1. Add a Low-level: Cumulus (3D /Volumetric) cloud layer.
2. Create a Distance Shader and pipe it into the Blending Shader input of the Cumulus' Density Fractal node.
2a. Pipe the camera into the Distance Shader. [Edit]: for got this one 3. Invert the blendshader in the Cumulus' Density Fractal node (and turn on Blend by Shader).
4. In the Distance Shader, set the Far Distance to 20,000 and the Near Distance to 19,000
5. Move the camera to roughly 2.2km from the surface, the clouds should be there.
6. Now move ever closer to the surface, until you are roughly 1.4km from the surface.
Result: the clouds disappear.
Cheers.