Frosts in New York 



5" 



Moreover, the records of cooperative observers are made mostly from 

 instruments located in shelters designed to screen the thermometers 

 from the direct rays of the sun; but such shelters act also as a hindrance 

 to radiation of heat, particularly on clear, quiet nights, so that shelter 

 temperatures are somewhat higher on such nights than open-air tem- 

 peratures. Professor Cox found that the average temperature of the air 

 at night inside a shelter five inches above ground was about 3 

 higher than the outside air temperature at the same elevation. Therefore, 

 it would seem that the practice of 

 recording a killing frost when the 

 temperature in the shelter at the height 

 of the thermometers falls to the freezing 

 point is justified. 



In this connection it may be pointed 

 out that for tree fruits this practice can- 

 not be justified on these grounds. In tree 

 fruits the blossom is the point where 

 injury occurs, and the blossom usually is 

 more than four feet above ground. Since 

 on frosty nights the temperature increases 

 with the elevation, a freezing temperature at 

 the height of the thermometers may not 

 mean a critical temperature at the height of 

 the blossom. In considering the accom- 

 panying frost data from the viewpoint of 



fruit culture, some allowance should be made for difference in temperature 

 due to difference in elevation. Just how much allowance should be made 

 in individual cases depends on the condition of the sky, the soil, and the 

 wind; but when the sky is clear and there is little air movement, a shelter 

 temperature of 30° should be considered critical at a height of 

 ten to fifteen feet above the ground. 



Fig. 137. — Thermometer shelter 

 and rain gauge for cooperative 

 observers 



EFFECT OF LARGE BODIES OF WATER ON FROST 



Under similiar conditions land warms and cools about five times as 

 rapidly as does water. For this reason, the air over large bodies of 

 water usually is cooler in the daytime and warmer at night than the air 

 over adjacent lands. During the winter months the temperature of the 

 water is lowered to such a point that seas and lakes remain comparatively 

 cool throughout the spring and exert a twofold influence on the air 

 temperattire for a considerable distance inland: (i) the cold air from above 

 the water tends to retard vegetation until the period of spring frosts has 

 passed; (2) since the air over the water partakes of its temperature, which 



