Sacramento is situated in California's Central Valley. This area is known in the winter to meteorologists as "Fog Central". It's a technical term. The place gets some pretty low visibility mornings. The photographs you see here aren't the worst mornings--such photographs just appear gray. I routinely wake up in the morning and cannot see across the residential city street my apartment faces on.
Despite the dense foggery, I still need to get to work in the morning. Sure, I can still take the bus. When I take the bus, though, I'd rather not bring my bike. The front fender doesn't like the bike racks on the public transit buses. Without my bike, I'm pretty much obliged to take the bus both ways. I'd rather be able to at least ride home in the evening.
All of this riding in the fog keeps me thinking about my visibility. As we move later into the winter, I find myself riding home from work in at least waning daylight... so the fog becomes my main obstacle to being seen.
Lights are as important in daylight with fog as they are at night. One difference that I have observed, though, is the TYPE of light that is important.
For my night riding, I have two headlights. One creates a powerful, beam that focuses well ahead of me. The other provides a wider beam that illuminates the pavement immediately in front of me and helps avoid road hazards such as potholes. In the fog, the wide beam is much less useful.
The fog is rarely so thick that I cannot see the pavement in front of me. The real issue is getting motorists and other cyclists to see ME. Thus, the focused beam. I find that, when following other cyclists in the fog, that focused taillights are also much more visible in the fog. Lights like Planet Bike's Superflash will cut through some amount of pea soup. Simple LEDs with no focusing lens just seem to get lost--absorbed into the mist.
The majority of other road users notice that visibility is impaired on foggy mornings. I say this because I notice that the cars move a little slower and a little more deliberately. As a cyclist, my immediate movements are less affected by reduced 100m visibility--I can see the pavement that I need to see in order to navigate. As a cyclist, I need to widen my sphere of attention on these mornings and become extra aware of what OTHERS can see.
Though I may feel that I do not need to slow down or be more deliberate in my actions on foggy mornings, I kick it down a notch anyway. If I signal for an EXTRA three seconds before turning, there is more chance that my intentions will be understood. If I take turns or change lanes more slowly, other road users have more opportunity to react.
Those are the take-homes for cycling in fog:
Be deliberate and communicate
Use focused-beam lights
I really can't justify NOT doing trips like this more often. There is so much beautiful countryside so nearby--and it's so flat that I can easily ride for hours without really having to psyche myself up for it. Hills are fun, but just rolling along a farm road is pretty fun, too. 
