Tuesday, July 03, 2007

Evaporation, condensation, and energy conservation

The earlier extensive discussions of evaporation and condensation, temperature inversions, and the "extra" latent heat dumped into the atmosphere by the closed cycle left one big issue unaddressed: there's no indication where that extra energy is coming from.

All the heat flowing through the Earth's atmosphere ultimately comes from input solar radiation.* But the Earth's surface absorbs and uses that incoming radiation differently, depending on whether the surface is water or ground.
  • If ground, essentially all the visible radiation is absorbed and re-radiated upward as infrared (IR) or heat.
  • If water, much of the visible radiation is absorbed and re-radiated upward as IR. But a significant part is used instead to evaporate the liquid water - that is, this fraction of incoming radiation is converted to latent heat.
That's why liquid surface water is colder than the immediately surrounding land. Water evaporates and needs latent heat to do so, taking its "cut" of the heat flow; soil doesn't evaporate and takes no such "cut."

Overall, these basic differences between water and soil create a major difference between how heat flows vertically over land and over water. Water, being colder, doesn't radiate quite as much as land, which is hotter. However, the water then makes up for the difference when it condenses at altitude and releases its latent heat. If the whole surface were land, and the incoming radiation flow were 100 units, the net heat flowing up would be 122 units. But in fact, compared to this imaginary all-land-surface Earth, 122 units flow into, through, and out of the real lower atmosphere; seventy percent of the Earth's surface is water. The "extra" 22 units is merely due to the fact that somewhat cooler but evaporating water is a better absorber and distributor of radiation/heat than is hotter but non-evaporating land.

Overall, the total energy is conserved and radiative power in = radiative power out. But the magical properties of water make themselves manifest again - and again climate is driven by differences, this time between land and sea.
* I'm ignoring the small heat flux arising from the decay of radioactive rocks in the Earth's core, as well as the tiny flux of radiation from the Galactic and cosmic microwave background.

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