Montana Earth Science Picture of the Week

During the winter, mountain valleys are prone to the development of "temperature inversions". They are called "inversions" because they are "upside-down" situations. 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.

Its a winter thing . . .
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.

Like a big baked potato . . .
The lower part of the Earth's atmosphere, called the troposphere, is heated from the ground up by heat given off by the Earth. As sunlight shines of the Earth's surface some of the energy is absorbed and changed into heat. Eventually this heat is given off in the form of waves (infrared). If you wanted to get real fancy, you could say that "the surface radiates infrared energy". You also give off heat, and so does a baked potato. The closer you hold your hand to the surface of the hot potato, the warmer your hand feels. The same is true with the Earth. Usually air that is closer to the surface (the source of heat) is warmer than air that is farther away from the surface. . . Unless there is a temperature inversion.

CLICK HERE to watch a 2-minute YouTube video, showing a temperature inversion in the Helena Valley during late December 2009.

Term: inverted