Notice: All answers here is about weather Von Karman Vortices that occur in nature, not about the vortex itself.
Q 1.What are Von Karman Vortices?
A 1. A Von Karman Vortex is a type of eddy, or a Whirlwind, also known as a Vortex street. First noticed in the laboratory by fluid dynamicists, it occurs when a more-viscous fluid flows through water and encounters a cylindrical object, which creates vortices in the flow.In this case, air is the fluid.
Q 2. How does a weather Von Karman Vortex forms?
A 2. When prevailing ocean winds encounter a mountain, the wind starts to pile up at the side of the mountain which faces the wind, and start to move around the slopes of the mountain. At the back of the mountain, which is facing in the opposite direction the wind is moving, a area of low pressure forms, which sucks the air that move around the slopes, causing a vortex to form. Also the winds has to be at the right speed, along with the structure, size, and height of the mountain. The mountain also needs to be above the flow of winds, in order to create such vortices.
Von Karman vortices form nearly everywhere that fluid flow is disturbed by an object. In the cloud images shown on this page, the "object" that is disturbing the fluid flow is an island or group of islands. As a prevailing wind encounters the island, the disturbance in the flow propagates downstream of the island in the form of a double row of vortices which alternate their direction of rotation.
Animation of the phenomenon:
Q 3. What causes an "eye" to form in the center of some Von Karman Vortices?
A 3.Clouds form around what is called a condensation nucleus, which could be a small particle of dust, ash, or smoke. As a Von Karman Vortex is filled with clouds, the center of the vortex concentrates the clouds and cloud droplets in the process of accumulation, where droplets within the cloud merge with others, forming larger droplets, which causes the larger droplets to fall, and evaporate again
which forms a clear "eye" of hole in the center of a Von Karman vortex street.