The shape that is formed by the free surface, which is called a hyperboloid, or "Gabriel's Horn". The core of a vortex in air is sometimes visible because a plume of water vapor that is caused by condensation in the low pressure and low temperature of the core. An example of this is the sprout of a tornado. Vortices form in stirred fluids and could be observed in phenomena such as smoke rings, whirlpools in the wake of a boat, or the winds surrounding a tornado or dust devil.
