This paper presents mean greybody infrared effective emissivity values of clouds deduced from 300 International Quiet Sun Year (IQSY) radiometersonde ascents. The cloud effective emissivity data are presented for two latitude regions: midlatitude and tropical. Mean cloud effective emissivity values for the surface to 300 mb layer ranged from 0.41 to 0.64 for the midlatitude data and from 0.54 to 0.84 for the tropical data. Clouds in the pressure interval from 400 to 600 mb exhibited the largest mean emissivity values. These data should he very useful for incorporation of realistic cloud radiative properties into modeling of atmospheric dynamics and climate.