An igloo is not only used as shelter from snow but also to keep warm. Perhaps, a simple igloo is made of ice and nothing else still, why is its interior warmer than the exterior?
1$\begingroup$ Though maybe not a large effect, the round shape provides the greatest inner volume to surface area ratio, which should also help minimize the heat transfer per unit of volume. Also heat rises, but space decreases with height. $\endgroup$– JoLJan 15, 2021 at 23:02
$\begingroup$ @JoL But volume is an only rough measure how useful a space is. There are surely shapes with the same surface area with more useful volume. The round shape is more likely for the structural support of the arch (imagine trying to build a log cabin, except out of blocks of ice rather than logs) . $\endgroup$– AcccumulationJan 16, 2021 at 22:19
$\begingroup$ If you understand why it is warmer inside a house, a cabin or any other kind of shelter, then the igloo is in no way different: it provides a thermally insulated space. $\endgroup$– nasuAug 29, 2021 at 22:30
An igloo is not made from ice, but made from compressed snow. Snow is basically semi-frozen water or frozen crystalline water.
Contrary to intuition, snow has actually got very good insulating properties. Solid ice on the other hand, is not a good insulator compared to compressed snow. This is because ice is actually solid but snow is filled with minute pockets of air. While snow on an igloo does indeed look solid, up to 95% of it is actually air trapped inside minute crystals. Because this air cannot circulate much inside these ice crystals, heat becomes trapped inside it.
Some engineering also goes into the design of the inside of an igloo. The inside is divided into levels, where the upper level is for sleeping, the middle one is for fire and cooking (yes, fire! and a little hole is built into the top of the igloo to prevent smoke inhalation) and a lower level is used as a sink for cold air. As we know, heavier colder air naturally drops, and since the lowest level is where the door is placed, this cold air stays there. And warm air which rises, collects where it is mostly needed - in the eating and sleeping levels. Also, since the entrance to the igloo is at the bottom part - the tunnel to crawl through whilst entering or exiting the igloo - freezing air cannot blow directly into its interior.
Temperatures can reach as low as -50°F (-45°C) outside the igloo but the temperature inside can be a “comfortable” 20°-70°F (-7° to 20°C) (when you’re exposed to those low temperatures outside, coming back in can be most pleasant!).
All of this can be explained if we consider heat transfer and convection. This is a process whereby when a fluid moves, it transfers heat along with it. When this fluid is stationary, it will transfer heat by thermal conduction which is the transfer of heat from one body to another when they are in contact. You can see an example of this when you touch an ice cube and seeing it melt right where you fingertip is. But the more a fluid moves, the greater is it’s Reynolds number since the flow patterns become more unpredictable. The greater the Reynolds number, the more heat transferred via convection. And because snow has a low thermal conductivity, as mentioned above for air and how the snow contains air pockets, an igloo stops the heat transfer into its outside surroundings. The compressed snow and stationary air both act as surprisingly effective insulators.
$\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$– tpg2114Jan 20, 2021 at 0:54
Although igloos are often associated with all Inuit and Eskimo peoples, they were traditionally used only by the people of Canada's Central Arctic and Greenland's Thule area. Other Inuit tended to use snow to insulate their houses, which were constructed from whalebone and hides. Snow is used because the air pockets trapped in it make it an insulator. On the outside, temperatures may be as low as −45 °C (−49 °F), but on the inside, the temperature may range from −7 to 16 °C (19 to 61 °F) when warmed by body heat alone.
italics mine .
Certainly they would use fire to heat also.
The crucial concept is that snow is an insulator.
2$\begingroup$ All true but simply being protected from wind-chill would be worth it. I know from personal experience that being outside (dressed properly) when it's below 0F can be tolerable if not moderately comfortable but only if the wind speed is nominally 0. If there's any significant movement of air, it's unbearable. $\endgroup$ Jan 14, 2021 at 19:16
$\begingroup$ "snow is an insulator" reminds me of the concept (falsely) termed "greenhouse effect" - greenhouses work by preventing convection of the air inside, shielding off movement of air inside by winds outside (CO2 does not work like a greenhouse, thus). In fact, the true greenhouse effect might work more like snow does, as a large pocket of insulating - non moved - air. This I can see anew as it'd be trivial to parallel the igloo to the greenhouse in shielding off the wind. Any material would do, in that respect. Thank you. $\endgroup$ Nov 13, 2022 at 9:24
Because it is heated up from the inside by people (or possibly small fires).
The snow walls work as a thermal insulator to the outside.
You are asking why it is warmer inside the igloo then outside.
Some experts say that a well-constructed igloo, coupled with a very small oil lamp and plain old body heat, can warm an igloo up to 40 degrees above the outside temperature. Hypothetically, if it is -40°C outside, the igloo has the potential to warm up to 0°C. It accomplishes this amazing feat thanks to several features:
You can see on the image an igloo illuminated by fire.
There are several reasons:
wind, there is no wind inside
people generate body heat inside
they combine heating and lighting (oil lamp), or they do use some kind of heating in certain cases, like fire
the igloo is not made of ice, but compressed snow (insulator)
The inside of an igloo is NOT warmer than the outside, it is merely very well insulated from the outside air.
It is only when you place a source of heat inside the igloo, such as a 300w "human body" brand heater or a 500w "oil lamp" class heater that the inside air will warm up.
Then, of course, this warm air stays warm because the snow (not ice) that the Igloo is build out of is a very, very bad conductor of heat.
$\begingroup$ "... merely very well insulated ... " So is it really different from the greenhouse that prevents convection but uses incoming UVA radiation as a source of heat? Just an idea: whereas water absorbs UVA as transfers UVA to heat and warms up, snow might even, like glass does, let pass through some UVA radiation from the sun. However, UVA radiation does not warm up the air but needs soil or water as a transformer (the latter, I'm not sure about any more). - Sure, there must be some melting inside of snow, as - paradoxically, considering the answer, air in snow is a good insulator. $\endgroup$ Nov 13, 2022 at 9:29