Math is the easy part

So I asked that math question earlier because I was looking at the clouds and realized that those moving wisps and curves and waves are just the boundary line at which the level of humidity in the air cross over into visibility: that gradient continues beyond that boundary, extending invisibly through the entire volume of air from the surface of our skin all the way to the outer edge of the atmosphere. The clouds just the one slice through that space-filling structure that happens to be visible to us.

And then I started trying to picture the humidity level at each point in space mapped as a function through that space, sweeping the visible slice from 100% humidity down to zero and back to reveal the whole shape, packed in the entire sky, one complexly-curved layer at a time.

Except I couldn't remember the word "function" and got hung up on the vocabulary and lost the train of thought completely.