How is our feared "tule fog" different from other more celebrated fogs, like the "little cat feet" stuff in San Francisco?
Coastal fog billows in on offshore winds. Ours is more like little frog feet; it just sits in damp, low-lying areas and refuses to budge.
What is fog exactly?
It's a cloud at ground level, tiny water droplets on dust particles suspended in air, says UC Davis professor Cort Anastasio.
Why doesn't it just fall to the ground?
It is falling, but the droplets – 50 times smaller than rain – and the particles are so light they drift downward in super slow motion.
What causes "tule fog?"
Not tule plants. Valley fog is really "radiation fog," meteorologists say. At night, the day's heat radiates up and away (as long as there is no cloud cover). If there is enough humidity, the air temperature hits the "dew point," and gas condenses into water. Often, that's around 40 degrees.
When does fog happen?
Fog season is generally December through February. Fog is at its most dense just before sunrise.
Then it burns off?
That's a bit of a misconception. Fog disappears when the air gets warm enough to turn the water droplets back mainly into gas. The moisture is still there, but doesn't block our view.
Where is fog thickest?
Usually near rivers, creeks and fields with vegetation. We get the name "tule fog" from the days when the Valley had many foggy winter wetlands where tules grew.
Where did the fog go the past few years, and why is it back this week?
Dry, highly windy, cloudy conditions keep fog away. Recent rains, still or slightly stirred air and clear skies are the recipe for fog. An average wet winter is forecast for this year.
What does it mean for drivers?
Watch out nights and mornings on any low-slung roads next to fields, farms or waterways. Around here, that could be anywhere. Interstate 5 south of Sacramento is notorious for fog and crashes over the years.
So what's the top advice for drivers to deal with fog?
Don't drive! So says the DMV handbook: "You should consider postponing your trip until the fog clears."
But that's not practical.
In that case, drive slowly, leave plenty of distance between you and vehicles ahead, and have your headlights on low. If you're going very slowly, put your emergency flashers on. Roll down the window to see out and guide the car in its lane by looking for the line.
Also, keep the radio off so you can hear other cars and possibly a crash ahead that you might not see.
What if it is just too thick to drive?
If you're on the freeway, try to find an exit. If you can't, then pull well off the freeway and turn off your lights so vehicles behind you won't veer into you, the CHP's Mike Bradley says.
Does the CHP do anything to help?
Yes, when visibility is really bad, as it was Monday morning, the CHP runs "traffic breaks." CHP cruisers will get ahead of vehicles before they enter the worst sections of fog and then weave across the road, slowing traffic down and guiding it through.
Call The Bee's Tony Bizjak, (916) 321-1059.
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