Start a new file in MAX or VIZ,

1) Create a plane for your ground surface. (CREATE>GEOMETRY/PLANE)

2) Create a big sphere, apply a NORMAL modifier to the sphere (MODIFIER>NORMAL) and select invert. (this is to make sure the sphere is viewable from inside) This big sphere is going to be used as the Blue sky background.

3) Position the sphere halfway, meaning centerpoint should be around the ground plane)

4) Select the Sphere and go into EDIT MESH mode (MODIFIER>EDIT MESH) and click on the VERTICES button, select the half the vertices underneath the ground plane and press delete. (We are basically chopping off the underside of the sphere, cause we don't need it.

5) Select Sphere and apply a spherical UVmap to it. (MODIFIER>UVMAP>SPHERE)

6) Create a gradient map and apply it to the sphere.

7) Now we are going to create the Gizmo apparatus. Create about 15 box gizmo's, all different sizes, different heights and different seeds. The seeds makes sure they are different so we don't get identical volumetric clouds.

8) After creating these we're going to add Volumetric fog to the gizmos, go to RENDERING>ENVIRONMENT and under "ATMOSPHERE" click on "ADD" and add a Volume Fog effect. Set the settings as I have there and select all the boxgizmos from the GIZMO>PICK GIZMO(under VOLUME FOG PARAMETERS).

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I always like creating two Volumetric Fog effects, and add different variations to different clouds. You can add more as you please.

9) Finally for an extra nice horizon effect you can add a FOG effect to smooth the horizon.

If you want to animate the Clouds, you can animate all the Gizmo to the direction you want, and Animate the "PHASE" parameter under the VOLUME FOG PARAMETERS. This will animate the volumetric effects of the clouds.

You may need to play with your settings to comply with your design and projects.
I may add a few animations of this tutorial, and also another cloud tutorial using a powerful plugin (Afterburn)