|MadSci Network: Physics|
The secret to warmth in an igloo is the fact that snow and ice are very good insulators. I have heard that, between body heat and one or two oil lamps, people can keep a relatively comfortable temperature (say, 10-15o C) inside. You won't be able to run around nude, but with warm clothing you'll do alright.
Apparently what happens is that the inside layer does melt, but turns to ice because of the contact with the snow next to it -- so you get good insulation both inside and out. Of course, eventually the igloo would melt, but igloos are typically temporary shelters anyhow, used only for a few days or, at most, weeks.
I was raised in the northern United States (Minnesota) and have read about winter survival. If you are wearing warm clothing and dig into the leeward side of a snowdrift, then curl up and remain still, you can stay reasonably warm for a long time. The same principle is operating when you stay snug and warm in a hut built of snow!
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