ORCHARD LOCATIONS AND SITES 647 



describes the following thermal belt: 'In Polk County, North Carolina, along 

 the eastern slope of the Tryon Mountain range, in latitude north 35°, the thermal 

 belt begins at the base of the mountain, at an elevation of 1200 feet. It is 

 about 8 miles long, and is distinguished by magnificent flora, such as would be 

 characteristic of a point 3° south of the actual latitude.' 



"Prof. John Leconte, of Berkeley, Cahfornia, in Science, Vol. I, p. 278, states 

 that at Flat Rock, near Hendersonville, Henderson County, North Carolina, on 

 the flank of the mountain spur adjacent to the valleys of the Blue Ridge, he also 

 observed a frostless zone. The valley is about 2200 feet above sea level, and 

 the thermal belt is 200 to 300 feet above the valley. 



"J. W. Pike, of Vineland, N. J., states that among the mountains of 

 California he has discovered that during the night the cold is much greater in 

 the valleys than on the terraces several hundred feet above, due to the settling 

 of the cold air, so that a thermal belt is formed at that height separating the 

 frosty valleys from the colder highlands. 



"In the Tennessee Journal of Meteorology for January, 1894, published by the 

 State Weather Service, the author describes a thermal belt between Los Angeles 

 and the Pacific Coast. It traverses the foothills of the Cahuenga range, and 

 has an elevation of between 200 and 400 feet and a breadth of about 3 miles. 

 It occupies the midway region of the range. 



"In the American Meteorological Journal, Vol. I, S. Alexander describes 

 a thermal belt in which the peach tree flourishes in the southeastern portion of 

 Michigan. He shows that the cold island discovered by Winchell in that region 

 is really the bottom of a topographical depression into which the cold air settles. 

 It is a long valley surrounded by a belt of elevated country from 50 to 600 feet 

 above Lakes Michigan and Huron. The valley and the isotherms trend north- 

 east and southwest from Huron County through Sanilac, Lapeer, Oakland, Liv- 

 ingston, and Washtenaw to Hillsdale Counties. The highlands of this region 

 are all much freer from frost than the lowlands, and all much more favorable for 

 early vegetation. He does not state that any point is high enough to be above 

 the thermal belt, but that, in general, two equal parallel thermal belts inclose the 

 cold island between them. 



"It is generally conceded that these thermal belts depend both upon the 

 drainage of cold air downward into the lower valleys and the freedom of radiation 

 from the surface of the ground to the clear sky overhead. During a still night, 

 when frosts occur, the surface of the hillside cools by radiation, and hence cools 

 the air in contact with it; the latter flows downward as long as its cooling by 

 radiation and conduction exceeds its warming by compression. Inasmuch as its 

 cooUng depends on contact with a still colder soil or plant, it soon accumulates 

 in the lowlands as a layer of cold air, which grows thicker during the night by 

 the steady addition of the thin layer of descending air in contact with the ground 

 on the hillsides. The warmer air, which has not yet had an opportunity to cool 

 by contact with the ground, floats on top of the cold mass; it spreads out toward 

 the hills, and is continuously furnishing its heat to the adjacent hillsides as fast 

 as it comes in contact with them before it also cools and descends. The formation 

 of the thermal belt seems to depend largely upon this gentle circulation during 

 the night time. 1 he lower limit of the belt is defined by the depth of the accumu- 

 lation of cold air in the confined valley and rises higher in proportion as the night 



