April 12, 2002
During the winter, mountain valleys are prone to the development of “temperature inversions”. They are called “inversions” because they are an “upside-down” situation. Typically the temperature of the atmosphere gets colder as you get farther away from the Earth’s surface. However, during an inversion, air at the surface is much colder than the air above it. Inversions tend to form during stretches of clear, calm, very cold weather. Without clouds, heat given off by the earth escapes easily into space, causing a layer of cold air to develop at the surface. If the inversion persists, air quality can become a problem as the stagnant layer near the surface fills with pollutants such as smoke from wood-burning stoves or emissions from automobiles. Eventually a storm passes through, blowing the polluted air out of the valley.
The photo on top shows an inversion that was present in the Helena Valley of Montana for several days during late December of 2001. The boundary between the colder air below and the warmer clear air above is very distinct. The bottom photo shows the same valley several weeks later. The “Sleeping Giant” can be seen on the horizon.