The mystery of the von Karman vortex street

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A von Karman vortex street captured by NASA's Modis sensor on October 30, 2010.

By Brian Jackson

Crop circles? Mini-hurricanes? Spaceships? What the heck are those things pictured to the right?

What we're looking at here is a classic example of a von Karman vortex street. Other than being really cool to look at, these phenomena are just another reminder of just how weird our atmosphere can be.

This image, captured by NASA's MODIS sensor on October 30, is of an area of the Southern Ocean between Madagascar and Antarctica. That little bit of brighter gray in the bottom part of the image is a remote, mountainous island called Heard Island, dominated by a volcano known as Mawson Peak.

It is a small island, approximately equal in area to Lanai in Hawaii, the sixth largest Hawaiian Island, yet it reaches over 9,000 ft at its highest point. It is this large topographical relief that helps to cause these strange vortex features to form.

So what exactly is a von Karman vortex street?

In technical terms, it is a repeating pattern of swirling vortices cause by the unsteady separation of flow of a fluid over bluff bodies.

In layman's terms, it's a series of whirlpools in the wake of something big. In our case, the flow of fluid is the atmosphere itself. Specifically the winds in the lower half of the troposphere, made visible here by the stratus clouds moving along with the flow, and our something big is Heard Island and its 9,000 ft. peak.

As the winds (clouds) reach the island, they are split by the volcano. As the flow moves around the volcano, it creates an off-centered area of low pressure in its wake, in effect a whirlpool or vortex.

As this vortex moves off, another vortex is created, alternately off-centered. This vortex again moves off repeating the process and creating these remarkable patterns in the island's wake as long as the wind speed and direction will support it.