#18 - Helena Valley Temperature Inversion

The photo above shows an inversion that was present in the Helena Valley of Montana for a few days in mid-December of 2023. That is Carroll College in the center. The boundary between the colder air below and the warmer clear air above is very distinct. During winter, mountain valleys of western Montana are prone to the development of inversions.

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.

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.

Upside-Down . . . .
During winter months, the mountain valleys of western Montana are prone to inversions. They're called inversions because they are upside-down situations. Since the atmosphere is warmed from the bottom up by heat given off by the Earth, the temperature of the atmosphere NORMALLY gets colder as you get farther away from the surface. However during inversions, air near the surface is colder than the air above. Local hikers know this means that it can be 5-10 degrees warmer on top of the mountain than it is down at the trailhead.

Recipe for a temperature inversion - The three "C's". . .
Mountain valleys serve as "sinks" where cold, dense air may sit for several days. They develop during clear, calm, cold nights - especially in December and January. Clouds act like a blanket, keeping much of the heat given off by the Earth close to the surface. However, on clear nights this heat escapes quickly out to space, and air at the surface becomes cold (and heavy). The low angle of the Sun in December and January prevents this valley air from heating up during the days. Snow cover, which reflects sunlight, and the shortness of winter days also help prevent it from warming. If the inversion persists for several days, air quality worsens as the stagnant, cold air fills with pollutants such as smoke from wood-burning stoves or emissions from automobiles.

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 1-minute VIMEO video, showing a temperature inversion in the Helena Valley during December of 2017.

Term: infrared radiation, emissions