It appears this is based mostly on noise, as nobody is "afraid" of playing in Minn... they have a cushy indoor stadium, and their Midwestern fans are overly nice, nothing like hard core northern east coast fans.
I'm surprised Denver isn't higher, just based on altitude adjustment alone
If I owned the Vikings, you better believe it would be an outdoor stadium. I know they have it indoors so they can host a SB every 10 years or so, but I'd say Screw That, and just have an outdoor icebox other teams/fans HATED, that would make Greenbay and Buffalo look like mild Miami winter. Get some wolves as mascots, run it 90% of the time, and make other teams hate life. As it stands, they're a sissy team nobody "fears".