Jules, you're gonna have to explain yourself here. What exactly do you mean "the 49ers play in California all year"? Their home is in CA, yes. But there are 8 more games that are NOT played in CA.
The thing is, in the modern NFL, I don't think any team is truly weather hardened. Yes a few teams in the north like GB play a one or two games at home in Nov/Dec snowy conditions, but we're talking about the depths of winter now... late January. Sub-zero Ice Bowl cold. Nobody is really ready for that. And most teams including the Packers have indoor practice facilities. AND, in todays NFL, a lot of players don't even live where they play. Aaron Rodgers is a native Californian who lives in SAN DIEGO!! So, nobody is really used to it.
And it's a time tested fact in the NFL that passing games suffer and running games rule supreme in the kind of weather I'm talking about.