system
Given the following post, respond as if you're commenting on Reddit:
human
Why does cold water feel, or seem to feel, so much colder than cold air? Why does 50°F water seem so much colder than 50°F air?

I can walk outside in a pair of shorts and a t-shirt when its 50°F outside just fine, but when I get in 50°F water, I feel like I'm going to die.

Thanks gang.
gpt
This has to do with conduction, or how quickly heat can transfer through a material.  Both air and water are touching you, and in both cases, they're leeching the heat from you - so the real question is why are some materials better thermal conductors than others.

This can be solved by a version of the First Law of Thermodynamics,  given as:

Energy to heat an object =  Mass(Specific Heat)(Change in Temperature)

Specific heat capacity is a material property of all materials (defined as the amount of energy it takes to heat some unit mass of the material by one degree C), and this multiplied by the mass of the mass of the object gives you the thermal mass.  That's essentially "how much energy does it take to heat up this thing."  Multiply it by the change in temp, and you have your total energy to heat.

This is important because water has a higher specific heat capacity than air, so it absorbs more heat.  Due to another material property, called thermal conductivity, it takes that heat and dissipates it across the rest of the water that isn't touching you.

So, this answer really has 2 parts:

* Water takes a lot of energy to heat up, and

* There's usually a lot more water nearby than there is of you, so it absorbs heat from you very quickly until (eventually) you and all the water you're in contact with are at the same temperature.

Air takes less energy to heat up, because it weighs less and has poor thermal conductivity.  It doesn't absorb as much energy as water, and once it does, doesn't distribute it to nearby air nearly as well as water dissipates heat.