View Full Version : How do blankets (or coats or whatever) keep you warm?
01-16-2009, 01:15 PM
It's a really cold day in Chicago today and I was just wondering how blankets work. How much does it matter what they're made of, or how thick they are? Does down work better if you fluff it up? Does layering your clothes ever become moot? I feel like the little brother from "A Christmas Story" these days when I leave the house.
01-16-2009, 01:20 PM
Blankets keep your body heat in, well, your body. Of course it matters what they're made of and how thick they are. The conductive heat transfer coefficient determines how good of an insulator or conductor of heat a material is, and that is well defined for almost any common material.
Layering only becomes moot when you pass what's known as a critical diameter. Since the surface area will be increasing relative to the square of the radius, and the radius is increasing linearly, at some point there will be much, much more surface area than there is thickness, and you will actually start to lose more heat by adding more insulation. The classical case is of a pipe with a hot fluid. The surface area is very important, that's why heat sinks and radiators have fins (to increase their surface area).
01-16-2009, 01:23 PM
Insulation keeps you warm by retaining your body heat (in the form of heated air kept close to the body). Layers are useful if you have to moderate how much heat you retain due to exertion. But to a simple approximation the more thickness of warm air you retain the less heat you will lose.
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