Inversion 101

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As high pressure builds overhead this week, the key local feature that will control our weather will be the low-level inversion. So lets do some inversion 101. Inversions occur when temperature increases with height. They are "inversions" because generally temperature decreases with height. Inversions represent very stable configurations in the atmosphere--preventing the vertical movement of air. An example would be cold, dense air with warm, lighter air above.
There are two main ways to produce inversion...and both are encouraged by high pressure overhead. High pressure areas are associated with clear skies aloft. This allows the effective emission of infrared radiation from the surface to space (clouds greatly lessen this emission). Good IR emission allows the surface to cool more than the air above...the result is a surface-based inversion.

High pressure can also producea "subsidence" inversion. In particular, highs are associated with sinking (subsiding) air and this sinking goes to zero at the surface. So strong sinking aloft and weaker sinking near the ground. Sinking causes warming because air is compressed when it sinks and compression causes warming (like in your bicycle pump). So strong sinking aloft produces lots of warming, but that decreases towards the surface. The result: an inversion. Both types of mechanisms can occur at the same time.

So lets see what is going to happen here. Below is the latest vertical sounding at Quillayute on the coast. Temperature is red and dewpoint is blue dashed. The y axis is pressure in mb (850 mb is about 5000 ft). At low levels the air is saturated (temp and dewpoint are the same), but temp decreases with height. A weak inversion is above the moist layer, with very dry air aloft. Now take a look at the computer forecast of the sounding over Seattle in 60 hrs (Thursday AM). A strong, surface-based inversion.

With such a strong inversion and a moist surface, the next issue is fog--and we will certainly get quite a bit of it. The inversion prevents the mixing from above, and the air in the surface layer can cool to fog. And once the fog forms it is very difficult to mix it out...particularly this time of the year when the sun is weak. And a stagnant layer of cool air can facilitate poor air quality. A really good place to get the latest air quality conditions and forecasts for the region is found at the Puget Sound clean air agency web site (

Reminder: I will be signing books on Saturday at 1 PM at the U Book Store in Seattle and will give a talk on NW Snowstorms at Third Place Books at 7 PM on Wed. January 21st (and signing books there too).
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