-30: Avoid going outside, if you do don't leave any exposed skin
-20: A cold winter day: toque, scarf, mittens, winter coat and boots are must have
-10: An average winter day: same load-out as above, but you'll be more comfortable
0: A mild winter day: snow possible, you may leave your winter jacket open, or skip the scarf
10: Early spring or late fall: you might be able to get away with a light jacket, but I'd leave that until 15
15: Long pants and light jacket
20: Leave the jacket home, maybe wear shorts or a t-shirt
25: Comfortable summer weather: shorts and t-shirt
30: Annoyingly hot summer weather, good for swimming
35 or higher: hole up somewhere with AC