Unit 3: Topography & Fire Behavior
If you live in a mountainous region, you surely have experienced the effects of a nighttime thermal belt. It's the result of a surface inversion of warmer air over colder air. Nighttime surface inversions occur over flat lands as well as mountainous regions; however, without differences in land elevation, the effects are not as readily realized. Have you ever wondered why orchards are often planted in the low foothills and not in the valley bottoms? Well, the chances of spring frosts are less here in the thermal belt at night than in the valley bottoms. Because nighttime temperature conditions are milder in the thermal belt, fire danger is usually also higher.
Thermal belts typically have the highest average temperatures, the lowest average relative humidity, and the highest average fire danger. Here's how they occur: During the night, cool, heavy air from higher elevations slides downslope into the valleys below. The warmer air in the valley is replaced and pushed to midslope by the cooler air. The midslope zone has thus cooled less rapidly than other portions of the slope. The midslope is then referred to as the thermal belt.
The development and strength of the surface inversion and thermal belt zone may depend on several influencing weather factors; however, the effects are generally most prominent during clear days and clear nights.