A Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over bluff bodies. They are named after the engineer and fluid dynamicist, Theodore von Kármán.

Relative motion between an object and a fluid is common occurrence. Simple examples are the motion of a plane in flight or the wind blowing on an obstacle. Obstacles disturb the flow and create particular shapes in their wakes. This phenomenon can be easily observed behind piers of a bridge, where eddies appear and are blown by the stream.

This wake (at right) might be complex depending on the shape of the obstacle. In order to understand this phenomenon, studies are done on a simple case: a two-dimensional flow past a circular cylinder. This case illustrates Strouhal instability and the particular wake known as Von Karman Vortex Street. It is a succession of eddies created close to the cylinder that break away alternatively from both sides of the cylinder. Vortex are emitted regularly and rotate in opposite senses.

When a vortex is shed, an asymmetrical flow pattern forms around the body, which therefore changes the pressure distribution. This means that the alternate shedding of vortices can create periodic lateral forces on the body in question, causing it to vibrate. If the vortex shedding frequency is similar to the natural frequency of a body or structure, it causes resonance. It is this forced vibration which, when at the correct frequency, causes suspended telephone or power lines to "sing", the antennae on your car to vibrate more strongly at certain speeds and it is also responsible for the fluttering of Venetian blinds as the wind passes through them, and causes these vortices.

Von Karman examples viewed from below

Each of these swirling clouds is the result of a meteorological phenomenon known as a von Karman vortex. These vortices appeared over Alexander Selkirk Island in the southern Pacific Ocean. Rising precipitously from the surrounding waters, the island’s highest point is nearly a mile (1.6 km) above sea level. As wind-driven clouds encounter this obstacle, they flow around it to form large, spinning eddies.

Clouds are made up of many small droplets of water or ice crystals. Clouds form around what is called a condensation nucleus, which could be a small particle of dust, ash, or smoke. Clouds reflect all visible wavelengths of sunlight, which often makes them appear white. However, clouds sometimes appear gray or even black, as they do in this image. This is caused by the process of accumulation, where droplets within the cloud merge with others, forming larger droplets. The space between droplets then becomes larger, allowing more light to be absorbed within the cloud, making it appear darker to the naked eye. A cloud vortex- the circular pattern seen here- is produced by the flow of air in the atmosphere. Heard Island (visible in the lower right portion of the image) is located in the Indian Ocean, about two-thirds of the way from Madagascar to Antarctica. The island is uninhabited by humans, although it is home to many birds and seals. Heard Island is rugged and mountainous, and is mostly covered with ice. It is also home to an active volcano, Mawson Peak. The island has been a territory of Australia since 1947.

The image of Isla Sorocco in the Pacific was shot from the International Space Station travelling at 17,000 miles an hour in one of its 15 daily orbits.

A vortex street streams slightly southeast of the Ilha da Madeira (Madeira Island) in this true-color Terra MODIS image acquired December 1, 2002. A vortex street forms when clouds over the ocean are disturbed by winds passing over land or other above-sea-surface obstacles, in this case the Ilha da Madeira. The southeastern movement of the low-level winds caused the clouds to line up in the same direction, called a street, and the wind's passage over the islands caused the swirls, called vortices. The particular kind of clouds forming the vortex street is referred to as "closed cell". These cells, or parcels of air, often occur in roughly hexagonal arrays in a layer of air that behaves like a fluid (as often occurs in the atmosphere) and begins to convect due to heating at the base or cooling at the top. In these closed cell clouds, warm air is rising at their centers and sinking around the edges to create this honeycomb-like pattern.

This wave cloud pattern formed off the Île Amsterdam in the far southern Indian Ocean, due to orographic lift of an airmass by the island, producing alternating bands–(Standing waves), of condensed and invisible humidity downwind of the island as the moist air moves in vertical waves and the moisture successively condenses and evaporates.

In mid-December 2005, the diminutive Amsterdam Island made waves—not in the Indian Ocean where it resides, but in the clouds overhead. Described casually as wave clouds, these features took on the shape of a giant ship before blending in with a larger cloud formation to the north and east.

The Moderate Resolution Imaging Spectroradiometer (MODIS) flying onboard the Terra satellite captured this image on December 19, 2005. The island itself is almost too small see in this image, but it serves as the starting point for the clouds that flow toward the northeast in a giant V shape. Amsterdam Island is a volcanic summit, the northernmost volcano on the Antarctic tectonic plate.

The volcano’s summit, poking above the ocean surface, conspired with atmospheric conditions to make these clouds. Pushed by wind, air ascended one side of this island then descended the other. As air rises, it cools and expands, and water vapor in the air condenses to form clouds. As air falls, the clouds evaporate. If the air is uniformly humid, it will likely form a uniform layer of clouds. If the air is dry, it may produce no clouds. But if the air contains alternating moist and dry layers, clouds form only in the moist layers of air. Known as lenticular clouds, they often look like flipped-over plates. Many of these clouds strung together form larger wave patterns like the one seen here.

Large Orographic Waves as seen from the ground