When a fluid, such as a current of air, moves past a barrier, like a region of high pressure or a volcanic island, as seen in the image below, a stream of alternating vortices can be generated under the right conditions. When this happens, the upstream vortices are shed one by one, which expand out as they advance downstream. The vortices occur because of rotating components which arise in the airstream as it moves past the mountain. The rotating component of the airstream is called the vorticity and results from the induced drag as the current shears past the obstacle. This is a highly coordinated process, so as one vortex is being shed, the other is in the process of forming, as seen in the image below, acquired by the NOAA satellite.
Vortex streets are a common occurrence in nature and can be seen imprinted in the cloud bands of Jupiter, airstreams in the Earth’s atmosphere and other fluid flows such as in relatively slow moving water currents in rivers and streams where perturbations are induced by an obstacle. Vortex streets can also be observed in the microscope under DIC illumination, when a fine dusting of debris can be seen swirling around, following the waters current contours induced by the ciliary and flagella turbines of microorganisms. Vortex streets are also responsible for the zig-zag motion of bubbles, rising to the surface in a fizzy drink. As the bubble rises through the fluid, it generates vortices in its wake. These vortices, shed one by one produce a force which pushes the bubble from side to side as they are being shed. Insects take advantage of the vortices shed from their wingtips by rotating their wings after the down stroke, to produce lift from the circulating eddies that they generate.