The net radiative flux density across a given point on the earth's surface, averaged over a specified time interval. Major components of the surface radiation budget are downward shortwave (direct plus diffuse solar radiation), upward shortwave (reflected), downward longwave (emitted from different levels of the atmosphere), and upward longwave (emitted from the surface). On a global annual average, the radiation budget for the earth's surface is positive, indicating an excess of solar heating over longwave loss, and is approximately 100 watts per square meter. Instantaneous values, however, may be positive or negative, and range through many hundreds of watts per square meter. In the annual global mean, the positive surface radiation budget is assumed to be matched equally by a negative atmospheric radiation budget so that the planet as a whole is in radiative equilibrium.