Temperature change...
I'll never forget the hike we took in college thru the "100 Mile Wilderness" in Maine. Much of it is in Baxter State Park, and it's the northern end of the AT, which terminates at the summit of Mt Katahdin. We were hiking at the base in cutoffs and t-shirts. By the time we were getting near the top of the Knife Edge trail, near the summit, we had taken every layer and scrap of clothing from our daypacks and put them on. The temp at the base was mid-70's, and sunny, and at the summit it was below freezing, a little sleet/freezing rain, and what seemed like 50 knot winds. That was in summer. I don't recall exactly what I had for extras, but, it was probably a wool sweater back then, a rain shell and wind pants. I always had a watch cap in the pack, and remember wearing my spare socks for mittens.
I've had similar experiences on other mountains, but, that was the one that I remember thinking, "I was swimming in the Penobscot River only about a mile from here earlier today in nothing but a smile, and now I wish I had my ski gear"
Ahhhh youth...
_________________________
- Ron