Use these physics tips on how to build igloos, and you can become the most popular person during the new ice age!
Perhaps an asteroid hit the earth and kicked up enough dust to block out the sun. Perhaps global warming caused extreme temperatures at the lower end of the scale in your neck of the woods. Perhaps a supervillain with a Persian cat got ahold of a machine that controls the weather. Whatever happened, it resulted in you wandering through a frozen wasteland without shelter. You need a place to stay the night, and io9 can help you build one. The first step is to avoid some common misconceptions about igloos.
Igloos aren't often made of ice. In fact, ice can be a pretty poor construction material. It's heavy, difficult to sculpt, and isn't any good for insulation. The best material to build your shelter out of is blocks of snow. Although snow and ice are made up the same stuff, frozen water, its structure is different. Ice is a rigid crystalline structure which transmits, and steals, heat rapidily. Snow is made up of small ice crystals interspersed with pockets of air, and air is a very good insulator. These air pockets can maintain a difference of 40 degrees between the inside and the outside of an igloo, and do it without sucking body heat away as dramatically as ice does.
Igloos also aren't the half-spheres that they're often shown as. A semi-circle is a very bad structure to build, engineering-wise. The principles that hold an igloo up follow the same ones that hold up an arch. The top and the sides have to be balanced against each other in order for the structure to stand. If the sides are sloping inwards, it's easy for them to prop the roof up. Although a half-circle looks like it has inward-sloping sides, when those sides hit the earth they are exactly perpendicular to it - standing straight up. This puts a lot of stress at the bottom of the structure, since that section of the wall is being pushed outward by the snow above it.
A parabolic shape lets the base of the walls hit the ground at an outward-sloping angle. The push outward from the top is countered with a push inward from the ground.
Not that the igloo should sit directly on the ground. Wind blows level with the ground, and heat rises away from it. Igloos aren't smooth domes inside. The best ones have interiors that take advantage of the way air moves. A doorway, even one covered by material, lets in the wind. The entrance of an igloo needs to be dug into the snow on which it stands. It should be so deep, in fact, that when a person passes into the actual dome of the igloo they have to climb up over a small ‘stair.' This keeps out most of the wind and makes a huge difference in temperature.
The stairs shouldn't stop once you enter the igloo. Heat rises, so the interior of the structure needs to be terraced to allow people to get higher, and closer to the heat. The center of the igloo should be the lowest area, and it should be small. The sides should be cut into wide benches or stairs that climb ever higher. These stairs will cut into the space inside the igloo, but space is not a commodity; heat is. The higher the area, the closer it is to the warm ceiling. High benches, close to the ceiling, provide warm sleeping spaces.
Before you settle in and await starvation or death by yeti, there is one finishing touch to add: holes. All that insulation can pay off too well, especially if the igloo is used a lot. Brief periods of heating cause the interior layer of snow to melt. Because of the parabolic structure, the water doesn't drip, but flows down the sides of the structure. When the igloo cools again, the water refreezes, this time into ice. The ice makes the igloo stronger, but it also seals up any cracks or conduits for air. After a few hours of breathing in a structure that doesn't breath along with you, and the carbon dioxide build-up can kill, so drill a few holes in the top and the sides, and only breathe when strictly necessary.
For a more detailed guid to building an igloo of your own, go here.