Obviously, there is more vapor close to the liquid surface that further away from it.

The higher gas velocity produces waves on the liquid surface.

Very probably, the opening is below the liquid surface - that's what those suits you saw were for.

"But how are we to get down below this liquid surface?"

As Sara leaned forward, images formed on the liquid surface.

The plain rippled again, as if it had suddenly been transformed into a liquid surface.

Horn could see a liquid surface ahead for as far as the foliage let him look.

Of the liquid surface of the sea there was no longer a glimpse.

The gas will form waves on the liquid surface, which may grow to bridge the whole cross-section of the line.

An important factor is the speed of the air being swept across the liquid surface.