What is a horseshoe vortex? How does it form? Can it become a tornado?

Question

I have heard about them and saw pictures of them but not really sure what they are.

Answer 1

The horseshoe vortex (a.k.a. trailing vortex) forms on the ends of the wings of a plane.  The reason that planes stay in the air is because the lower surface of the wings has a high pressure while the upper surface of the wings has a lower pressure.  If you look at the tips of the wings, the low pressure air is in direct contact with the high pressure air.  This causes the air below the wings to move upward and around the tips of the wings.  This flow results in a vortex.  Sometimes you will see little panels at the end of wings that stick up called winglets.  These act to stop the flow around the tips of the wings.  

You can think of the horseshoe vortex as similar to a tornado, but it cannot cause a tornado to form.  These vorticies can become a problem when two planes fly close enough to one another - that's why planes have to wait a few seconds to take off of a runway if another plane has just taken off.  The vortex will eventually die due to losses from viscosity.