An igloo is a sturdy structure made of easily obtainable materials. It protects its inhabitants from the elements and is easy to build. It is a prime example of man's ability to adapt to and use nature to his advantage.
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Igloos are constructed with one of the weakest materials known -- snow. Its spiral design and self-supporting domed shape make it strong enough to withstand arctic blizzards, lose half of itself and still continue to stand. It requires no interior support system yet can support the weight of a man balancing on its roof. The key to its strength is the structure of the snow; it must be windblown and compact enough to cut and shape into blocks.
Inside the igloo, heating causes the walls to melt. Continous melting and refreezing causes layers of ice to build up on the inner walls, adding to the overall strength of the igloo. A tunnel at the entrance traps cold air, keeping it away from the living area.
Snow and ice are natural insulators. Once warmed, an igloo maintains its inside warmth with little more than body heat and the light of an oil lamp.