What do they call that cloud??

Larry J. Kosch

Hey, I got some great storm cloud pictures last weekend. I especially liked the ones where it shows the thunderheads breaking thru what looks like a icy/snow layer type of cloud formation. Anybody has the name of that cloud formation on their tongue tips?? When you see the photos, the thunderheads of the clouds looks like they were snow-capped. Thanks for any answers and explainations why they form. 8)

P.S. Check out my avatar. The cloud top on the left has a good snow top!!
Correct term!!

You were correct in using the term "Cap Clouds", but I wanted to know more about how it forms, etc.

Did a little Internet search and found this definition:

Cap clouds form when air containing water vapor is uplifted on the windward slide of the slope and reaches saturation producing liquid water cloud droplets and a cloud which can "cap" the summit.

This was a description of Orographic clouds, as the name implies, are produced by the flow of air interacting with mountainous terrain.

Thanks for coming up the right term!!
I believe they are due to stratified air aloft (in the upper troposphere and tropopause) being pushed up due to the updraft. As air is pushed upward, it cools adiabatically, and if it is near saturation it will condense. Since the air is already stratified, it gets a very laminar sheet like apperance. Imagine a sheet of paper representing the slightly more moist air aloft being pushed upward by someone blowing it from underneith...

Re: Correct term!!

You were correct in using the term "Cap Clouds", but I wanted to know more about how it forms, etc.

I'll take a stab at this. Pileus clouds are often seen above rapidly developing cumulus congestus - these thin smooth cloud caps are often short-lived though, as the cloud developing just below them often overtakes the pileus clouds, or development moves into a vertical region of drier air. While when we look at a developing tower we see the top of the cloud and think that must be the top of the rising air, this is actually not true, as air is be forced upward just above the rising thermal. When this forced air above the developing cumulus is sufficiently moist, this upward push can lead to adequate adiabatic cooling to form a cloud. The laminar appearance of the cloud is evidence that the air has been forced upward, and did not rise buoyantly. Also, pileus clouds will almost entirely be composed of liquid cloud droplets, not ice, as temperatures need to be around -40 F for a cloud to develop as mostly ice, and this temperature is generally higher up in the atmosphere than where pileus clouds are typically seen (Take a look at a sounding and look for the height where -40 C temperature is, around 230 mb this morning at OMA).

Pileus are photogenic, but hard to catch on film unless you are stopped and have the camera ready. If you're driving and see one starting to form, the top of the updraft almost always blasts through it before you can stop. There can also be multi-layered pileus clouds.

Here is a short RealVideo clip of a storm punching through pileus in Lexington, Kentucky:


Most of the pileus I've seen are very short-lived. One day this year, however, the caps formed on very low-topped congestus in Charleston, WV and stayed steady-state for about 20 minutes:

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Their formation is very similar to lenticulars over mountain ranges. Unlike lenticulars the conditions for persistence are short-lived.