Extraordinary Lenticular and Mountain Wave Clouds

The Northwest is a place of great physical beauty and that allure extends to the sky.

There is perhaps no better example than the lenticular and mountain-wave clouds that are frequently seen here.   The displays during the past several days have been extraordinary.

During the recent days, I have viewed some stunning lenticulars (lens-shaped clouds) over Puget Sound, but the most impressive imagery has occurred west of peaks of the Cascades. 

Let me show you a few produced by Mount Adams, provided by the most accomplished lenticular photographer of the region:  Darlisa Black.   And I will give you a bit of mountain-wave 101 as well.

Here is a picture taken by Darlisa on Friday (looking north, west is to the left).  

Look closely and you will see one lenticular right over Mount Adams, nearly symmetric over the peak.  That is often called a cap cloud, and it results from air being pushed up by the mountain until it reaches saturation (100% relative humidity).  

Picture by Darlisa Black

But even more impressive are the stacked lenticular clouds downstream (east) of Mount Adams).  These clouds extend to great heights in the atmosphere and are associated with vertical propagating mountain waves.   Translation:  vertically propagating means waves that can extend through great vertical distances.

Here is a close-up view of the vertically propagating wave clouds.  Just wow.

They looked like stacked plates.  This may be due to variations in moisture in the atmosphere.  I also suspect that there is some kind instability mechanism that contributes as well.

Mountain Wave Cloud 101

There are actually three types of lenticular clouds that are viewed around here.

The first is a cap cloud right over the mountain peak, as noted in the first photo.  A schematic is shown below.  Clouds form on the windward side from the upward motion on the terrain and dissipate on the downstream side as the air sinks.

Then there are waves that can be produced downstream of the mountain peak that remain or are trapped in the lower atmosphere (see below). Known as trapped lee waves, this situation can produce lenticulars well away from terrain.....sometimes even over Puget Sound or the Columbia Basin of eastern Washington.

Sometimes these trapped lee waves are very apparent on high-resolution visible satellite imagery as lines of clouds parallel to the crests of mountains.    Exactly this situation was occurring yesterday downstream of the central Cascades (see satellite imager below, arrow shows the feature).

The final configuration is shown below:  vertically propagating mountain waves.  When the flow approaching the terrain has the right properties, the mountain barrier can produce a mountain wave that moves upwards and amplifies, producing clouds (often lens or lenticular shaped ) in the upper troposphere (roughly 15 to 30 thousand feet above the surface).   This is the process that created the magnificent clouds downstream of Mount Adams shown above.

These vertically propagating clouds are also quite evident in satellite imagery, as shown by the Friday afternoon image shown below.  The red arrow indicates Mount Adams. The white globs to the west are the vertically propagating mountain wave clouds.

Mountain wave clouds can be stunningly beautiful at sunset (see image taken by UW Physics Professor David Kaplan from Cle Elum).   Some provide useful for glider pilots, providing welcome lift,  But they can also produce powerful turbulence, particularly high amplitude versions of the vertically propagating variety.

Picture by David Kaplan