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.

However, one of North Carolina's leading year-round resorts, Tryon, in the edge of the Blue Ridge Mountains, has developed in an area known as a thermal belt. In neighboring Rutherford County, the town of Rutherfordton enjoys 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. Tryon grapes are still a well-known product of the Tryon area, but in recent years their place in the town's economy has been far surpassed by the value of the travel industry and the increasing popularity of the region with people seeking home sites for their retirement years.

*W. N. Hutt, former horticulturist for the State of North Carolina, writes in an article on "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 writes:

"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 zones 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 describes 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 advances the explanation that this is because one-third of the state's area is made up of rolling Piedmont hills stretching up to another third, containing 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. At Tryon, however, thermal belt climate has contributed heavily to resort prosperity.

Quoting from the U.S. Weather Bureau Report: "Because of the peculiar topographical features at Tryon which affect the flow of air, there are some remarkable variations in the extent and the amount of inversions during the course of a year. Normally, the middle of the thermal belt lies at an elevation of only 400 feet above the valley floor. On no other slope is the center of the thermal belt found so close to the valley floor."

It is on such slopes that the famous Tryon grapes ripen in late summer. The same climate that produces the grapes contributed to Tryon's development as a resort.

*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.