Sharp Cut-Out Height Fog?

Hey guys:,

Just wanted to know if it is possible to make a Height Fog image effect that is completely cutout at the height point, the standard Height Fog bleeds a little bit far from the max point of height.

I would like to have a height fog that doesn't have any gradient on the top, that it could go from 1 fog, to 0 fog in just a height point value.

 F
 -----                  -------                            - Height Point.
 F                          F                              F Fog.
 F       ----->             F
 F                          F