648 FUNDAMENTALS OF FRUIT PRODUCTION 



is clearer and longer, and also in proportion as the valley is more or less perfectly- 

 inclosed. The upper limit of the thermal belt may depend upon the strength of 

 the wind, and the general temperature of the air. But if there be no wind, then 

 it depends equally on the freedom of radiation to the clear sky and on the above- 

 described circulation of air." 



Influence of Bodies of Water. — After elevation, probably the next 

 most important factor influencing air temperature and drainage is 

 proximity to bodies of water. The specific heat of water is high; it 

 absorbs heat slowly and gives it up slowly. Consequently in the spring 

 a large body of water warms more slowly and in the fall it cools more 

 slowly than the surface of the adjacent land or than near by vegetation. 

 It is slower even than the atmosphere in responding to changes in tem- 

 perature. Relatively the air shows a great variation in temperature 

 between night and day, while a body of water of considerable size shows 

 no appreciable change. The air warmed during the day, coming in 

 contact with the surface of a body of water, is cooled; consequently the 

 air in close proximity to such a body is cooler than it would be otherwise. 

 On the other hand, at night air cooled to a temperature below that of 

 the water, is warmed by contact with its surface and in turn gives up 

 that heat to vegetation and other bodies with which it comes in contact. 

 Consequently points close to bodies of water are frequently somewhat 

 cooler during the day and warmer at night than corresponding inland 

 points and are freer from frosts, while blossoming is at the same time 

 retarded in their proximity. 



Influence of Distance from Water. — Some measure of this influence 

 may be obtained from data presented in Table 12 showing the air tem- 

 peratures, atmospheric humidity and dewpoints for three stations in 

 New Jersey and one on Kelley's Island in Lake Erie for the months of 

 July and August, 1866. Vineland is about 30, Haddonfield, 50 and 

 Greenwich 5 miles from the ocean, or from wide ocean tributaries, while 

 Kelley's Island, as the name indicates, is surrounded by water. The 

 daily range of temperature is higher the farther the station is removed 

 from the influence of water and also the more remote the station the 

 lower is its mean atmospheric humidity and the lower its mean dewpoint. 

 In other words, those stations close to large bodies of water enjoy a 

 climate more equable in temperature and consequently less subject to 

 frost injury. 



The interchange of heat and equalization of temperature in the 

 vicinity of bodies of water is favored by a gentle breeze but it will occur 

 to a certain extent when there is practically no air stirring at inland 

 points. The water is itself responsible for a certain amount of air move- 

 ment and the attendant air drainage. It is almost needless to state that 

 the larger the body of water the greater is its influence on air movement 

 and air temperature. Much, too, depends on the topography in the 



