This has profound consequences for the biosphere. Clouds also participate in the greenhouse effect, but they are opaque and behave as non-selective absorbers. Hence, they close off the atmospheric window and trap in even more heat energy. Clouds provide a nice illustration of the concept of feedback within a system. Feedback occurs when flows of energy or matter in a system influence the basic operation of the system.
Net radiation is determined by inputs and outputs of solar and terrestrial radiation.
As a global, long-term average, incoming energy must equal outgoing energy. Otherwise, temperatures would be progressively increasing or decreasing. In daytime, net radiation of a surface is generally positive, dominated by incoming solar radiation. At night, net radiation of a surface is negative, dominated by outgoing terrestrial radiation. This is how much solar radiation strikes the earth or atmosphere but is never absorbed.