Most of the solar energy reaching the Earth originates from a
relatively thin layer of the Sun (
km thick) known as the photosphere.
The Sun can be considered as a blackbody of temperature 5800 K, that of
the photosphere, with the
maximum intensity in the solar spectrum occurring at 500 nm.
Absorption and re-emission within the solar atmosphere gives rise
to a fine structure, called Fraunhoffer lines, in the solar spectrum.
It takes this solar energy, in the form of electromagnetic radiation,
eight minutes to reach Earth's atmosphere. Upon entering,
the electromagnetic radiation, or sunlight, interacts with the
atmospheric molecules and is progressively absorbed and scattered.
The number of satellites measuring either direct or scattered sunlight has greatly increased over the past twenty years. These measurements are used to infer information about atmospheric composition and meteorology. As such, the ability to understand and predict how sunlight interacts with the atmosphere is essential to the correct interpretation of these measurements. The most useful tool from which this understanding can be gained is the mathematical model. The equations which describe the interaction of sunlight with the atmosphere are well known but difficult to solve. Through the use of computers, these equations may be solved numerically. The specific goal in the mathematical modeling of atmospheric radiation, or radiative transfer as it is more commonly known, involves taking the `physics' of the scattering and absorption phenomena and applying it to the Earth's atmosphere in order to quantitatively predict the characteristics of the radiation field. To accomplish this, knowledge of atmospheric composition, temperature, and pressure is required beforehand.