130 Principles of Plant Culture. 



Opposite 47°, iu the left hand column, and under 7 in 

 the top line, we find 2S°— the dew point. If these 

 readings are obtained toward sunset on a clear, still even- 

 ing, we should expect frost, because the dew point is 4 

 degrees 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 expected. 



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 colder air accumu- 

 lates 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." 



211. Wind Tends to Avert Frost because it prevents the 

 settling of the colder air and thus keeps the temperature 

 of the lower strata of the atmosphere nearly uniform. 



212. Clouds, Haze and Smoke Tend to Avert Frost be- 

 cause they act to some extent like a blanket in prevent- 

 ing the radiation of heat from the earth, and thus check 

 the fall in temperature (217). 



213. 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 atmosphere in 

 the vicinity; also because it keeps the neighboring at- 

 mosphere moist, thereby raising the temperature of its 

 dew point (206). The proximity of buildings and trees 

 tends to avert frost, probably because tliese objects give 



