# Fixed Surface Temperature¶

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 here 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 $$T_{\rm surf}$$.