Stagnation
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The dictionary states that "stagnate" means to become stale or foul from standing around too long. Well, that's a pretty apt description of what happens to air during a prolonged temperature inversion.

Hi, I'm Bryan Yeaton and this is The Weather Notebook.

Think of an inversion this way--as an ordinary weather parfait turned upside down: instead of warm air near the ground and cooler air aloft, which is the usual structure of the atmosphere, in an inversion colder air settles at the bottom and stays there, held in place by a ceiling of warmer air.

For a day or two, this can be quite pleasant, with chill starry nights, sunny afternoons, and an eerie absence of wind. But, if an inversion persists in an urban area, pollutants get trapped in the cold bottom layer and stagnate there. Haze takes on a brownish tinge, fog degenerates to smog, and air quality agencies begin issuing stagnation advisories. Inversions are most common in river valleys and basins - Fairbanks, Alaska, with hills on three sides, has inversions more than 70 percent of the time in winter.

In a way, inversions are too much of a good thing weather-wise. They often come in winter with the arrival a high pressure system, and they herald a spell of calm, settled weather - a welcome relief from winter storms. The problem is that inversions tend to hang in for days at a stretch, which is not only tedious but potentially dangerous if pollution gets bad enough. The only remedy for a stuck inversion is a radical change in the weather, preferably in the form of a blast of wind and rain.

Thanks today to writer David Laskin of Seattle, Washington. The Weather Notebook is a production of the Mt. Washington Observatory and is supported in part by the generosity of the National Science Foundation.