One of the underlying factors that motivates much of the research in atmospheric radiation arises from the need to understand the role of radiation in shaping not only our present climate but also in its influence on climate change. Recent research on this topic has focussed on producing quantitative estimates of the radiative forcing of climate especially associated with changes in greenhouse gas concentrations, ozone depletion, sulphate aerosols and volcanic aerosols. What is meant by radiative forcing is itself an issue of debate [ Houghton and Filho, 1994] and estimating the radiative responses associated with cloud is difficult.