system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does it feel so miserable when it's 100°F/37°C outside, but so comforting when the water of your shower meets or is even above that temperature?
gpt
Well, first of all it doesn't feel that comforting to take a 100 degree shower when it's 100 degrees outside, does it?  It's nice when it's room temperature, below that, or maybe a little above.  It's because your body is constantly trying to keep your body at a relatively regular temperature.  This temperature varies across your body but we commonly accept that it's around 98.5 degrees.

Now when you're cold, your body has a very efficient way of keeping that temperature up; it burns calories.  However our method of cooling down when we're too warm is less efficient, we sweat.

So, if the temperature in your bathroom is 70 degrees, your body is warming constantly.  It's very efficient at this, but to then be thrust into a warm shower that's only a few degrees above our body temperature, it's like... wow now i don't have to work so hard!  On top of this, you're not actually getting 100 degrees most likely.  Most water heaters are set to around 110, but it cools quite a few degrees as it gets to the shower head.  Plus then on top of that you're not getting that on your entire body at once, you're getting sprayed across maybe 1/4th of your body.  Plus you can spin in place, duck your head, step forwards or backwards, etc all to direct that glorious warm water wherever you want.  Then when you start to get too warm, you turn and guess what happens?  That's right, similar to sweating the water runs down your body or evaporates off, carrying heat away from that spot.

Now, lets compare it to being in a room that's 100 degrees.  First of all, there's no break.  You can't turn and avoid the heat, it surrounds you.  It's why people put fans on themselves, to generate airflow to encourage evaporation of sweat from skin.  Plus there's a further issue at play here.  A lot of times when it's 100 degrees, it's also humid.  The more humid the environment, the less effective your sweating is at cooling you.

Ever hear someone say "yeah, it's 100 degrees out, but it's a dry heat!" as if that matters?  Well, it does!  A lower humidity means your body can cool itself more efficiently.

Anyways, hope that helps.