This is a really quick, off the top of my head reply. Try adding a filter node that would constrain the altitude values before the blender. (not talking about the altitude constraint in the influence of environment tab of the filter editor, but the filter itself bringing any value above a certain height down to a lower value so nothing appears above your set height)

But functions are really visual. If that sounds wierd or doesn't work, can you give me a screenshot of the function you made?

Thanks.

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3D Art Essentials: The Fundamentals of 3D Modeling, Texturing and Animation Vue 7 From the Ground Up