Well, just anything that takes a height and can be adjusted, such as a distribution or surface shader, or perhaps even get altitude and a smooth step function, where you can input the heights preferred.
If you have a cloud that goes to 5000m high, but with unchecked taper top and base, you can cut it off smoothly at say 4500m with such a distribution shader with fuzzy zone. And thus keep the total underside of that cloud, because tapering is unchecked.