Effective Terrain

Definition

The National Weather Service Glossary, using the term effective topography, defines the concept as "the topography as seen by an approaching flow, which may include not only the actual terrain but also cold air masses trapped within or adjacent to the actual topography." In mountain valleys or larger basins between mountain ranges, the presence of dense cold air prevents warmer air from sinking into the lower elevations. The warmer air will tend to move over the top of the cold air. Cold air lodged against the side of a mountain range causes warm air to rise before it actually gets to the mountains themselves.

Implications

Heavier precipitation falls at lower elevations since air cannot sink into the lower elevations to warm and dry the air which in turn erodes the precipitation through the evaporation and sublimation processes.

Temperatures have a better chance of remaining cold enough for precipitation to stay snow as it falls into the lower elevations.

The potential for freezing rain exists if the cold air pooled at lower elevations is below freezing and the air riding over the top is warm enough for the precipitation to be liquid when it enters the cold air.

Examples showing how effective terrain can influence the weather

The Appalachian cold air damming discussion on this website

The Puget Sound Overrunning discussion on this website

The The St. Patricks Day Snowstorm, 2002, Anchorage, Alaska - Joel Curtis, John Papineau , Carven Scott, Weather Forecast Office, Anchorage, Alaska; Kent Johnson, Meteorological Service of Canada