The Thermal Belt
The mountains and valleys provide a variety of weather conditions. In fact, Polk County is widely known for a unique weather phenomenon known as the “thermal belt”. The thermal belt refers to a mountainside zone where frost or freezing temperatures are less likely to occur than they are at either higher or lower elevations.
Confused? Here is a more detailed explanation: In order to have a thermal belt you must live in the foothills or in a mountainous region. A typical thermal belt is formed on a mountainside and not on flat land. Heat absorbed by the soil during the day radiates from the soil surface of the mountain at night and rises into the free air.
The radiation of heat actually makes the air closest to the soil surface colder than the free air. The cold air situated near the ground then moves downward into the valley below (cold air sinks). This movement of cold air to the valley, forces what warm air there is in the valley upward. During the night there is a continuous interchange of cold air from the mountain surface and warmer free air from the valley.
Meanwhile there is a continuous movement of air above the mountain ridges. This rapidly moving air traps the rising warmer free air. Thus, a band of warm air is created with colder air above it and colder air below it. This band is a thermal belt.
In Polk County, the Tryon-Columbus area is protected on the north and northwest from the cold winds of winter by a crescent of 2,500 to 3,000 foot mountains. These same mountains that help to form the thermal belts. According to a US Weather Bureau Report, there are some “peculiar topographical features near Tryon that affect the flow of air” causing some remarkable variations in temperature. The report also states that “on no other slope is the center of the thermal belt found so close to the valley floor.”
Finally, it is true that gardens located in a thermal belt will normally have a longer growing season between killing frosts.