In numerical models the infra-red emissivity of ice clouds is generally computed from the optical depth. The convex relationship between these two quantities implies that if ice clouds are inhomogeneous, emissivities calculated from a grid-box mean optical depth will be biased high. An analogous effect in stratocumulus albedo and liquid water path is compensated for by multiplying liquid water by 0.7 to calculate albedo. However it is unclear that a similar correction is valid for all cloud types. In this paper profiles of radar reflectivity in mid-latitude winter ice clouds are analyzed to derive the variability of optical depth and quantify any biases introduced into emissivities calculated from grid-box mean optical depths. The bias is small for grid-lengths of 10km, but the average optical depth should be multiplied by a correction factor of 0.44 for calculating emissivities in a 300km box.