We investigate the grain opacity Kappasubgr in the atmosphere (outer radiative zone) of forming planets. This is important for the observed planetary mass-radius relationship since Kappasubgr affects the primordial H/He envelope mass of low-mass planets and the critical core mass of giant planets. The goal of this study is to derive a simple analytical model for Kappasubgr and to explore its implications for the atmospheric structure and resulting gas accretion rate. In agreement with earlier results, we find that Kappasubgr is typically much lower than in the ISM. In retrospect, this suggests that classical giant planet formation models should have considered the grain-free case to be as equally meaningful as the full ISM opacity case. This corroborates the result that core accretion can explain the observed increase of the giant planet frequency with stellar Fe/H.