Stellar atmospheres are heated from below, are so hot that most elements are found only in atomic form, and have no clouds. Planetary (and satellite) atmospheres are heated from above, and feature molecular gases. Some key points of planetary atmospheres include the following.

• Energy source: the top of the atmosphere is irradiated by the Sun. Some of this radiation is absorbed or scattered by the upper levels of the atmosphere. The temperature profile is determined by this process, in conjunction with radiative losses and conduction.

• Energy from internal heat sources, for the gas giants, and re-radiation of absorbed sunlight from the surface below will modify the temperature profile.

• Chemical reactions in the atmosphere will alter the composition, changing the opacity and the thermal structure.

• Clouds, haze, and sedimentation will change the opacity and temperature through release (cloud formation) or absorption (evaporation) of latent heat.

• Chemical interactions between the atmosphere and the planetary crust or ocean can affect the atmospheres of terrestrial planets (and of satellites).

• Volcanoes and geyser activity can likewise modify the atmosphere substantially.

• The composition, opacity, and thermal structure can all be strongly affected by biochemical and anthropogenic processes.