as the difference between the clear-sky and total-scene radiation results. This difference is defined as cloud-radiative forcing. Optically thick clouds reflect more short-wave radiation back to space than the darker surface would in the absence of the cloud. Thus, less solar energy is available to heat the surface and atmosphere which tends to cool the Earth's climate. In addition, roughly 20% of solar radiation is absorbed by atmospheric gases and clouds. The combination of CERES top-of-atmosphere radiation data with surface radiation measurements will allow unprecedented studies of the absorption of solar radiation within the atmosphere. Optical depth is a general measure of the capacity of a cloud or a region of the atmosphere to prevent the passage of light. Greater optical depth means greater blockage of the light and a larger cooling of the Earth-atmosphere system. The intensity of the thermal emission from a cloud varies with its temperature and the optical depth or thickness of the cloud. The top of the cloud is usually colder than the Earth's surface. If a cloud forms in a previously clear sky, the cold cloud top reduces the longwave emission to space, and energy is trapped beneath the cloud top. The trapped energy increases the temperature of the Earth's surface and atmosphere until the longwave emission to space once again balances the incoming absorbed shortwave radiation. This process is called the "greenhouse effect" and, taken by itself, causes a heating of the Earth's climate. High, thin cirrus clouds have a warming effect because they transmit most of the incoming solar radiation while, simultaneously, they absorb some of the Earth's infrared radiation and radiate it back to the surface. Deep convective clouds, such as those associated with thunderstorms, have neither a warming nor a cooling effect because their cloud greenhouse effect, although large, is nearly balanced by the effect due to the convective clouds' high albedo.