Definition

A Karmen wind, or Von Karmen vortex street, is linear chain of alternating spiraling whirlwinds produced by a process called vortex shedding which is responsible for the unsteady separation of flow of a fluid around a blunt object. 

Von Karman vortex street generated to the south west of Santo Antao island, Cape Verde, 20 December 2020 [Source: NASA earth observatory]

Description

Under specific conditions, the atmosphere can produce Von Karman vortex streets as prevailing winds blow around an object such as a mountain peak or island that sticks up into the flow.  If there is a thin deck of clouds within the air stream, those clouds act like dusting fingerprints, revealing the details of the disturbed flow.

The vortex street can reach over 250 nm from the obstacle and the diameter of the vortices are normally 12–25 nm.

Flight safety considerations

Severe turbulence and wind shear can be experienced in a vortex street. Destinations where this phenomenon is a hazard should publicise the direction from which the prevailing wind will cause Karmen vortices and include it in weather forecasts. 

Von Karman vortices being generated to the south southwest of the Canary Islands and Madeira, 20 May 2015 [Source: NASA, Author: Jeff Schmaltz]

Example airfields where karmen winds might be encountered

• Jan Mayan Island in the Arctic Ocean.
• Robinson Crusoe Airport in the eastern Pacific.
• Ascension Island/Wideawake Airport in the South Atlantic
• Saint Helena Airport in the South Atlantic
• Rishiri Airport in western Pacific

• Madeira/Funchal Airport in the North Atlantic