September 30, 2010

Fall and Low Clouds

We are in for a stretch of beautiful fall weather, but many of our days will start off with low clouds and fog. And that is absolutely typical for this time of the year. In fact, we are now in the foggiest time of the year for central Puget Sound.


Night is getting fairly long, giving a good opportunity for the surface to cool off by emitting infrared radiation. And with a lack of clouds aloft, that radiation has a good shot at leaving our planet. Cooling the lowest layer of the atmosphere often produces saturation..and clouds or fog.

Take a look at the visible satellite imagery this morning at roughly 8:30 AM--lots of low clouds over the lowest elevations: Puget Sound and the Willamette Valley. You can also see some fog-filled valleys in BC.

One of the interesting things we learned when satellite imagery became available is that fog and low clouds tend to burn in from the sides towards the center. Watch it happen today in the images below:

11 can also see the arid areas of eastern WA and Oregon (lighter colors)

2 PM...almost all gone.

The low-level cool air associated with clouds is quite apparent on the Seattle profiler (see below). This figure shows you the temperature variation with height from 5 AM (yellow) through 11 AM (black). Height is in meters. The cold air is about 400 m (1300ft) deep, with a strong inversion (temp increasing with height) above.

If you had hiked up Tiger Mountain or some other lowland peak this morning, the temperatures would have warmed by nearly 10C (18F) in 200 m (650 ft)--something you would have noticed. Such inversion conditions are often evident in fall. So if it is cloudy in the lowlands, don't give up on your hike. Check the satellite picture and it may be clear and warm above.


  1. Very informative post with to the point graphics and concise commentary. Thanks, Cliff.

  2. Lowland fog can make for some really interesting photography oportunities.

    I have been hearing that we can expect a winter worse than '55? Is that really true? What can Seattlites do to be more prepared for what is coming?

  3. We just finished a backpacking trip in the Olympics (outside Sequim)and the weather was spectacular, absolutely clear all 3 days. That could be explained by elevation, but the skies were also cloudless when we got down to Sequim. By the time we got home to the Seattle area, we were back in the clouds. I'm curious, why wasn't it foggy in Sequim, right on the water?

  4. Had a great time hearing you speak in B'ham tonight! I'm up north of town a bit, right up against the foothills and smack in the path of the Fraser River outflow. Definitely stocking up the pantry and other winter preps in advance of the La Nina shenanigans :)

  5. Loved the lecture tonight in Bellingham!!! Book was sold out at the talk, so we stopped by Barnes & Noble Bellingham and picked up a copy. I love the book. Our weather is so unique that It is awesome that now we have a reference for it!!! I will be interested to see what the next book has to offer. I think you need to come to Snohomish County and talk to us in greater depth about the quirks of the convergence zone, since it is something that residents here battle with all the time (sunshine, snow, hail WHAT??!!!).


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