A temperature spectrum of heterogeneous freezing nuclei concentrations in continental air in the upper troposphere was determined based on airborne measurements. Numerical model simulations incorporating ice formation by heterogeneous and homogeneous freezing of deliquesced soluble aerosol particles were performed to investigate the effect of the heterogeneous process on the microphysics of upper tropospheric clouds. Heterogeneous freezing nuclei were predicted to cause lower maximum concentrations of ice particles formed in clouds. These nuclei also initiate the first ice formation and act to broaden ice crystal size distributions in upper tropospheric clouds. Observations of ice formation in an orographic wave cloud supported these predictions. ¿ 1998 American Geophysical Union