Mad about Science: Inversion

By Ben Olson
Reader Staff

Publisher’s note: Brenden Bobby is out again this week, so you’re stuck with me. He’ll be back to the usual soon.

My partner and I share an inside joke about inversion. Whenever we encounter a weather-related matter we don’t quite understand, we just shrug and say, “It’s probably just inversion.”

The truth is, inversion is a naturally-occurring phenomenon that happens most often during winter months.

Normally, within the lower atmosphere, or troposphere, the air temperature decreases gradually the higher up in the atmosphere one is. This is largely due to the atmosphere being heated from below as solar radiation warms the Earth’s surface that, in turn, will warm the layer of atmosphere directly above it by thermals. In fancy science terms, this is measured by the adiabatic lapse rate.

An increase of altitude means higher air is at a lower pressure, and lower pressure generally results in a lower temperature. These follow the model of the ideal gas law, an equation that explains the behavior of many gases under a variety of conditions.  

However, when inversion is present, the air doesn’t mix height-wise like it usually does. In fact, among the troposphere, it does the exact opposite: cold air hugs the surface of the Earth while the air temperature gradually increases with altitude. 

A real-world scenario of inversion is when it might be 25° Fahrenheit in Sandpoint and 35° on top of Schweitzer. The inversion cap is a layer of relatively warm air that exists above the inversion and, as air parcels rising into this layer become cooler than the surrounding environment, that inhibits their ability to ascend. The cold air is essentially “trapped” beneath the warm layer above it.

A temperature differential of up to 20 degrees can be recognized during inversion.

It’s common during meteorological inversion to stand atop Schweitzer under bright blue skies while the valley below is obscured by a thick cloud layer that almost resembles the surface of the ocean.

Inversion often occurs when the sky is clear and the wind is light or calm. Valleys that are sheltered from the wind will see a more pronounced cold temperature because cooling near the surface of the Earth happens quicker there. Inversion often happens in the late afternoon or early evening and lingers into the next morning for a few hours. This is because the heat of the sun is not sufficient to dissolve the air layers that have formed.

While it might make for a pretty photograph from the mountaintop and a warmer day of skiing when it’s brutally cold in the valley below, inversion can have negative effects as well.

Since warm air rises, air under and inversion cannot escape because it is cooler, and that causes smoke and pollution to get trapped and hug closer to the ground when it usually gets dispersed into the higher atmosphere.

High pollutant concentrations can accumulate near the ground, which is why haze might be observed on a winter day. All the smoke from wood stoves and vehicle exhaust has nowhere to go, so it hangs low to the ground where we breathe it more directly.

Many cities even have “no burn” days where they ask residents to forego lighting the wood stove to improve air quality during prolonged inversion events. The negative air quality effects are substantially increased in cities which are surrounded by hills or mountains since they form an additional barrier to air circulation. The Great Smog of 1952, for example, was a severe air pollution event that affected London that lasted from Dec. 5-9, 1952. 

The smog disrupted daily life in London by reducing visibility and even penetrating indoor areas. Government medical reports in the weeks after estimated the smog killed 4,000 people and made 100,000 sick. More recent estimates put the death toll at closer to 10,000-12,000 people. 

Finally, inversion is also the cause of why you might observe Fata Morgana, or a mirage, of a ship “floating” on a distant horizon. This is because as the temperature of air increases, the index of refraction of air decreases, meaning distant objects will often appear shortened vertically (if you’ve ever seen the sun set over the ocean, you’ll notice it looks like an oval). Fata Morgana is also responsible for the “green flash” some sailors have claimed to see the very moment the sun dips under the horizon. This rare phenomenon, also occurring at sunrise, is when the sun’s green light is isolated due to dispersion. The shorter wavelength is refracted most, and the blue component of sunlight is scattered, making green the first or last light from the sun to be seen.

The National Weather Service outlined several signs of temperature inversion, including the presence of clear skies, calm winds and proximity to sunrise or sunset. Dew might also be present, horizontal smoke patterns might be evident and ground fog will exist in low-lying areas.

Stay curious, 7B.