 Solution

It is convenient to use the optical depth above a level,Thermal radiation escapes mainly from levels with $t$ of order unity: emission much deeper is attenuated, while the column far above contains little emitting material. Hence, in an optically thick atmosphere,For a particular viewing direction, $t\sim \mu$ instead gives $p_{\mathrm{rad}}\sim p_s\sqrt{\mu /{\tau }_{\infty }}$. These are characteristic levels, not sharp emitting surfaces; the contribution function depends on the temperature exponent $a$. If ${\tau }_{\infty }\lesssim 1$, the formal level lies below the surface and the surface itself contributes strongly. A useful estimate then clips $p_{\mathrm{rad}}$ at $p_s$.