After a week away from school, I took the opportunity to catch up with spring on the campus for a half-hour during first period. Walking out the door in shirt sleeves (having forgotten my jacket this morning), I was surprised at how comfortable it was compared to my arrival less than an hour before. By the time I was headed back for the door, I was chilled. Why?
We commonly predict our comfort outdoors by the air temperature. This had not changed, but when I came out of the building I was in sunlight, so I benefitted from another source of heat than my own. Further, the building sheltered me from the wind initially. Then again, I began to warm up a little as I walked briskly back to the building, generating increased body heat.
So a better question than, "what's the temperature?" might be "how fast will my body lose heat?" or--even better--breeze, sunlight and exercise considered, "what will my body's net heat flow be?"
I returned from a boating trip a few days ago that reminded me of another lamentable (and embarassing) factor: wet clothing. I had overturned my kayak in cold knee-deep water by a simple-minded error in climbing into it. I spent the next half-hour wet and wind-blown, until I could get aboard the bigger boat, and get below and change. Being wet in cool weather can be positively dangerous. Worse still is staying in the water: you are out of the wind and its evaporative cooling power, but water is so effective at removing heat from the body that even an hour in really cold water can be a death sentence. Indeed, water robs heat so effectively that a person can die of hypothermia in New England waters even in high summer, if he stays in the water long enough.
In physics terms, you can think of your body as a container; inside the box thermal energy is generated by respiration fueled by the food you've eaten and enabled by the oxygen you inhale. Clothing varies in its ability to insulate the box, slowing the flow of heat outward. Outside the box are a variety of conditions that increase or decrease (on a very warm day, even reverse) heat flow. The air itself is a fair insulator, but far less if it is moving air; water, by contrast, is an effective absorber of heat. The idea of wind chill gets at just one of these conditions. "Net heat flow" encompasses all the factors.
So it seems temperature is only one factor in comfort and safety outdoors. Besides dressing for the weather, we must consider sun and exercise, and wind and water. As your heat balance can change many times on even a single outing, the wisdom of dressing in layers becomes obvious, as well as the importance of being able to keep dry when shelter is not close by.
And shivering, although uncomfortable, is one way your body automatically increases heat production when you're cold: you muscles vibrate, so increase energy output without actually going anywhere--like taking a brisk walk while standing still.