Volumetric Clouds

Set up a scene with a half geosphere with a gradient ramp for the sky and a plane for the terrain or water:

Top view and front view:

 

Add a mess of atmospheric gizmos:

 

Each gizmo needs a new seed:

In environment effects set up ONE volume fog ... pick ALL the atmospheric gizmos via the H key.

Here are my settings. Yours will probably vary:

Here is a camera viewport:

Avoid setting up gizmos like this shown below; when the edges “line up” you end up with a hard in your clod:

Here is a cropped render from the camera:

Add a general fog effect ...

You can set the background fog to exponential ...

... and get the effect of the fog gathering density in the distance:

Add layers set to the default settings ...

... and achieve yet more complexity and controls: