I don't think a lot of the attend/don't attend decisions are made a year in advance. I think they're made within 24 or 48 hours of the game.
The vast majority of the season-ticket holders show up for the games. some of the corporate accounts may stay away depending on weather and the opponent. Obviously, if there are more season-ticket holders, more people will be in the seats.
But, the big swing group for attendance are the single-game ticket buyers, fair-weather fans and bandwagon jumpers, along with fans of the visiting team. I think a lot of those people make their decisions based on the weather forecast. if it's expected to be cold, raining or snowing, they stay away.
To really change the situation, Gopher football has to become a real attraction - one that people do not want to miss no matter what the weather is like. That involves winning games, but it also involves the overall perception of the program, game-day atmosphere, concessions, tailgating, etc.
We'll know Gopher football has "arrived" when 40,000+ people are willing to sit outdoors on a cold, rainy, sleeting miserable day - because they cannot bear the thought of missing a game. In the era of 60" TV's with 4-K resolution, that may be a pipe dream.