From AMS Glossary
A streamline or flow boundary representing the height that separates stable flow approaching a hill or mountain into two regions: a lower region, where the flow is horizontal and splits around the obstacle (i.e., the flow is partially blocked), and an upper region, where the flow is three-dimensional and goes over the hill.
This occurs when the relationship between the flow and the obstacle height H is characterized by a Froude number Fr less than 1. The height of the boundary Hc has been found to be Hc = H(1 - Fr ). Thus, it lies between the top and bottom of the hill and is lower for more stable flow. This concept has been used in air pollution studies to determine where pollution plumes are apt to impinge on a slope surface.