The video shows a classic high-amplitude mountain wave with plunging flow producing a "waterfall cloud" on the lee side of the ridge with a hydraulic jump and rotor developing immediately downstream. The cloud structure on this occasion is ideal for tracing the flow. A schematic of the flow in these situations is depicted below, although you have to mentally flip it and reduce the scale a bit to match that in the time lapse.Aye. Here’s a peek at rough unfinished timelapse I captured of it. pic.twitter.com/yuLubcQR6r— Terry Abraham (@terrybnd) June 19, 2020
|Source: Whiteman (2000)|
In the downslope flow, the air sinks and warms, leading to a decrease in relative humidity and cloud evaporation. However, evaporation takes time. In the time-lapse above, the amount of warming, resulting decrease in relative humidity, and time needed to evaporate cloud droplets are not large or long enough to create a large cloud-free region (although there is some clearing). Thus, the clouds provide a pretty good tracer for the low-level flow.
What you can't see is the wave-breaking region aloft, as depicted in the schematic. While rotors and other features associated with downslope flow are hazards for aviation, the wave breaking region, which is often cloud free, is another as it is often accompanied by turbulence, which can be severe.