Planck's Law deals with absorbance of photons by so-called greenhouse gases. Under conditions of thermodynamic equilibrium, photons are absorbed and emitted by radiatively active gases such as CO2 and H2O vapour. Such condtions happen everywhere in earth's atmosphere. To model this: one would make a mean free path model of the photons as they move from their origin to space. Many originate at earth's surface. Others originate when H2O vapour condenses to droplets and releases latent heat somewhere at the top of the cloud layers. It follows that a "greenhouse gas effect" can only keep the atmosphere warmer than it would otherwise be. It happens as radiation transitions from its origin to space.
There can be no greenhouse gas effect making the surface warmer. Because cold does not heat warm. At everywhere on earth, the surface is warmer than above. As one rises above the surface a Lapse Rate of -6½ C per km (on average) shows the atmosphere cooling the further one gets from the surface. The very definition of "to heat" in physics means the transfer of energy from warmer to colder. In physics it is formulated as the Second Law of Thermodynamics. It is called a law because it's a formulation of something which is always observed. No counter example to the Second Law of Thermodynamics has ever been shown.
- photon : the quanta of enery, as EMR, absorbed or emitted by substances.
- Lapse rate : Can vary from about -3C/km to -9C/km mainly depending upon humidity and cloud but is always cooling as one elevates above the surface.
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