Terryd23 wrote:svanarts wrote:That's a normal weather phenomenon here in the valley in winter. You learn not to look up but to look out at an angle to see if you still see blue sky. It's especially dangerous if you do this just before sunrise here in the valley. It can be clear with just a little haze and then as soon as the sun comes up it can go to 0/0 in about 5 minutes. Luckily it's a shallow layer and you can usually fly to local foot hill airports above the fog. But then you have to make the "call of shame" as you phone your wife or friends to come pick you up.
Scott - Just to be clear, I'm not being critical of your observation at all (with my previous post). Just trying to understand the weather ingredients that make this happen.
No offense taken. I'm no weather expert but I'll try to explain. Someone who knows about this phenomenon better can correct me.
They call it Tule (too-lee) fog. It's basically radiation fog. We have an inversion layer here in the central valley pretty much constantly and in the winter time a really cold layer of air gets trapped near the valley floor. The sun comes out and warms up the ground and the fog appears. It's powered by wet warm ground and cold air. Usually a day or two of sun after a good soaking rain. Sometimes it hangs around for weeks. It's only a couple hundred feet thick but it is incredibly dense. I can tell you it's maddening to hear all the foothill and Bay Area pilots flying in the sunny skies above the fog.

Edit: Just found a couple of other good explanations...
Tule fog usually forms the 1st or 2nd clear night after it has rained, the skies have cleared, and the winds light. Rain + rapid cooling + longer nights. Inversion occurs, colder air nearer the earth's surface. The cold mountain night air descends into the valley and becomes trapped. The air above the tule fog is warmer, drier, lighter, further serving to trap the heavy, humid fog. Low air drainage may result in several days of fog, while sunny breezy days may clear the fog by noon.
Also go here and scroll down to Tule:
http://www.weather.com/glossary/t.html