Ive noticed this when I go cliff jumping. When I land it hurts more than at a pool.
-Water depth is deep; negligible
-Water temp in river is 9C
-Water temp in pool is about 28C
So does air solubility have anything to do with it? Or water density? Or is it just a placebo effect?
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Cold water is more viscous than warm, but I have no idea if the difference affects diving into it.
I’m sure there are physical reasons why the cold river water is technically more dense than warm pool water, but I would guess it’s pretty negligible as far as human perception is concerned.
My thought is this has to do with your nervous system’s Cold Shock Response to being thrust into a bunch of cold water. So I’m not sure that I would call it a placebo, as there is a meaningful difference between jumping in the two different bodies of water, but it doesn’t have to do with the physical “hardness” of the water. It instead has to do with your neural reaction to the sudden change in temperature. To grossly over-summarize, you have a ton of cold receptors in your skin and when you suddenly flood them with activity by jumping in cold water it leads to funky (and sometimes dangerous/deadly) bodily responses.
Interestingly, once you make it through the initial Cold Shock Response there may be some anti-depressive benefits to having your neural system activated all at once in that way! Almost similar in theory to shock therapy, though obviously a whole lot different in many ways haha. Source for this: [source](https://pubmed.ncbi.nlm.nih.gov/17993252/)
Short answer is, yes, cold temperatures can in fact make water more dense, salt water is also more dense, so jumping into cold water is denser than warm, and jumping into salt water is denser than fresh, so cold salt water is definitely going to be more dense than a warm pool.
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