Tule Fog

Fog
The fog comes
on little cat feet.
It sits looking
over harbor and city
on silent haunches
and then moves on.
.............Carl Sandburg

This is the time of year when fog blankets the Sacramento area at seemingly random times and at random intervals. You sometimes awaken to find your neighborhood in a cloud of mist and other times you find your field of vision shortening in the late afternoon. When it is really thick here, it is called a “tule” fog referring to the tules or reeds of the marshland habitats. The following is from the San Jose Mercury News Weather Corner:

It is commonly known as ""tule fog'' because of its prevalence in marshy areas populated by tule reeds or cattails. Technically it's a radiation fog, which forms as the ground cools off at night and radiates heat into space.

Tule fog usually forms on the first or second clear night after it has rained, skies have cleared, and winds are light. This happens when high pressure returns, creating an inversion with colder air near the earth's surface than aloft. This in turn causes moisture on the ground to condense into a low-lying layer of fog that develops from the ground up. Because the coldest air settles into the lowest elevations, such as along a river or stream, this is where fog is most likely.

Twenty years ago the tule fog would ground all airplanes in the area and prevent landings as well. I’ve flown from Orange County airport to Sacramento and back again without landing because of the fog. Nowadays, the danger is more on the ground with freeway accidents being especially bad. People just can’t believe their visibility range really is less than 100 feet at times. Even with visibility at 1/8 mile, a driver going 65 only has 7.5 seconds after seeing an obstacle to react and avoid it. At 100 feet, a driver has only 1 second.