This is a bit complicated... it has to do with thermal transfer rates as well as state change energy costs...so here goes.

Imagine the temperature outside the igloo is 0F. When the ice is warmer than 0F, it will want to radiate its heat out into the colder 0F air. For sake of this argument lets say that 0F Ice has Zero thermal energy. 10F ice has 10 thermal and so forth. I'm making the thermal numbers up for simplicity here...

You sit inside the igloo, warming the air up to 50F - but the air inside the igloo is far less dense than the ice. While ice at 10F has 10 Thermals (T) air at 10F only has 0.1T because there's so much less mass to the air.

That warm air wants to transfer the thermal energy to the ice - and it does - which absorbs it. Not the 0F ice has 0.1T (so its temp changes to 0.1F). It's not even - the ice inside the dome is warmer than the ice on the outside - but the internal temp of the ice is also trying to equalize

Over time the ice on the inside of the dome warms up to, oh, lets say its 10F so its got 10T but the outside ice is only at 2F. That 2F ice is radiating heat out to the cold 0F air.

Heat comes into the ice on the inside of the igloo and leaves the ice on the outside. It's slow but it only has to be fast enough that the rate of change never lets the ice get higher than 32F.

If the air inside got hot enough, it could overwhelm the interior, melting it - but one other factor is at play here. It only takes 1T to take ice from 30F to 31F, and 1T to go from 31F to 32F - but to MELT requires a state change. State changes in water take a lot more energy. Call it 5T. IF the surface of the ice reaches 32F, then more heat will start triggering a very very slow melt but at the same time the heat is still being sucked into the middle of the ice.

AND the warmer the ice on the inside is? The faster the heat moves to the middle. A bigger temp difference means a faster rate of change. Thus the heat that would melt the ice is moving away from the warmer ice faster than the ice can absorb it for the state change.

Of course the same is true for the temp diff between the warm air and the ice - so like I said - if the air is hot ENOUGH the ice will melt - how hot exactly? I have no idea - but warm enough to be able to survive apparently isn't warm enough to overwhelm the ice when it's super cold outside.

If the air outside is warmer, well then of course that works against the ice too. Igloos would work better at sub freezing temps, otherwise that heat wouldn't leave the ice - it'd stick around for the state change but on both sides of the ice.

Warm is relative term. If it's 10° below zero and -30 with the wind chill, you will automatically feel 20° warmer in an igloo since it blocks the wind. Then it will also trap body heat which could get it up to 25 or 30°. At that point the feels like temperature could be 60° warmer than outside, but still be below freezing. Also, the blocks of an igloo are quite thick, so even if the air inside is a bit above freezing, it won't completely melt the igloo. The inside layer of snow and ice might start to warm up a bit, but it will also constantly be cooled by the rest of the ice block.

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It seems like you know what you're talking about but I don't understand. I have no idea what thermal energy or thermals are, so I'm totally lost with what role that plays. Are you able to explain this like you're talking to a 5 year old?

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No - but I can explain some terms further. ELI5 isn't a *literal* requirement here - 5 year olds don't generally have the language yet to understand thermodynamics.

That said - thermal energy is just a fancy name for heat - and thermals is me making up a unit of measure for the amount of heat.

You have to measure heat differently from temperature. Temperature tells you how hot a substance is but different substances hold different levels of heat. (Strictly speaking the correct term is Joules but I called them Thermals because I was making up numbers and didn't want to make you think my numbers were anywhere close to correct).

The measurement of heat capacity is actually based on joules of energy per kilogram per degree Kelvin but in simpler terms it means if you have a pound of water and a pound of air (yes, air has weight) both can be the same temp but the water will hold more than 4 times the amount of actual heat energy than the air. What happens when the amount of heat hits its limit? The temperature goes up.

Looking at it another way - if you have a pound of air and a pound of water - but the water is at 0F and the air is at 10F - if you moved ALL the heat from the air into the water, the water would only go up to just under 2.5F, not 10F - because water can absorb so much more heat than air. Same amount of energy but in different materials - so less temperature change in the denser material.

Now in reality temperature won't move 100% from one to the other - instead they'll reach a balance where the temperatures equalize - which will still work out to about 2 deg F for both the air and the water since the water will absorb most but not all the heat.

If that doesn't clarify please lmk what you need help with, happy to keep trying to come up with examples. :)