Monday, February 7, 2011

Monday Morning: Inversion Layers

An inversion layer trapped over Lake Tahoe.

I woke up a couple weeks ago and while stumbling towards the kitchen to make coffee before a day hike I glanced out the window to find a gray soup outside the window. A thick fog had cut visibility down to about a hundred feet in any direction. Ordinarily I might have been disappointed but I recognized this not as thick fog, but an inversion layer, perfect for a peak climb. Although

Inversions, as the name implies, flips the ordinary layering of the lower atmosphere trapping cold air, normally higher, low to the ground. The warm air which sits near the ground in normal atmospheric conditions is forced up and becomes a cap.

Even a weak storm can break up the cap and push away the fog. Typically the inversion fog will also burn off slowly as the day goes along but unless the cap is broken it will resume the next morning.

However If you can get above the inversion layer the skies are often crystal clear and when we reached the summit that afternoon after our hike we could see hundreds of miles all the way from the Bay Area to the summits of the Sierras.


  1. Neat. We usually see this in play with wood / stove smoke. You can see the trapped layer in the morning on some days. Also learned back in my wildland fire class that the lifting of an inversion usually means an increase in fire activity (from winds).

  2. Thats good info to know. Although the clearing of winter inversions here in the Bay likely mean a very low fire danger - because it's about to rain.

  3. How could you tell the difference between fog and an inversion layer by just looking out your window?

  4. @Steven - Fog, the kind SF is known for, rolls in and out as a mass. It crawls over the mountains and does not usually hang low to the ground. Even when it does it hardly ever does so in the winter. Because it was a winter day, because the fog was so dense, I just made the logical jump. To be totally honest I was not absolutely certain, but it seemed pretty likely to have been an inversion layer.