The role of clouds in the radiative balance of the atmosphere and their effects on climate

Recent measurements of the reflection (back-scattering), transmission and absorption of solar radiation and also the emission and absorption of terrestrial infrared radiation by liquid-water and ice-crystal clouds are reviewed. The measurements are made by a suite of radiometers on board the Meteorological Office flying laboratory operating at visible, infrared and microwave wavelengths. Values of radiative properties such as cloud reflectance/albedo transmittance, absorptance and optical depth derived from these measurements are compared with corresponding theoretical and model computations. The radiometric measurements are interpreted in terms of simultaneous measurements of cloud height, thickness, temperature, the liquid-water and ice contents of the cloud, the concentration, size and shape of the cloud droplets and ice crystals. The limitations and uncertainties of the radiometric and microphysical measurements, and the deficiencies of the models are described. The effects of clouds on the Earth's radiation budget, and hence on the global climate, are discussed, together with their influence on model predictions of global warming attributed to emissions of greenhouse gases such as CO 2 and methane.