To invert something is to turn it upside down from its normal condition. When we talk about a temperature inversion, we are speaking about an atmospheric condition that is upside down from the way things normally are. In a "normal" atmosphere, the temperature decreases the farther up you go from the earth's surface.
This creates the temperature inversion situation pictured to the right. The effect can be especially pronounced in the winter months when towns like Custer or Hill City can have temperatures near 50 while Rapid City struggles to reach 30 degrees. The sun heats the ground during the day, creating a warm layer of air near the ground. As the warm air rises away from the heated surface, it cools. Sometimes, a layer of air is so cold and dense that it resists the warming effect of the sun and hugs the ground while less dense air above it warms at a faster rate.

Temperature inversions are common in cities located in mountain valleys or nestled up against a mountain range. Cold air sinks to the valley floor or base of the mountains and becomes trapped there, a process known as "cold air damming". Unfortunately, air quality suffers in an inversion situation because pollutants like dust, smoke and vehicle emissions are trapped close to the ground by the warm layer of air above. The warm air layer acts like a lid and prevents pollutants from rising and dispersing. This can cause medical problems for people with respiratory ailments and irritate the eyes, noses and throats of others.

An early morning inversion traps smoke and smog over Rapid City.
Photo by Dan Carlson

Thick smoke shrouds Rapid City as a rare summer inversion traps smoke from a fire on the south side of town over the community. As this photo was taken, the west side of Rapid City was experiencing crystal clear skies. The smoke never made it west of Skyline Drive. The top of the inversion layer can be clearly seen at the upper portion of the photograph. This picture was taken from the KEVN/Fox 7 Powercam.