Right click on the ground, and there's a new option: Add Cloud.

This adds a cloud region, a very small one by, and opens the cloud editing menu:

There, you can edit various things, allowing you to make all kinds of cloud layers.

By default they ignore the wind, but you can make them move with local, target, or custom wind.

To re-edit clouds, you'll find them in the Objects menu.

Bugs and suggestions are, as always, appreciated.

One immediate application of this is the Tedesco case, where a plane flies up into some clouds.