People love to talk about Lenticular clouds, typically created by air moving up (and down) downstream of a mountain barrier (see figure below). Such clouds are often found in the lower atmosphere at or just above the mountain barrier that produces them.
Photo by Joel David-Aldridge
As shown in the diagram below, lenticular clouds, also known as trapped lee wave clouds, often grows in multiples downstream of mountain fences and is lenticular in shape (see below). Such clouds with mountain waves are called “trapped” because of the clouds and the motion associated with them trapped in the lower atmosphere. Some people think they look like flying saucers and many UFO scares have been caused by such clouds.
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Why does air rise and fall downstream of mountains? Think about a child’s swing set. You push the swing and it swings back and forth for a while. The air pushed upward by the mountain top does the same thing.
Interestingly, there is another type of mountain wave cloud that is found much higher in the atmosphere but still below the mountain barrier. In this case, the mountain wave energy propagates vertically into a very high amplitude wave, with high-level clouds immediately downstream of the mountain (see diagram below). Such clouds can be associated with significant turbulence aloft.
Some conditions favor high-amplitude mountain waves, such as strong winds approaching mountain peaks and supporting changes in wind and temperature with altitude.
Yesterday was a great day to see the mountain wave propagating vertically just downstream of the Cascades.
Image at 8:41 a.m. clearly shows the feature (below, see red arrow)
The same goes for the 11:01 a.m. image (below).
To show you how good the weather models have become, here is the simulated cloud field at 8 a.m. yesterday–the exact mountain wave cloud type was clearly shown.
Living here in the mountains of the Pacific Northwest, we see ridge clouds almost every day, so it’s important to know these two types of clouds. And now you do.
