The biggest bout of hypothermia I had to deal with was on my first serious year of group rides. My brother and law and I participated in the local shop 2 hour ride early in the season. I was familiar with the route and by bro was lagging so I hung back.
The weather was turning as he was slowing. We were by ourselves when it began to downpour. My bro became cold quickly, but I was feeling fine. With him cold and shivering we made it back to the shop in about half an hour into the deluge. I was still feeling ok, we loaded up the car and blasted the heater.
After dropping him off, I headed home. That is when I started to react to the cold. By the time I got into the house, I was shaking uncontrollably, so much so I had a hard time speaking. I jumped into the jacuzzi and spent an hour shivering.
The delayed reaction to cold, is a common occurrence for me. I figure that I must be pretty effective at shunting blood away from the skin in the cold and generating heat during exercise. After stopping, my heat production plummets while at the same time the blood in the skin starts to flow more freely to warm it up - resulting in a drop in core temperature. It happens on most of my colder rides and cold swims (anyone up for a dip in Lake Superior?)
I commuted to work last Tuesday on a 10 degree morning. I wore a baselayer and two long sleeve shirts, it felt cool upon starting but I was soon adjusted. An hour fifteen minutes later I was at work, my chest, abdomen and legs were bright red and cold to the touch, but I was feeling fine. Not until after showering, changing clothes and running off to a department meeting did I actually feel cold. I got a few quizzical looks as I sat shivering in the room.