Weather Whys; Thermal Belt

This is weather-wise. Mountains are generally thought to be cooler than the valleys below them, but on clear-winter nights valleys can actually be colder. That's because of a basic physical property of the atmosphere. Cold air tends to sink. When air is warmed, it tends to expand, and when it's cooled, it contracts. So cooler air is denser than warmer air. That means if you have air that's not all the same temperature, the cooler air will tend to sink below warmer air. Picture a valley between two mountains. The ground on the mountain sides cools by giving off heat and the cold air in contact with it drains down the mountain sides and settles in the valley. In turn, the warmer air in the valley rises up the sides of the mountains, expanding and cooling as it rises. So eventually it'll be as cool as the air in the valley. That produces a region on the hillside, which is warmer than the air both above and below it. That region is called a thermal belt, and in hilly country, farmers often plant their most cold sensitive crops in those areas.

The drainage of the cold air creates some problems for valley residents. The cold pool of air is essentially trapped in the valley, since it's too dense to rise. So any pollutants such as factory smoke plumes, dust, and so forth are trapped there too. Air quality can get pretty bad under those conditions, and residents just have to wait for some kind of atmospheric disturbance to come through and clear things out. We'd like to know what you think of weather-wise. Please write us and weather-wise KGOU radio, the University of Oklahoma, Norman, Oklahoma, 73019-0250. Weather-wise is made possible by a grant from the National Science Foundation. For weather-wise, I'm Drew Barlow.