I was hiking on the A.T. back in college mid-March and it was just a bit above freezing and rainy. My group (all from Florida) was bundled up inside a cabin to get out of the rain. In wandered some other college kids in shorts and tees, soaking wet. They were from Minnesota. I really don't know how they didn't have hypothermia, but I guess you can acclimate to quite a range of temperatures... for a while.
Yeah at the top of Mt. Baldy we discovered a group of Georgian Scouts huddled shivering under their space blankets in a rock grotto. Meanwhile our Scouts (from Iowa) took off their coats and shirts and ran around like wild men. All about what you're used to!