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.

A Strong Surge of Marine Air Brings Relief to the Pollen Afflicted

During the past weeks, I have gotten several emails from folks with pollen-related allergies.  

When will the suffering end they asked?

Early spring is the time of tree pollen, and the recent warm weather brought high values and substantial suffering to the sensitive.

As shown the pollen levels at Seattle for the last 30 days (below, from pollen.com), the values during the past several days reached a high level (red colors).    

The rise began in late March, with several ups and downs.  Ups during warm periods, downs in cooler periods with onshore flow of marine air.

Compare Seattle's pollen levels with those in Wenatchee, east of the Cascades in the Columbia basin (below).   Much more consistently high, with a major drop on April 4th.  A big drop that day in Seattle as well.

What happened on April 4th?   A major cool down on both sides of the Cascades, as illustrated by the temperatures at Wenatchee this month  (below).

The cool-down was associated with a powerful front that moved through the area during the previous day (see the map below, the blue line indicates the front).

And the same thing is happening as I write this on Saturday evening.

A very powerful front is moving through the region now, bringing much cooler air and strong winds into the region.   The satellite image his afternoon at 4 PM was dramatic, with a well-defined frontal band crossing western Washington and the swirling clouds with an offshore low-pressure center being evident.

Gusts this afternoon and early evening have surged to 20-50 mph (see below).  Tens of thousands lost power in western Washington, the result of the strong winds combined with the newly leafed-out trees.

Expect much lower pollen levels on Sunday and Monday.  

Finally, Seattle is one of the best major cities in the U.S. for the pollen afflicted, sitting in the 88th position on the Allergy and Asthma Association's list.    Thank our location downwind of the Pacific Ocean, mild climate, and favorable mix of trees.   

And yes, the Seattle Times had an article last month saying that the pollen problem will get worse here under global warming:

Scientists predict the pollen for some trees, like birches in the Seattle area, will be eight times more abundant in our region by the end of the century, according to the state Department of Health.

The culprit? Climate change.

I will deal with the problems with these claims in a future blog.