In the series of interesting aircraft contrails spotted, here's one that particularly caught my interested in the weekend.

Sighted Sunday from the Windsor Great Park near Heathrow (where, inevitably, there is a high frequence of flyovers by various sized aircraft), this contrail was shaped like "smoke rings" from a giant cigar. I whipped out the camera and took a few shots. A close-up of the phenomenon is seen here:

Some searching online tells us that this is caused by the so-called Crow Instability in the vortexes caused by aircraft; whilrling the otherwise straight contrails into spirals then donut-shaped rings. More about this in Scientific American:

The mixing gases contained in the contrail rotate with respect to the ambient air. These regions of rotating flow are called vortices. (Any sharp surface, such as the tip of a wing, can cause vortical flow in its wake if it is sufficiently large or the flow is sufficiently fast.) On occasion, these trailing vortices may interact with one another.

In one well-known example of this fact, the Crow Instability causes the vortices to develop symmetric sinusoidal oscillations and eventually to merge and form vortex rings behind the jet. This instability can be triggered by turbulence in the surrounding air or by local variation in air temperature or density, which may itself be the result of the stratification of the atmosphere. When the contrails are visible and strong, it is possible to see the white streaks become wavy and then leave rings floating high in the sky, like smoke rings from a giant cigar.