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bacchus2
10-24-2008, 06:24 AM
Any thoughts on why this occured??

Bill O
10-24-2008, 07:06 AM
I'm sure the observers have more detailed explanations, but its just air rising over the summit, reaching saturation and condensing into a cloud. After passing over the summit it begins to descend, warm and the cloud dissipates. I'm sure there are lots of great photos of MWN out there, but I don't have any. Here's my photos of Mount Adams in WA with a lenticular.

http://www.easternlight.net/photos/wash0001.jpg

Knapper
10-24-2008, 05:27 PM
Are you asking how lenticulars in general form or why the one in Thursdays comment formed?

bacchus2
10-28-2008, 04:05 PM
Ryan was most interested in how the one in the picture occureed.

Have some great pic's of lentiuclars over yellowstone, I'll share with you when I am up Feb 11.
John

Patrad Fischroy
11-12-2008, 11:55 AM
So was the cause of the Lenticular/cumulus combination ever discussed. I ask because I saw a similar situation ove rthe mountains here the other day. I always thought that the lenticular clouds were associated with a relatively stable horizontal flow of air up over an obstruction while the cumulus clouds were from more vertical movement of air. They would seem to be mutually exclusive.

Knapper
11-12-2008, 11:35 PM
No the thread kind of died without a possible explanation at the formations.

To start with the comment picture, my best guess is the cap cloud that formed over the cumulus cloud was a pileus cloud. I say "best guess" because pileus clouds usually are more common in the mid and upper atmosphere and I had never seen one in the lower atmosphere, let alone below the summit. They are usually hand in hand with severe thunderstorms, especially out west. But a pileus cloud is formed as a moist layer is lifted due to a rapidly rising cloud below. The air is cooled rapidly until it reaches dew point in which it condenses and forms a small cap cloud over the cumulus cloud. Eventually, the cumulus cloud "breaks" through this cap cloud and the capping will either dissipate or incorporate into the cloud that formed it. This is the case with the cloud we saw.

The picture of Mt Adams in this thread is basicly the same idea and goes by pileus, cap cloud, crest cloud or banner cloud. These are seen on the summit from time to time and are very common on the volcanoes of the west. The principle of formation is the same. A moist layer is lifted due a flow over a rapidly rising object (a mountain instead of a cumulus cloud). The air cools and condenses into a thin veil cloud.

The air associated with these types of clouds is considered "stable" but depending on what you are flying in, these are either something to avoid or something to seek. If you are in a fuel powered aircraft, you would want to avoid these or any other type of lenticular clouds because of the turbulance that is contained in them. If you are in a sailplane, these are seeked out because to the trained sail pilot, these can provide a lot of lift which can provide a better distance for sailing.

Patrad Fischroy
11-13-2008, 09:55 AM
Thanks Ryan, that makes a lot of sense. So at the top of the convective airflow there could be some high shear winds right?

Knapper
11-13-2008, 01:25 PM
So at the top of the convective airflow there could be some high shear winds right?

No, if high wind shear was present, the convective cloud would have started to create a mini anvil shape that is so familiar in severe thunderstorms over the plains. You would need a type of shear (wind increasing with height) but only a slight increase. In this case, air below the summit was barely moving allowing for the cumulus cloud to build poking into a layer that was blowing a bit more. For the type of cloud that was seen, winds were probably only an increase of 25 kts/1000 ft or less at the time. Lenticular clouds, rotor clouds or K-H clouds indicate shearing usually but cap clouds are not always an indicator of shearing.

Steve M
11-17-2008, 03:13 PM
But a pileus cloud is formed as a moist layer is lifted due to a rapidly rising cloud below. The air is cooled rapidly until it reaches dew point in which it condenses and forms a small cap cloud over the cumulus cloud.

We see these clouds quite often here in Florida during the summer, very often the storm below reaches severe limits.

Great question!