When the eruption of Mount Etna began last week, I witnessed an extraordinary sight. There, all alone, gazing down from the ground, is a cone-shaped mountain. Then suddenly, all hell broke loose: the hillside broke off, pillars of smoke reached as high as five miles into the sky, volcanic ash spewed forth, and ash plumes rose as high as 20,000 feet, the highest point on earth, where it has been suspected that Mount Etna has its own moons. The flight directly over Etna crashed us into that surreal first moment of seeing this spectacular volcanic phenomenon for the first time.
Etna has an enormous amount of gas, and some of that gas causes clouds to form all around the volcano. These clouds of gas consist of tiny pieces of molecular gas (known as particles) that fall to the ground as volcanic ash. A volcano’s explosion can also cause the eruption of these particles, known as ash clouds, as they form in a process called thunderstorming—in which microscopic particles, speeding around the eruption, generate massive enough rainfall to break up even larger bits. In order to explain how such clouds are formed, we need to study how they work.
Our story begins in the lab. In addition to the volcano itself, eruptions trigger a subterranean reaction where magma—a mixture of salt and other particles—comes back to the surface, forms a dark, solid rock called pyroclastic. To understand this, imagine pyroclastic super-heated gas over an immense area, sticking to everything in its path.
A cloud of superheated gas (material left over from a volcano’s eruption) erupts from a mountain. (Photograph by MARCEL BOUTON, annochetr/iStockphoto)
To study thunderstorming and ash clouds in Mount Etna, we can’t conduct scientific research from the ground, but we can take a plane over the volcano. Depending on weather conditions at the time of departure, the change in altitude can produce different conditions in the ground below (or the debris from the mountain behind). When the plane is above ground, it can study how dust (which is not part of the volcanic eruptions but instead grounded particles of ash) covers the ground. We can then map how much dust covers the ground and the ash clouds it creates. We can then use what we learn to predict how fast the clouds and ash will move, and when they might erupt from the crater.
More and more airports, including New York’s LaGuardia and John F. Kennedy, are working with researchers from the German aerospace research institute DLR to add a feature that’s called Infrared Aperture Radar (IRAR) to the flight path of planes. IRAR is a radar system that uses heat from the ground beneath the plane to map the terrain. This is great for predicting volcanic eruptions, for the researchers who are studying the dynamic processes that cause them and for pilots who are flying into the volcanic plumes. IRAR has been in use for aviation for more than 15 years, and pilots have become very familiar with its use, as they are also starting to get used to other infrared technology.
When an aircraft approaches Mount Etna, the jet in front has to fly without looking up or out of the windows at any point of the approach. This is harder than it sounds. When an airplane uses IRAR to map the terrain from above, the image it creates is so much brighter that it can be hard to see all of the ground directly beneath the plane and where the plume is coming from. The plane will have to wait until it is so high above ground that the vista becomes clearer before the pilot can even look down at it. This takes longer than the plane is being flown (or, just as important, the time it is on the ground) but it also comes much more quickly than a plane cruising in a volcano’s plume. If an airplane can’t see all of the ground, this makes it difficult to learn about the dynamics of the plume and of eruptions, and could contribute to its unpredictability. In order to reduce the risk of getting near an erupting volcano, pilots are generally trying to fly under clouds by remote control—but this might reduce the information that IRAR can provide about how far in advance a plume has been developing and how far apart the clouds are from the plane.
Because of the way lightning is transmitted around the volcanic surface, like water in a river, IRAR can provide another way to survey it. With the right atmospheric conditions, IRAR can