Since my recent move to Missoula from the sunny state of Florida, I had experienced many unfamiliar weather conditions. Montana residents might be well accustomed to snow, black ice, negative temperatures, and the season known as winter, but these were still novelties to me.
“Looks like the inversion is acting up today,” my driving companion commented, briefly glancing at the sky as we sped along on the highway.
“Hmm,” I agreed, glumly staring out the passenger-side window at the thick, murky, low-hanging cloud cover.
The snow was fun and the black ice added a hair-raising thrill to highway driving, but the cold weather had been dismally challenging, as had the inversions. Following their appearance with winter, the thick foggy curtains of clouds that intermittently blocked out the sun had cast a melancholy pallor over me. Given their dreary appearance, I was convinced inversions were caused by a malignant winter spirit rather than some concrete meteorological process.
Of course inversions are not supernatural, so what really causes them and why do we see them most often during the winters in western Montana? Under normal conditions, air temperature decreases with distance from the Earth’s surface. During the day, the sun heats the ground which, in turn, radiates heat into the air. As the warm air rises, it mixes with cooler air, creating a temperature gradient.
Inversions occur when a cap of warm air forms over a lower layer of cold air, essentially inverting the temperature gradient. The layer of cold air close to the Earth’s surface does not rise, which prevents vertical mixing of air, trapping moisture in the upper warm layer and cold dry air close the ground. Temperature inversions are common in winter because, as the sun sets, the air closest to the ground loses heat faster than the air above it.
Over flat terrain, the resulting inverted temperature gradient returns to normal the following morning after the sun once again heats the ground and stimulates vertical mixing of the air layers. However, in areas such as the Missoula valley, the walls of the surrounding mountains cause hot air to flow upward, causing a cap of warm air to form during the day. If this cap forms high enough, cumulus clouds will accumulate beneath it, spread out, and block sunlight from reaching the ground during the day. Without sunlight to heat the low layers of air, the inversion and cloud cover will persists.
Beyond the occasional case of the doldrums caused by the cloudy weather, inversions cause a number of other phenomena. In large cities, inversions trap fine particles and pollutants in the lower cold layer because the upward movement of air ceases and prevents the dispersal of air-borne contaminants. This is particularly problematic in cities with traffic congestion because the formation of inversions temporally coincides with rush-hour traffic and contributes to the formation of smog.
In addition to exacerbating pollution, inversions also increase the transmission of high-frequency radio waves. Although FM radio signals are powerful enough to be received hundreds of miles from their broadcast source, the majority of the signal power is lost as it is projected skyward. However, when an inversion is present, the cloud layer prevents the radio waves from traveling upward and, instead, disperses them outward for up to a thousand miles. For this reason, during an inversion, it’s possible to pick up radio stations that are not available during normal weather conditions.
As well as altering the transmission of radio waves, inversions also modify the way we perceive the refraction of light waves. As air temperature increases, it decreases in density which increases the speed that light travels through it. This causes light waves to bend away from warmer air and results in distant objects appearing flat and stretched out. During sunsets, this mirage will cause the sun to appear oblong and flattened.
In summary, the presence of inversions explains a number of interesting phenomena that we might witness during winter. However, my favorite consequence of inversions in Missoula is that, with enough driving, it’s possible to leave behind the cold cloudy layer of air in the valley and travel to the upper, warm and sunny layers of air in the mountains. For those of us who are enthusiastically anticipating the spring months, this provides a bit of sunlight at the end of a long winter tunnel.
'Field Notes' is produced by the Montana Natural History Center.