Electromagnetic radiation emitted from the Earth and its atmosphere. Terrestrial radiation, also called thermal infrared radiation or outgoing longwave radiation, is determined by the temperature and composition of the Earth's atmosphere and surface. The atmosphere is composed of two groups of gases, one with a nearly permanent concentration, consisting principally of nitrogen (N₂) and oxygen (O₂) molecules, and another with variable concentrations of other gases. Although considered to be permanent constituents, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and carbon monoxide (CO) have been observed to increase in association with anthropogenic activities. One of the principal variable gases is water (H₂O) vapor, the major compound that modulates the hydrological cycle involving evaporation, cloud formation, and precipitation.

Water vapor concentration decreases rapidly with latitude, almost following an exponential function. Ozone (O₃) concentration also varies significantly with space and time and occurs principally at altitudes of about 15–30 km (10–20 mi). A significant variable gas is a mixture of chlorofluorocarbons (CFCs) produced by industrial activities.