High pressure is moving in, winds are light, skies are mostly sunny, warmer air moves in atop the snow pack; sounds like a great day! However, when these conditions combine with terrain features and the presence of an activity that pushes pollutants into the air, you have a recipe for stagnant air and poor air quality.
|This image shows fog and stagnant air settling in a valley. The top of the inversion is so shallow that even some of the telephone poles in this image rise above it. The steep inversion causes a mirage in the distant mountains making them look like mesas, a phenomenon known as "Fata Morgana" - Photo by Kelly Allen|
Warm air moving atop cold air creates what is called in Meteorology an “inversion layer.” In the lowest layer of the atmosphere, the troposphere, the air typically cools with height. So the natural state of the air within the troposphere is buoyant and well mixed because warm air (at the surface) is lighter than cold air (aloft). When cold air is trapped at the surface, this goes against the natural rules of the troposphere and the air becomes stagnant as the heavy cold air pools near the ground.
When visualizing an inversion, it helps to think of the area as a garage. In Sublette County, the Upper Green River Basin is surrounded by higher terrain; these terrain features create the “walls” of the garage. When warmer air moves in over the area, this warm air acts as the “roof” of the garage. So when the usual human activity from wood burning, driving cars, and oil and gas exploration pump pollutants into this garage, the pollutants become trapped until the “roof” erodes away and allows the air to escape.
So what erodes the roof away? A lower layer of clouds will weaken the inversion by creating less of a temperature difference between the surface and the air above, as will precipitation. Strong winds that can break through the inversion could mix the air. Also, colder air moving in aloft would break the inversion in the same way that a low cloud layer would, if cold air aloft replaces the warm air aloft, then you return the atmosphere to its normal state.
For more information on the current Ozone Advisory in Sublette County and for more information on the dangers of ozone, please visit the Wyoming Department of Air Quality website.
Air Quality Alerts are disseminated through the NOAA Weather Radio and the National Weather Service website. Keep an eye and an ear on these sources for the latest on this Air Quality Alert and for information on future Alerts.