Abstract

The International Satellite Cloud Climatology Project (ISCCP) dataset is used to correlate variations of cloud optical thickness and cloud temperature in today's atmosphere. The analysis focuses on low clouds in order to limit the importance of changes in cloud vertical extent, particle size, and water phase. Coherent patterns of change are observed on several time and space scales. On the planetary scale, clouds in colder, higher latitudes are found to be optically thicker than clouds in warmer, lower latitudes. On the planetary scale, winter clouds are, for the most part, optically thicker than summer clouds. The logarithmic derivative of cloud optical thickness with temperature is used to describe the sign and magnitude of the optical thickness-temperature correlation. The seasonal, latitudinal, and day-to-day variations of this relation are examined for Northern Hemisphere clouds in 1984. The analysis is done separately for clouds over land and ocean. In cold continental clouds, optical thickness increase with temperature, consistent with the temperature variation of the adiabatic cloud water content. In warm continental and in almost all maritime clouds, however, optical thickness decreases with temperature. The behavior of the optical thickness-temperature relation is usually, though not always, the same whether the temperature variations are driven by seasonal, latitudinal, or day-to-day changes. Important exceptions are noted. Some explanations for the observed behavior are proposed.