Depending where you are, you can get some pretty good fog in Seattle.
We ride year-round. Wool and rainproof stuff are your best bet. Fenders are required for many club rides, and the vast majority of commuters have fenders.
It rains. A few years back we had 186 days straight of rain here in the north Sound, which thrilled a lot of folks because it was a new record.
You won't see the sun from October until about February. A "bright" day is one where there is enough light coming through the clouds that you cast a bit of a shadow.
Algae (like the kind that grows on fishtank walls) grows on the sidewalks and MUPs because they don't dry out for months. It can be very slippery, so it's good to be cautious in some spots.
Algae also grows on cars. My car had both algae and moss growing on it, which I thought was quite cute.
The chill and damp will soak into your bones, so you may want to do what a lot of folks do and wear light long underwear during the rainy season just to fight the damp.
You will want to keep sunglasses handy, because when the sun does come out after a few weeks it is actually quite painful. Seattle has the highest per capita sunglasses ownership in the country.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson