It's 7C/45F and rainy right now where I live...in Canada. This is shorts-wearing weather, actually.
'SAD' in theory should affect me year round; I rarely go outside during daylight hours, but as I'm super sensitive to ultraviolet light, my vitamin D levels don't drop. Nor do I get affected by cabin fever - I love solitude as it's great creative time. Winter seems to make me more active than the peak of summer - I feel restless in cold temperatures and super sluggish in hot.
I'm actually looking to gain a dozen pounds this winter, too. Been eating a ton more food (w/ extra helpings of creams, butters, fats) and still stuck at 126lbs. I need that extra weight to keep my stamina going through the day as my metabolism is just wonky.