For many years the expression "isothermal belt" or "thermal belt" has been used to describe certain sections of North Carolina which enjoy a more equitable climate than neighboring regions of comparable altitude and latitude. The questions "what are these isothermal belts?" and "why do they exist?" arise frequently and not all are clearly answered.
In Rutherford County, the towns of Lake Lure, Rutherfordton, Spindale and Forest City enjoy a thermal belt climate, and a nearby community is named Thermal City. Smaller thermal belt areas across western North Carolina are favored locations for apple orchards.
W. N. Hurt, former horticulturist for the State of North Carolina, wrote in the article, "Thermal Belts from the Horticultural Viewpoint,"** that, until he came to North Carolina in 1906, he had never heard of a thermal belt or of a verdant zone. Fruit growers used the terms, and Mr. Hurt wrote:
"Practical men who make their living from Mother Earth in fruits, vegetables, grains or other products are close observers of nature and her laws. They may not always be able to correctly interpret her ways and define her laws, but if they have observed any phenomenon and formulated any practice from it you may be pretty sure there is something in it, and you will be unwise if you disregard it without investigation."
Mr. Hurt made a lengthy study of thermal belts and verdant zone, which appears as an appendix to the U.S. Dept. Of Agriculture's Monthly Weather Review Supplement No. 19, published 1923, on "Thermal Belts and Fruit Growing in North Carolina."
Mr. Hurt described the thermal belt as being similar to a will-o'-the-wisp, which always seemed to elude his grasp, but he did draw the conclusion that thermal belts are a reality and that North Carolina seems to have a monopoly on them. He advanced the explanation that this was due to the fact that one-third of the state's area is made up of rolling Piedmont hills stretching up to another third that contains the highest elevations east of the Mississippi River. His preliminary survey resulted in the U.S. Weather Bureau's setting observation stations at 16 western North Carolina areas where apples, grapes or peaches are grown, and making a study of frost pockets, high top freezes, and the fact that sometimes, in a year when weather conditions were so unfavorable over the state that it would seem impossible that any fruit could survive, some section, or some orchards in a section, would bear a phenomenal crop.
In the 1941 Yearbook of Agriculture, published by the U. S. Department of Agriculture, Herbert E. Kichline, associate meteorologist and climatic section director for the North Carolina Weather Bureau, Raleigh, included a statement about thermal belts in his summary of North Carolina's climates:
"An outstanding characteristic in western North Carolina is the thermal belts," he wrote, "which are probably more pronounced here than at any other place in the eastern United States. Frequent observations have shown temperature inversions of 20 degrees or more along some mountainsides."
Inversion is the term used to describe a condition occurring when on certain cool nights the temperature is relatively high on the slope of a mountain -- much higher than at the base.
Over a period of four years, records kept by the U.S. Weather Bureau on six selected long slopes (having a vertical height of 1,000 feet or more) showed a total of 860 inversions for the six slopes together in one year (1913) and in no year did the number fall as low as 800. The largest for the entire period was Ellijay, in the southwestern section of the mountains. The greatest single inversion at any hour was 30 degrees at Cane River; Altapass in Mitchell had the smallest number, 173. Globe in Caldwell County near Blowing Rock, and Tryon, in Polk County at the southern tip of the mountains, had 26. Inversions occur most frequently in the spring and autumn, and August generally has the smallest number.
While inversions and thermal areas occur in a number of localities in the mountains, the results in most areas are evident only in the successful growing of apples and other fruits.
*W. B. No. 796, U.S. Department of Agriculture Weather Bureau, MONTHLY WEATHER REVIEW SUPPLEMENT NO. 19., "Thermal Belts and Fruit Growing in North Carolina" and "Thermal Belts from the Horticultural Viewpoint", Government Printing Office, Washington, D.C.