The Other Type of Mountain Wave Cloud

 Folks love to talk about lenticular clouds, which are generally produced by air moving up (and down) downstream of a mountain barrier (see picture below).  Such clouds are generally found in the lower atmosphere at or just above the crest level of the mountain barrier that produces them.

Picture by Joel David-Aldridge

As shown by the schematic below, lenticular clouds, also called trapped lee wave clouds, often come in multiples downstream of the mountain barrier and have a lens shape (see below).    Such mountain-wave clouds are called "trapped" because the clouds and the motions associated with them are trapped in the lower atmosphere. Some folks think they look like flying saucers and more than one UFO scare has been initiated by such clouds. 

Why does the air go up and down downstream of the mountains?   Think of a kid's swing set.  You give the swing a push and it goes back and forth for a while.  Air pushed upwards by the mountain crest does something similar.

Interestingly, there is another type of mountain-wave cloud that is found much higher in the atmosphere, but still downstream of a mountain barrier.   In this case, the mountain wave energy propagates vertically in one very high amplitude wave, with high-level clouds just downstream of the mountain (see schematic below).  Such clouds can be associated with substantial turbulence aloft.

Certain conditions favor high-amplitude mountain waves, such as strong winds approaching the mountain crest and supportive changes in wind and temperature with height.

Yesterday was a super day to view a vertically propagating mountain wave just downstream of the Cascades.

 The image at 8:41 AM shows the feature clearly (below, see red arrow)

As did the image at 11:01 AM (below).

To show you how good the weather models have become, here is the simulated cloud field at 8 AM yesterday--the correct type of mountain wave cloud was evident.

Living here in the mountainous Pacific Northwest, we get to see mountain-wave clouds nearly every day, so it is important to know the two types.   And now you do.