Diathermancy

Diathermancy (from "dia" through and "thermē" heat) is the property of some fluids that allows rays of light through them without itself being heated. A diathermanous fluid is thus "permeable" by heat.[1]). Diathermancy was first described by German physicist and chemist Heinrich Magnus in the 1800s ([2]). Atmospheric air is diathermanous; therefore, air is not heated by sunshine, but by long wave heat reflected by soil, and especially, by water on the Earth's surface. Water, on the contrary, is not diathermanous, and it is heated directly by sunshine.

The Amazon river delta showing no clouds above river waters because water is not a diathermanous fluid.

Atmospheric heating from oceanic waters
Northeastern Pacific Ocean Surface Water Temperature

Atmospheric heat comes from long wave radiation from the soil and, mostly, from the water surface (oceans, lakes, rivers), since water is a not diathermanous body and covers three quarters of Earth's surface. Diathermancy cause subsidence on damp or water surfaces. That is because these areas tend to absorb heat radiation directly from the Sun but very slowly and also emits this radiation to the atmosphere very slowly. Therefore, cold ocean currents have very clear skies, without clouds, because subsidence from cold and heavy air avoids or limits convection since they are opposite processes.

References
  2. Magnus wrote initially four papers on Diathermancy )


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