system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come being outside in 90 degree weather is uncomfortable, but 90 degree water is amazing...65 degree weather is very comfortable, but 65 degree water is awful?
gpt
The thing is, your body doesn't really measure temperature - it measures a little something known as heat flux.

Heat flux is pretty much the rate at which heat enters or leaves your body. For us, the most comfortable state is when we have a little bit of heat leaving our body. 

In all of these cases, heat is moving from the body to the surrounding environment, since your body is always warmer, but the important part here is the rate. Different materials touching have different willingness to transfer heat to each other, as it turns out, water transfers heat easier than air does. The farther the temperatures between the two materials is, the faster the heat transfer will occur as well.

So, 90 degree air is uncomfortable because your body wants to move more heat from your body to the surrounding environment, but water, which conducts heat better, will transfer the heat from your body to it much faster, making it a comfortable feeling.

The opposite is true. 65 degree air pulls quite a bit of heat from you, but still it is within normal comfortable range. Water on the other hand will pull much more heat from you, making it cold.