Flying Fires -- Fog, Smoke, and VFR or IFR: Inversions

Cold air is denser and heavier than warm air, so cold air sinks and warm air rises.

In a normal airmass, the air cools as altitude is increased. That is, the air is cooler aloft, and helps to move smoke and pollutants out of valleys. An inversion is the opposite of a normal airmass. In an inversion the air aloft is warmer then the air near the ground. Remember that cold air sinks, so the warm air aloft acts like a trap door, trapping the cool air below.

If the airmass is holding enough moisture, fog will form in the inversion because the temperature of the air near the ground is at or near the dewpoint. Smoke and pollutants sitting near the surface are trapped in an inversion.

Like clouds and fog, inversions inhibit visibility, hampering VFR operations.

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Note (added July 2012): I'd like to thank a couple of my tanker pilot friends whom I corresponded with as I was preparing the articles in this series for their time and help. Here is a complete list of articles in this series with hyperlinks:

1. Introduction (Jan 4)
2. VFR, I can see (Jan 6)
3. IFR, I can't see but I have instruments (Jan 8)
4. Fog (Jan 11)
5. Inversions (Jan 12)
6. Flying fires: VFR or IFR? (Jan 13)
7. Reflections (Jan 18)