When the atm_option control has the value 'fixed_Tsurf', MESA
sets T_{\rm surf} to the value specified by the
atm_fixed_Tsurf control. The effective temperature is determined
from the Eddington-grey T(\tau) relation
T_{\rm surf}^{4} = \frac{3}{4} T_{\rm eff}^{4} \left( \tau_{\rm surf} + 2/3 \right).
See :ref:`here <tau-surf>` for a discussion of how the surface optical depth \tau_{\rm surf} is evaluated.
Then, MESA sets P_{\rm surf} to equal the radiation pressure associated with T_{\rm surf}.