The first was from Ellen Baker from Glacier, Washington taken late Friday afternoon. There was a growing cumulus with what appears to be a lenticular ("lens shaped") cloud right above it.
And here is one taken by Catherine McConnachie around Marrowstone Island on Friday evening. A bit different, but the same idea...there appears to be a lenticular cloud right above a growing cumulus. And these clouds have soft boundaries...they are made of ice.
What is going on? How can you get a lenticular cloud without a mountain barrier?
You can....and these are called Pileus cloud (which means "cap" in Latin).
This is how it works. Cumulus clouds, particular growing cumulus congestus clouds as seen above, are associated with strong upward motion. This upward motion can push the air above the cloud upwards. Air being pushed upwards cools because it expands as it shoved toward lower pressure, since expansion is associated with cooling (like air coming out of bike or car tire). If that air aloft is relatively moist and close to saturation, the cooling can cause the air to saturate, producing clouds.
OK... let's see if this makes sense. Here is the upper air sounding on the WA coast for 5 PM Friday. Temperature is in red, dew point is in blue and the air is saturated with they are on top of each other. The height coordinate is pressure, with 500 being about 18,000 ft and 700 about 10,000 ft. The cumulus in the picture were perhaps 10,000 to 15,000 ft high. The air was pretty much saturated to 550 hPa (about 17,000 ft) and fairly close to saturation in some layers above (e.g., about 450 hPa). Strong lifting from the growing cumulus could have lifted and saturated that upper layer--which as cold enough to form clouds made of ice.
Pileus clouds are more frequent that you might think, so keep you eyes open for them. And now you know a new Latin word.
I've never seen it happen on the eastern seaboard (or in the midwest for that matter). I wonder if it's just that I haven't been looking for them or if there is something different about our typical atmospheric conditions.
ReplyDeleteCascades got an unexpected hammering by thunderstorms and heavy rain in many places on Friday afternoon/evening. Was hoping you'd have a post on that. I'm guessing the conditions you describe here had something to do with it.
ReplyDeleteThese look similar to clouds that form(ed) during above ground nuclear tests, presumably by the same mechanism.
ReplyDeleteSo pileus clouds look the way they do because they are made of ice, not water droplets?
ReplyDeleteI've shot my share of these over the past dozen years or so around Portland and SW Washington. It's amazing how quickly they can form and dissipate - no longer a surprise thanks to your chart showing moist and dry air at different levels. At least once I've had a camera in the car and seen a fine example, only to have it nearly dispersed by the time I reach a pullout.
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