As the summer heats up, many people will head to local lakes to refresh themselves. In a temperate climate, the surface layer of a lake often warms to above 20 °C. However, if you were to dive down into the lake, you would soon be in water with a temperature close to a frigid 4 °C. That cooler water settles in the deep, partially isolating itself from the warmer, sunlit upper layer that you enjoy. You can thank the physics of the water molecule for that—pure, salt-free water is at its most dense at 4 °C.

Bodies of water tend to settle into a state in which the fluid density increases with depth. That tendency, called density stratification, is often a dominating influence on the physics of lakes and oceans. The phenomenon, however, is more complicated when the water contains dissolved salts. Along with heat, salts act to change the...

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