A retrieval of ice crystal effective diameter (De) near the tops of active deep convective clouds (Cb) throughout the Tropics over a 12-year period is presented based on radiances from the 3.7-micron channel of the AVHRR. Due to the great optical depth of this particular cloud type, independent information on optical depth is not crucial to retrieval of De and subtle microphysical variations can be retrieved accurately without reliance on a poorly-calibrated visible channel. The methodology yields the angular distribution of backscattered radiance empirically as a function of sun-satellite viewing geometry, which may be compared with ice-scattering model calculations or aircraft observations of other cloud types. The methodology also yields spatio-temporal variations of De. We find that De is 10-20% smaller over land than ocean, and exhibits other patterns correlated with aerosol presence in the lower troposphere (particularly, biomass burning), indicating that tropospheric aerosol can suppress hydrometeor sizes all the way up to the tops of the deepest cloud systems.