Snow does not equal an Under. If anything, it helps the Over. Two reasons: 1) Everyone and his uncle jumps on the Under, giving you more value on an Over as the number is driven down; 2) The players on offense know their routes. The players on defence have to react to where their man is running. On a snowy field it's much more difficult for a defender to stay with his man & make a quick cut when needed. Look at all those big plays tonight. Once the defence gets running the wrong way, it's nearly impossible to reverse direction and catch up.
Rain doesn't equal an Under either. Same story...but to a lesser degree (you'll still get people piling on the Under, but the field is not as difficult for the defence).
As for the turnovers-leading-to-lower-scoring argument I often see...I don't see any validity to that either. Sure, turnovers increase in rain and snow, but who is to say teams are turning over the ball when they are about to score? They are just as likely to turn over the ball in their own end of the field, fielding a punt, etc, which actually increases scoring as the other team quickly has a short field to deal with now.
Mud, a la New England today...yes, Under. Nobody can move in that slop when the field is REALLY REALLY bad (which happens about once a season that you see a field that bad). And high winds, definitely consider the Under, because it affects both the kicking game and the passing game.
