That doesn't mean that trainer workouts are warm. My trainer is set up in the garage, and winter temps in the garage are typically in the upper 40's or lower 50's. At such temperatures, I don't wear a shirt when I'm on the trainer, there is a pool of sweat all over the floor, and I don't even know how much sweat I wring out of my halo band during the workout. I can do this for hours.

If I rode outside in 50 degree temps in shorts with no shirt, I'll bet I'd get hypothermia in less than an hour. I certainly wouldn't be sweating much.