Fog forms when the air is saturated with water vapor. Saturated air more dense than drier air and sound travels better as air density increases. Thus, air chock full of fog is much better at conducting sound than drier air.
Bottom line, the screams and cheers from the 12th man will travel greater distances in a fog and won't fade as quickly than in just plain old regular-humidity air -- just what the St. Louis Rams wanted to hear, I bet. (It's also why thunder seems so much louder during a snowstorm -- cold, dense air filled with thick snowflakes lets the thunder travel farther and maintain its intensity over greater distances.)
Secondarily, we'll likely have a temperature inversion where cold air was pooled at the surface (helping to create the fog), and warmer air was sitting aloft.
This boundary between the cold air and warm air can actually reflect sound waves back toward the ground, allowing sound that would normally radiate out into the hinterlands of the higher atmosphere to bounce back and travel near the surface. (Ask anyone who has served on a submarine, and they can tell you about the importance of those temperature boundary layers in the water and how they can affect sound transmissions when tracking via sonar.)
The only bummer is it appears the inversion will be weakening and the fog will be lifting as the day progresses in the afternoon and the game doesn't begin until after 1 p.m., although CenturyLink Field is close enough to Puget Sound that we can hope the fog will linger a bit longer than elsewhere. But even if the fog isn't visible, if the air at least maintains that cold, dense feeling, it'll still be better at carrying crowd noise than on an early season game when it's in the 60s and dry.
And even if the fog disappears before kickoff and the sun breaks the inversion and warms the air up a bit, I have full confidence the crowd won't let the Rams be able to tell the difference!
P.S. It's likely last Sunday night's game against San Francisco got a bit of an atmospheric assist as well. Many raved that it was one of the loudest crowds of the year, if not ever, and there was a cold, steady drizzle through the game that likely mimicked the effects of fog.