29 June 2009
Pileus clouds form as air is lifted above a rapidly rising thunderstorm. The holy grail of Pileus clouds are the ones that have blue sky between the cloud and the top of the cumulonimbus. The storm usually blasts right through the Pileus in a few seconds, so getting a pic before this happens is rare. If you do, the story goes, you will get your pic on the cover of Weatherwise Magazine! You can see one here: http://severewx.atmos.uiuc.edu/01/11-clouds/08-pileus2_romine.jpg
Thermodynamically, what is happening is this. The layer of air above a rising storm tower is lifted and cools. The cool air cannot hold as much moisture and a strati-form type cloud forms. The storm usually pushes right on through it, and it slowly evaporates.
Enter NASA and the International Space Station.
The ISS Astronauts took this incredible photo of an erupting volcano in the NW Pacific a few weeks ago.
Yes, That is one incredible pileus cloud. A pyroclastic flow?? is visible on the slope of the volcano as well.
According to NASA, there has been a lot of discussion among Geologists, and Meteorologists about the hole in the cloud around the island. (This is just the type of thing we weather geeks love to argue about too!)
The question is whether the erupting volcano “punched” a hole in the cloud deck, or was it already there. If it did punch the hole, what was the process by which it did so?
NASA rightly points out that many times air being lifted over an island, will cause clouds to evaporate as they are pushed into the dry air above the marine layer. I think this is not likely the case here. The volcano did it.
This happened in one or two ways. Probably both. The rising plume of hot volcanic ash lifted the air, and any cloud quickly evaporated. The other likely scenario is that the plume of ash lifted the air around it. The atmosphere strives for hydrostatic balance, and so air descended around the plume. This causes adiabatic warming, and the clouds would again disappear as the humidity dropped below 100%. The curve of low cloud seems to match the plume, so this seems likely to be part or most of the answer.
Either way, it goes down in history as the most INCREDIBLE pileus cloud ever photographed. I’ll bet you a pizza it makes the cover of Weatherwise!