Those watching the footage of the Tavurvur volcano probably noticed appearing and disappearing clouds along the edge of the shock wave. They're called Wilson clouds, and they used to flicker in the sky above nuclear bombs, too.

Adiabatic cooling is the drop in temperature that happens when a gas is suddenly given lots of extra room to expand. It doesn't have to be a violent process. It happens in your home. Refrigerators work by running compressed, warm liquid into big spaces, where it turns to gas, expands rapidly, and cools down - cooling your fridge down in the process. Dry ice manufacturers make dry ice by putting pressure on carbon dioxide until it is compressed, and then pumping it out into a large empty space. The carbon dioxide rapidly expands, and as it does so, it cools down until it condenses into snow. There are plenty of examples of safe, domestic adiabatic cooling in your life.

Then again, sometimes, adiabatic cooling can be violent and dramatic. We all got a good look at a dramatic example when we watched the footage of the Mount Tavurvur eruption.

Above the eruption, clouds flickered in and out of existence as the shock wave moved through the air. The shock wave essentially pushed a lot of the air outwards, leaving a very low-pressure zone in its wake. A low-pressure zone gives the air inside it or near it lots of room to expand. The air expanded and cooled. The water vapor it was carrying condensed into droplets, forming clouds. As the air in the surrounding atmosphere came rushing back it exerted pressure on the clouds and the air around them. That heated up the air, and made the clouds disappear.


It's rare to be able to see these clouds, called Wilson Clouds, pop in and out of existence.

It used to be a lot less rare. Wilson clouds were a common sight over nuclear explosions. They were relatively uncommon in desert tests, but when governments tested nuclear bombs in the tropics, scientists saw them form rings around explosions, or caps over the mushroom cloud.


[Via Atmospheric Shock Waves and Condensation Clouds.]