The fluid is always approximately in radiative – convective equilibrium () for the atmospheric structure through which it is moving. The upflows transfer their internal energy to radiation between optical depths and . Between those depths they have a temperature gradient close to but slightly less than the grey radiative equilibrium value of (Figure 14). Their temperature gradient is slightly less than the radiative equilibrium value because the radiative flux is increasing as the optical depth decreases due to the transfer of energy from convection to radiation. This well known gradient on an optical depth scale corresponds to an extremely steep gradient on a geometric depth scale (Figure 13), because of the extreme temperature sensitivity of the dominant H– opacity.
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