  Stellar models often use relations between the temperature $T$ and optical
depth $\tau$ to evaluate the structure of their optically-thin outer layers. We
fit a novel analytic function to the Hopf function $q(\tau)$ of a
radiation-coupled hydrodynamics simulation of near-surface convection with
solar parameters by Trampedach et al. (2014). The fit is accurate to within
0.82 per cent for the solar simulation and to within 13 per cent for all the
simulations that are not on either the low-temperature or low-gravity edges of
the grid of simulations.
