Tropospheric clouds can impact the radiative heating of the lower stratosphere substantially, though their effect over a winter has been estimated by previous studies to be small. In this study clouds were incorporated into radiative transfer calculations by matching computations of outgoing longwave radiation (OLR) with collocated satellite observations of OLR. Narrowband (cm-1 resolution) heating rate calculations reveal that differences in clear and cloudy heating rates occur in the ozone 9.6 μm band, where radiation exchange with the lower surface dominates the heating rate terms. By reducing the temperature of the emitting surface, clouds can decrease the heating rate by 0.6 K d-1 (d&thgr;/dt) at 450 K for single profile calculations and have the greatest impact when over the warm, open ocean. Monthly mean calculations in the Southern Hemisphere show regions where heating rates decrease (cooling rates increase) by 0.2--0.3 K d-1 when clouds are added; these regions include areas that are equatorward of the sea ice edge but within the stratospheric polar vortex. Zonal mean heating rates display a maximum just poleward of the vortex edge during August and September 1994. Zonal monthly mean comparisons of the OLR-matched cloudy heating rates with those using a cloud climatology show that the climatology captures 50--75% of this decrease in heating rates compared to clear skies. Differences between zonal monthly means of the daily heating rate and heating rates computed using zonal monthly inputs are small, validating the use of mean radiative heating rates in two-dimensional monthly calculations. ¿ 1999 American Geophysical Union