On 3-7-14 at 12:41 pm a shot of a transport helicopter was taken over Copper showing clear vortices from the blade tips.
Blade tip vortices form at the outermost end of a rotor blade; a zone of
reduced pressure forms above the blade and a region of increased
pressure forms below the blade. As the air flows around the blade tip to
reduce the pressure difference, a concentrated vortex is created behind
the blade tip.