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The atmosphere consists of molecules and aerosols. An aerosol constitutes anything other than a molecule suspended in the atmosphere, typically within a size range between 10 to 10 m (Measures, 1992). This includes wind blown dust and pollen, various forms of pollution (e.g., ash, sulfites) which may change through photochemical processes, and all varieties of clouds. Together these particles scatter, absorb, and emit radiation from the ultraviolet through infrared.
The presence of an equator to pole gradient of incident solar radiation, coupled with infrared radiative cooling, induces a thermally driven global circulation. Spectral characteristics due to thermal differences between solar and terrestrial emission allow independent observation of this radiative imbalance; where the visible and infrared Planck emission curves exhibit negligible overlap. The following sections will detail and differentiate the radiative principles required to analyze remotely sensed observations in the infrared and visible domains.