, a killing frost might be ex- 

 pected. 



209. 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." 



210. Wind Tends to Avert Frost because it prevents 

 the settling of the colder air and thus keeps the tem- 

 perature of the lower strata of the atmosphere nearly 

 uniform. 



211. Clouds, Haze and Smoke Tend to Avert Frost 

 because they act to some extent like a blanket in pre- 

 venting the radiation of heat from the earth, and thus 

 check the fall in temperature (216). 



212. The Proximity of a Body of Water Tends 

 to Avert Frost because the water cools slower than the 

 air and thus checks the fall in temperature of the at- 

 mosphere in the vicinity; also because it keeps the 

 neighboring atmosphere moist, thereby raising the tem- 

 perature of its dew point (205). The proximity of 

 buildings and trees tends to avert frost, probably be- 

 cause these objects give up their heat gradually and 

 thus temper the surrounding atmosphere. 



