The atmosphere can be enabled in the atmosphere sub-panel. This will add an atmosphere domain around the scene and change the scene's world background.
The elevation of the sun in degrees (zenith angle). At 90 degrees, the sun will be directly above the viewer, like at midday. Sunsets/sunrises occur at elevation angles approximately less than 7 degrees.
Note that the scene brightness changes drastically between 0-90 degrees of sun elevation and this will often need accountng for in the exposure and gamma settings.
The angle of the sun relative to the horizontal in degrees (azimuth angle).
The sealevel density of air. Higher values produce a thicker looking atmosphere. Be careful changing this value from the default, as the exact appearance of the sky is very sensitive to the atmospheric density.
Air Exp Altitude
Controls the exponential falloff of air density with altitude. At this altitude, the air density will be 36.7% of the sealevel density.
The sealevel density of aerosols. Aerosols are larger particles in the air such as water vapour or smog that produce white foggy scattering.
Aerosol Exp Altitude
Controls the exponential falloff of aerosol density with altitude.
Controls the proportion of light that is scattered forwards by aerosols towards the camera, producing a glow effect around the sun.
The amount of light that is absorbed by aerosols, making the distance appear darker. For a polluted air look, this value can be made higher. Humid air on the other hand does not absorb as much light, but could have a higher density.
PlanetSet generates an 'atmospheric shell' around the scene when the atmosphere is enabled. This shell extends upwards into the sky until it reaches the ceiling altitude, which can be changed in the atmosphere panel.
You may want to make this value much larger to recreate scattering for taller mountains or for high-altitude clouds. However, as the ceiling gets higher, the atmosphere starts becoming more like a true sky. Eventually, it becomes unphysical to illuminate the sky volume with the Nishita sky background. This would be equivalent to outer space looking like the sky, whereas in outer space there is just the direct light coming from the sun and everything else is black. Therefore, the physical atmopshere option simply sets the background to black with a sun disk, and the colours of the sky are recreated naturally through volumetric scattering.
The Nishita sky model is the default sky model built into Blender which users may be familiar with. PlanetSet adds a small scale medium around the planet to add the effect of Rayleigh scattering and haze to the terrain and distant clouds. The sky color comes from a combination of volume scattering and the sky lamp, which is fast but less accurate.
Enabling the physical atmosphere will yield a slightly different result since all the sky color contribution comes from the atmosphere volume shader. It is particularly effective at high altitude shots and sunset/dawn shots. It also makes very high-altitude clouds look more realistic. You can also control the ozone layer density, which gives more control over the lighting color at sunset/sunrise. Note, because all of the lighting contribution comes from the volume shader, it takes much longer for the renders to converge when the sun angle is low.