These strange loops in the sky over Great Crag in the Lake District, England that resemble giant smoke rings reveal a phenomenon of aerodynamics known as Crow instability.
 
Nothing to do with wobbly corvids, Crow instability is named after the scientist who first explained them. It occurs when two counter-rotating vortex trails, like those from an aircraft’s wingtips, interact to form a wave pattern that builds in its wake. The disturbance from one vortex is amplified by the other thanks to the springiness of the air and the alternating effect of the vortices. In the right conditions, this back and forth can turn straight contrails into undulating loops that break into vortex rings as the turbulence unravels. All loops and wiggles, when Crow instability kicks in, nothing goes ‘as the crow flies’.