Injury from Cold During Growing Period. 125 
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 accumulates 
in the lowest places, which explains the familiar fact that 
hollows and vallies are colder in still weather than ridges 
and mountains. Ina falling temperature and in the absence 
of wind, gentle currents of the colder air tend to follow the 
natural water courses. This explains 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 tempera- 
ture of the lower strata of the atmosphere nearly uniform. 
212. Clouds, Haze and Smoke Tend to Avert Frost 
because 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 off 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 and thus raises the temperature of its dew 
point (206). The proximity of buildings and trees tends to 
avert frost, probably because these objects give up their 
heat gradually, and thus temper the surrounding atmosphere. 
214. The Localities Most Subject to Untimely 
Frosts are narrow and deep vallies that are inclosed on all 
sides, and inclined vallies that serve as channels through 
which cold air flows to lower levels. Partially cleared dis- 
tricts usually suffer more from frosts than those fully cleared, 
because the remaining forests obstruct the air drainage. 
