Monday, January 12, 2009

Inversion 101

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 saturation..giving 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 (http://www.pscleanair.org/)

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).

16 comments:

Anonymous said...

Ugh!!!! I hate the fog! I'd rather have the Snow. Or rain and wind. But this fog is driving me nuts!!!!

How high up will one have to go to escape it?

Also, you mentioned something about what is coming after this week of high pressure weather. Any hints?

Anonymous said...

FOG is the BEST weather phenomenon EVER!! I absolutely LOVE it!!!!!

Anonymous said...

Will we ever see snow again this month?

Anonymous said...

I am looking to contact you to talk about the possibility of delivering some training. How do I do that?

casej@wsdot.wa.gov

Adam said...

What does all this warming mean to skiers? It's really frustrating watching all the snow melt away rendering spring conditions in January! Will we ever see winter weather again?

Fall City Joe said...

It's always interesting reading weather predictions from someone in Seattle from my home in the Snoqualmie Valley. No fog here. My guess is that we're under the influence, again, of eastern Washington air flows westerward, but I'd love to hear you validate.

Anonymous said...

The warmth means spring skiing!
Could be a little icy, unless there is good grooming. Actually, the shadows are so long this time of year, you're not likely to see snow actually melt. It crisps up at night, loosens up a bit daytime, but the dew points will drop a lot in the mountains and crunchy snow will be the rule. If it was going to be windy it might be slushy, but winds ought to be calm enough for the layer of air against the snow to stay chilled. See if it isn't a lot warmer on the ridges versus in a shady calm spot.

These conditions will be just fine for my old K2 KVCs, ha. I actually like the Mission Ridge refrozen icy corduroy. What I don't like is where the snowboards chew it up and it isn't groomed enuf.

Isaac Molitch said...

Cliff, At what elevation should we be above the fog zone this week?

Josh-B said...

Cliff,

How is the economy going to be in the next quarter?

MarkM said...

For those who think it's better elsewhere, the lower Methow Valley is socked in with fog, though if you go up to Sun Mountain or even to Mazama you rise out of the fog. About 40 in afternoon and 25 at night makes it impossible to walk around b/o ice everywhere.

Mike of MLT said...

For Fall City Joe--there isn't much fog in the city. There is a moist mixed layer that extends up to about 1000M, so a few spots of light drizzle, some patchy fog, but mostly a moist air mass in the lower levels but not yet a classic fog pattern. When the inversion sets up and the mixed layer becomes shallow, then dense fog will develop. Whether the fog penetrates up the Cascade valleys will be hard to say, maybe not if the cross Cascade gradient gets up to -5mb (easterly). Right now it is +6mb (westerly) so you are not getting any influence from dry air working westward yet.

Anonymous said...

thank you for inversion 101!!!

Anonymous said...

The NWS threw out the "r" word this morning...

.LONG TERM...PREVIOUS DISC...THE GFS AND ECMWF BOTH SHOW THE BLOCKING RIDGE PERSISTING THRU SUN. A WEAK COLD FRONT MAY MOVE THROUGH THE AREA MON NIGHT OR TUE. THEN THE ECMWF IN PARTICULAR HINTS AT RETROGRESSION OF THE RIDGE TO 145W...OPENING THE DOOR TO A POSSIBLE COLD AIR INVASION. THE GFS MAINTAINS THE BLOCK ALONG THE WEST COAST LONGER.

Retrogression is a Pacific NW snow lover's best friend. Whether it will happen remains to be seen...

Anonymous said...

I'm ready for a good wind storm....snow's ok....but I love the wind....especially in the day time so I can watch the trees blow! (I only like to watch the trees because I don't have any within falling distance of my home! :) ) Any idea if we will possibly have a good "Mountain Wave" East wind event when this high pressure moves east? I live in Eatonville....we don't get the gap winds but we do get the wave winds! Also usually get hit hard with southeast and south winds.

Joseph Ratliff said...

Ahhh calm weather...love it.

Keep the lights on inside the house, and get out and go for a long walk...this is all good. :)

Kevin Purcell said...

Isaac Molitch asks "At what elevation should we be above the fog zone this week?"

Above the inversion ... you can see the current height of the inversion on the virtual stemp plot (or the Stuve plots.

See the Upper-Air Observations: Soundings and Profiler Data

http://www.atmos.washington.edu/~ovens/loops/

Today the inversion is down to around 800m (2500 feet) so you need to get onto a decent sized foothill to guarantee it. The passes should be above it.

That said not all places under the inversion have fog. The sun is shining in Seattle (as the song says) on Cap Hill at 300 feet.