I try to ride all year around outside as I get too bored riding indoors on a trainer. I'll ride in the winter down maybe to the teens (unless it's a one or two day cold snap - then I may just take the day off). My winter rides are shorter than my long spring/summer/fall long rides. Generally, not more than 25 miles or so if the temperature is in the 30's - maybe around 15 if it's colder than that so really just trying to keep up with some base conditioning for when spring rolls around. Regarding heat, I've done a few rides in the 85 - 105 mile range where the temperature got into the mid-90's along with high humidity and I found that really difficult. If I limit the ride to 50-60 miles and get a fairly early start before the heat really ramps up, I'm ok even if the last hour is getting too hot for my comfort level. Just really need to be more conscious of staying hydrated. The 50's through 70's/low 80's is kind of my preferred sweet spot.