Video

The Dry Wet Valley Phenomenon

A Föhn wall and Rotor Clouds are Created by Orographic lift  when the air mass is forced from a low elevation of the San Luis Valley to a higher elevation as it moves over Sangre de Cristo Range.

As the air mass gains altitude it quickly cools adiabatically (lowered pressure), which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.  As this air descends on the leeward side, it is warmed by adiabatic compression at the dry adiabatic lapse rate. Because the air has lost much of its original water vapor content, the descending air creates an arid region on the leeward side of the mountain, sometimes as little as 15 miles away from high precipitation zones, annual precipitation can be as low as 8 inches per year.

On the leeward side of the mountain, the air flowing downward is known as a foehn wind. Because some of the moisture has condensed on the top of the mountain, the foehn (or föhn) is drier, so any suspended moisture quickly evaporates as the air descends. Föhn winds can raise temperatures by as much as 54 °F in just a matter of hours. Winds of this type are called “snow-eaters” for their ability to make snow melt or sublimate rapidly.
The distinct cut-off line which forms along and parallel to the ridge line is sometimes known as a foehn wall (or föhn wall). This is because the edge appears stationary and it often appears to have an abrupt wall-like edge.  Rotor clouds are sometimes formed downwind and below the level of the ridge.  They have the appearance of the ragged cumulus cloud type but it is caused by a turbulent horizontal vortex.  Thankfully the Winter Snow accumulation on the Sangre de Cristo Range melts and flows into the Westcliffe valley were it is treated as a precious gift from nature and regulated by complicated water rights. It is not unknown to see men come to blows or bullets over a water issue.