Seeing as I live in Minnesota I think I can add to this discussion. The biggest issue is not staying warm - that is no problem for any outdoor aerobic sport no matter what time of year.
The real issue is that once the temps drop below 50 F the balls do not bounce normally - it really changes play. I've been tempted to try pressureless balls in these conditions to see if they bounce more normally, thereby extending the outdoor playing season.
Anybody tried that?