The phenomenon of instability applies to all fluids. Measurements of surface temperatures of turbid water in a ricefield in New South Wales showed stable conditions from 8 a.m. to 2 p.m.
in late spring (i.e. the surface film was then warmer than the water a centimetre below), but thereafter there was instability as the surface film cooled as much as 5 K lower. The water layers would then mix turbulently, and the entire water column cool off uniformly.
Similarly, lake water is warmest on top in summer but cools uniformly in autumn, when cooling at the surface makes the density profile lead to instability. The water density in oceans is complicated by the dissolved salt (Chapter 11), yet the same concepts of static stability apply. The effect of salt on water's density, and hence its pattern of stability, enables the capture of solar energy (Note 7.I).
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