Atmospheric controls. These parameters control the nature of the light source (the sun) and the way it's light is is changed as it passed through the atmosphere or reflected/shaded.
The atmosphere model in GenesisIV is extremely powerful and allows many atmospheric effects, some of which deviate dramatically from normal atmospheric conditions. Atmospheres can easily be reset using the Quick Settings dropdown list.
These parameters are not completely independent and changes in one may effect the behaviour of others.

Atmospheric Colours. The colours used to create the atmosphere. Click on the coloured button to change.
Most colours are self explanatory in their effect, with the exception of Decay, which controls the decay profile of the light rather than a direct colour.

This drop-down list contains a set of preset atmospheric conditions which can be quickly applied to the project. The large number of atmospheric controls that GenesisIV presents make it very easy to quickly become lost - this drop down provides a simple group of common settings.

Sun position as Azimuth and Altitude. Azimuth denotes the number of degrees around the circle clockwise from north, so if the sun is shinning directly from the east this will be 90, south 180 and west 270.
Altitude denotes the angle between directly overhead and the horizon, so 90 would have the sun on the horizon. Generally altitude angles of around 40 to 75 degrees work best. To bring out the maximum relief in an landscape position the sun so it is about 135 degrees further west than the viewer, i.e. if looking northwards then shinning from the north west.
You can change the sun position using either the boxes or clicking on the blue circular sections.

These controls set the nominal top and bottom of the atmosphere. Generally they can be left as the defaults of 200 for the ceiling and -100 for the floor, but if your landscape is particularly high or low you may need to change these as the terrain can protrude through the boundary leading to unusual effects. Reducing the height span in some cases, where the landcape height range allows, can produce more dramatic skies.

These two parameters, the Atmospheric Control Distance and Infinity, input into the mathematics used to calculate atmospheric light decay. They can interact in complex ways with some of the Atmospheric Effects parameters, so should be changed with care. However some adjustments can produce interesting and unusual atmospheres.

GenesisIV draws the sky as a hemispherical dome over the landscape, with part of a further dome inverted from the horizon downwards so as to simulate a distant horizon beyond the defined landscape. At a displacement of 0 the bottom edge of the sky dome is at zero altitude, but because the camera is often used many metres above the landscape surface this may look incorrect with with flat landscapes. Adjust this figure to displace the sky dome downwards (negative), or in case of high altitude landcapes, upwards