One of my favourite ever pictures taken from space is a picture of the 2009 eruption of Sarychev Peak (Ostrov Matua island, Japan) taken by an ISS astronaut during a lucky fly-over.
Image Source: Earth Observatory Image of the Day. I heartily recommend clicking through and seeing the animation.
I always assumed that the hole in the cloud cover around the island was caused by some sort of shockwave originating in the eruption plume, most probably as a result of increased temperatures evaporating the cloud, or some such. However, upon revisiting the IOTD page, I was surprised to learn that this is not necessarily the case and that the origin of this hole is a matter of some controversy:
**Editor’s note: Following the publication of this photograph, the atmospheric and volcanic features it captured generated debate among meteorologists, geoscientists, and volcanologists who viewed it. Post-publication, scientists have proposed—and disagreed about—three possible explanations for the hole in the cloud deck above the volcano.
One explanation is that the hole in the clouds has nothing to do with the eruption at all. In places where islands are surrounded by oceans with cool surface temperatures, it is common for a sheet of clouds to form and drift with the low-level winds. When the cloud layer encounters an island, the moist air closer to the surface is forced upward. Because the air above the marine layer is dry, the clouds evaporate, leaving a hole in the cloud deck. These openings, or wakes, in the clouds can extend far downwind of the island, sometimes wrapping into swirling eddies called von Karman vortices.
The other two possibilities that scientists have offered appeared in the original caption. One is that the shockwave from the eruption shoved up the overlying atmosphere and disturbed the cloud deck, either making a hole or widening an existing opening. The final possibility is that as the plume rises, air flows down around the sides like water flowing off the back of a surfacing dolphin. As air sinks, it tends to warm; clouds in the air evaporate.
Has this controversy been settled? Is there a convincing, accepted explanation for the origin of the hole in the cloud cover?