Volcanic eruptions produce some of the largest flows on Earth. These towering ash clouds were imaged from orbit in May 2017 as an eruption began on Alaska’s Bogoslof Island. The clouds are a beautiful example of a turbulent flow. Turbulence is characterized by its many length scales. Some features in the plume are tens or hundreds of meters across, yet there are also coherent motions down at the centimeter or millimeter scale. In a turbulent flow, energy cascades from these very large scales down to the smallest ones, where viscosity is significant enough to dissipate it. This is part of the challenge of modeling turbulence; to fully describe it, you have to capture what happens at every scale. (Image credit: DigitalGlobe, via Apollo Mapping; submitted by Mark S.)