About Lapse Rates in Planetary Atmospheres

Lubos and Goddard seem to be confused about the role of the adiabatic lapse rate in planetary atmospheres. In particular, they seem to consider it sort of a law of nature. That’s the case only in a very limited way – the adiabatic lapse rate is rather a limiting condition: if the lapse rate becomes super adiabatic, then convection will occur. Nothing special will occur if the actual environmental lapse rate is less than the adiabatic lapse rate. The fact that the environmental lapse rates in the Terran, Martian, and Venusian atmospheres are all decidedly subadiabatic (on average) ought to be a clue as to that fact. Isothermal and even temperature inverted hunks of atmosphere are common, even near the surface of the Earth.

If an adiabatic or semi-adiabatic lapse rate is not some sort of consequence of the ideal gas law, then what does cause it? It’s caused by the fact that the atmosphere is heated from the bottom and cooled mainly from the top, and that is in turned caused by the opacity of the atmosphere in the infrared. If the atmospheres of Earth, Mars, and Venus were composed purely of some gas nearly transparent in the infrared (nitrogen, for example) then they would probably be nearly isothermal, and the surfaces of the planets would be at more or less the same temperature that they would be in a vacuum. If those atmospheres were almost perfectly opaque (i.e., for large greenhouse effects), then convection would dominate, and the portions of the atmosphere at an optical depth greater than one would closely approach the adiabatic limit. For the Earth and for Venus, the lower atmosphere is pretty opaque, and convection plays an important role.

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