Commonly known around the Front Range area as a ‘mountain wave’, ACSL clouds are created when relatively fast-moving air that is relatively moist hits a topographic feature, typically mountains. The air is deflected upward, and cools slightly to below the dew point of the air mass, condensing into a cloud. The stable air moves downstream and starts to descend back down to its previous altitude, warming a bit back below the dew point, and the cloud disappears. Due to the physical uplift of the air by the mountain, the cloud stays relatively stationary, even though the air is moving anywhere from 40 to 100+ knots. In the two videos below, you can see the air movement downstream (right to left) through the cloud, even though the cloud remains in the same position.
These are the kind of conditions that all the sailplane pilots in Boulder wait for, as the rapidly-descending stable air allows them to rise to upper altitudes very rapidly. In the case of this morning’s ACSL wave shown below looking south of Golden, the winds were fairly moderate for ACSL development, only about 30-40 knots at 13,000′, where the cloud you see below lies beneath a broken ceiling at 22,000′. Even though I cannot see west from my vantage point, I’d guess that this cloud is on the lee side of Mount Evans, which stand tall above the surrounding foothills.
Timelapse video, 8:02 to 8:41 AM, 11/19/13:
Close-up view of lee side of ACSL cloud: