SEATTLE - There are two main types of fog, and the Northwest gets both varieties.
Ground fog usually begins on clear nights. As the temperature drops, the air cools to where it becomes totally saturated. The air condenses into water droplets, which we see as fog.
Advection fog is created from a warm air mass that moves over a colder area. This is common here in spring and summer. Over on the coast, warm air reacts with cooler air near the ocean water. It then cools and condenses, creating fog. It's a near-daily occurrence on the coast, but that fog can get carried into the Puget Sound region by the overnight marine winds, allowing our days start with fog or a low overcast.
For More Information:
KOMO Weather FAQ