Thermodynamics is the study of the relationships between work, heat, and the different forms of energy. The first law of thermodynamics states that energy cannot be created or destroyed, though it can be converted from one form to another.

In discussing energy and other thermodynamic concepts, it is useful to simplify the problem by imposing boundaries. Within a boundary lies a system; the remainder of the universe is referred to as the surroundings. An open system is one in which matter and energy are exchanged with the surroundings. Precipitating clouds are a good example of an open system. A closed system does not exchange mass with its surroundings. For example, if we impose a boundary at the top of the atmosphere, then the Earth is a system and space its surroundings. This is a closed system (if we neglect meteor impacts and rockets blasting into space).

If we define a planet as an closed system, the only way it can exchange energy with its environment, space, is by radiation. For this reason it is     important to understand the radiation budget at the top of the atmosphere. This includes incoming solar energy, solar energy reflected back to space by Earth, and terrestrial energy emitted by Earth out to space.

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