Montana Earth Science Picture of the Week

An Upside-Down Situation - Move your mouse over the image to see the inversion.

Move your mouse over the photo above to see a temperature inversion. During the winter, mountain valleys of western Montana are prone to the development of these 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. The inversion shown in above is a bit unusual in that there are cloudy skies above it. The inversion formed during a period of clear skies, and then the moist warmer air moved in without disturbing the cold air in the valley below. If the inversion persists, air quality can become a problem as the cold, 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 above (mouse-over) shows an inversion that was present in the Helena Valley of Montana for a few days in mid-December of 2012. The boundary between the colder air below and the warmer clear air above is very distinct. The "Sleeping Giant" can be seen on the horizon. Below: Members of the Helena High Outdoors Club look down on the city of Helena. A foggy inversion layer is illuminated by city lights.

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.

Can't get warmed up . . . .
Inversions are much more common in cities located in mountain valleys because valleys serve as sinks where cold, dense air may sit for several days. This air may originate as cold air from surrounding mountains settles into the valley , or it may spill in from the north as Arctic air moves into Montana. The low angle of the Sun and the short length of daylight in December and January prevent the air from heating up. Snow cover, which reflects sunlight, and short winter days also help to keep the air cold.

Check out this album of photos from a November 2015 temperature inversion, including several of a mirage known as Fata Morgana. The mirage photos were taken on a hike between Stemple Pass and Flesher Pass, and those overlooking Helena were taken the next day.

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

Term: infrared radiation, emissions


*More about inversions
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By Rod Benson
Earth Science Teacher at Helena High School

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