Eli5: How come wood stays cool to the touch in high temperature environments (e.g. saunas) when all other common materials would be hot?
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Tl,dr your body doesn’t sense temperature. It senses the rate of heat flow. When heat is flowing out of your body you interpret your surroundings as cold, and the faster the heat loss the colder your interpret your surrpundings to be. Vice versa for heat flowing in.
Go indoors. Put one hand on some metal. Put your other hand on some wood. They are the same temp, but they don’t feel like it. That id because your body doesn’t sense temperature, it senses heat transfer. Your skin is warmer than room temp. If you touch something that is cooler than your skin and easilly conducts heats, such as metal, it will feel cold. Wood has much lower rates of thermal energy transfer because, unlike metal, it doesn’t have lots of free electrons zipping around through it to conduct that heat away fro your hand (not ELI5, but one of the main aspects of metals is their freely moving valences electrons). So your hand loses heat less quickly, and you interpret the wood as being warmer, even though it isn’t.
This is also why you get acclimated to cool water on a hot summer day. Yiu’re sitting in the sun and your body is trying to get rid of this by floeing blood through capillaries near the surface of your skin. You hop in the pools, and it feels cold because your warm skin is rapidly losing heat to the cool water. Your skin cools off, and your body tries to reduce heat loss by constricting blood vessels to reduce flow through those capillarirs near the surface. As a result, you are now losing much less heat to the water, and you interpret tjis as the water being warmer, even though it isn’t.
This is good, because heat flow direction and rate is what really matters to a living organism for temperature regulation.
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