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I have a lot of free time and often observe and take photos of the sky. Along with that, I always try to figure out what exactly I saw. I think that I have the basic understanding of clouds and cloud classification, but sometimes I see a situation which doesn´t fit the articles about clouds and there is a problem in classification. I have the biggest problem in case of the strange altocumulus lenticularis - pileus / velum clouds. See the photos below.

enter image description here The first photo - several pileus clouds forming enter image description here Second photo - I would say this is not a pileus. The two photos were taken in the same day, few minutes difference. enter image description here The thirs photo - there is a lenticular cloud on the left. On the right there is.. I dont know. I would not say it is a pileus cloud.

According to definitions, there is no strict difference between pileus and velum. I am fine with it. I have seen some pileus clouds and those are often very thin, sometimes multi-layered. Velum is, in fact, a large (horizontaly expanded) pileus. I am fine with it. But, according to some definitions, it would be classified as altocumulus / altostratus, if there were no connections to the cumulus top. That makes me confused. I have seen many "pileus" clouds that were not tha classic, pileus. Those looked like altocumulus lenticularis AND isolated Ac len were present on the sky in these situations. Definition of altocumulus lenticularis doesn´t say anything about connection with low level clouds, though.

A situation I came across many times.. the sky is full of isolated cumulus mediocris clouds & altocumulus lenticularis clouds. Ac len are supposed to form above terrain elevations, but I believe that there is a mechanism which allows them to grow above cumulus clouds. I have seen many photos all over the world showing a typical cumulus mediocris connected with a relatively large altocumulus. Like this one:

https://www.flickr.com/photos/jsjgeology/27681312683/in/faves-130789578@N06/

My intuition says that it should not be classified as a pileus or velum cloud.

Have you ever seen the same thing happening ? What do you think ?

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  • $\begingroup$ Second photo would make a nice Windows XP wallpaper... $\endgroup$ Commented Feb 26 at 9:21

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I’m just now learning about velum clouds, but have been familiar with pileus clouds for a while. It seems to me that the main difference is size if I’m understanding correctly. That doesn’t really help your question, but I mainly wanted to address the ending of your post.

When an updraft occurs, this essentially acts as an obstruction in the horizontal flow. When this reaches high enough in the atmosphere to perturb a stable, near saturated environment in the upper levels, that can lead to lenticulars.

While the trigger is vastly different, a mountain vs a cloud, the mechanical mechanism is almost identical. Your inducing wave like motion to a nearly saturated layer and the rising condenses, the lowering evaporates.

Being in a few weather groups on Facebook, you see a lot of people post from flat areas (relatively) and wondering how they are seeing lenticulars… absent of large cumulonimbus updrafts or mountains, you can also get them downwind of mountains, for several hundred miles, or even an atmospheric trigger. Two airmasses with different densities interacting and sending some of that energy/motion upwards is all it takes.

As for the pileus/velum debate, moving forward, I’ll hold off on using “velum” unless the cloud covers an area bigger than the parent cloud. But that’s just an interpretation.

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