Frosts in New York 515 



This phenomenon is easily explained; it is due mainly to the fact that 

 the outlet and slope of the valley are not sufficient to carry off, as rapidly 

 as it accumulates, the cold air that slides down the mountain sides. Thus, 

 the depth of the cold air in the valley increases as the night goes on, for 

 the same reason that water rises in and fills a valley when a dam is thrown 

 across its outlet. Now the warmer air, being lighter, floats on the surface 

 of the colder air that fills the valley, and, coming in contact with the 

 surface of the mountain sides, yields up its heat to the vegetation and 

 thus gives immunity from frost. The upper limit of the verdant zone 

 probably depends on the strength of the wind and the general temperature 

 of the free air. 



The effect of air drainage on frost conditions is often quite as marked 

 where the differences in elevation are much less than in the foregoing 

 examples. The so-called frost island, discovered by Winchell in the 

 southeastern part of Michigan, is really the bottom of a depression into 

 which the cold air settles. The surrounding elevated lands are much 

 freer from frost, though the difference in elevation is small. 



The lesson to be learned from these facts is that tender crops 

 which are liable to injury by frost should not be planted on low lands, 

 particularly when there is no outlet through which the cold air may drain 

 away. A low hill or a gently sloping hillside somewhat above the valley 

 floor is to be preferred as a location for an orchard or a garden. Just 

 how high one should go on the hillside can be determined usually by care- 

 ful observation of frosts for a single season. So much depends on the 

 lay of the land and the grade of the slope that no rule can be given which 

 will apply in all cases. With due regard to the foregoing principles of 

 air drainage, experience and careful observation are the best guides. 



EXPOSURE 



In the latitude of New York State, hillsides exposed toward the south are 

 warmest; next come those facing east, then west, and finally north. Lia- 

 bility to frost follows in the reverse order, being greatest on the north 

 side. It is pointed out, however, that the fact that frost isless likely tooccur 

 with a southern exposure is not sufficient reason of itself for choosing the 

 south side as a location site for an orchard, for the higher temperature 

 on that side brings the blossoms out at an earlier date than on the north 

 side ; hence, a frost may kill the crop on a southern slope before the buds on a 

 northern slope are in condition to be injured. On the other hand, the argu- 

 ment is advanced that when the trees on both sides are in bloom, the greater 

 amount of heat stored in the soil on the warmer side may be sufficient 

 to ward off a frost that would kill the crop on the colder side. There 

 are numerous observations to show the temperature of the air and soil 



