Everything radiates—the Sun, the Earth, the atmosphere, and you. The energy provided by the Sun is reused in the Earth system to provide the energy that drives weather and climate. But ultimately, the infrared radiation radiated by Earth into space must balance the solar visible radiation coming into the Earth system. From the point-of-view of the Earth system, we are most concerned about how atmospheric radiation interacts with matter. Matter is simply molecules and atoms and the structures that they build, such as the air, the clouds, the Earth, and the Sun.
When radiation encounters matter, three things can happen. The radiation can be transmitted through matter; it can be absorbed by the matter; it can be scattered by the matter. One of these three things must happen, so we can sum them up to one:
where \(τ\) is the transmissivity, the fraction transmitted; a is the absorptivity, the fraction absorbed; and s is the reflectivity, the fraction that is scattered or reflected.
Scattering vs. reflecting
Scattering and reflecting are related but are different because reflection is scattering in a particular direction whereas scattering tends to go in a range of directions.