Crow Instability

The sight of contrails left by commercial or military jets across a bright blue sky is one of the common sights of a warm summer morning or afternoon on one of the many flight routes across the country. These streaks of cloud, formed by moisture condensing out of the hot engine exhaust into the cold air of the sky, may melt away almost immediately if conditions are very dry, or linger for minutes, or even expand into broad ribbons of cloud that merge with natural forms as they drift in the wind.

Sometimes, however, you can see a contrail twist in on itself, break into sections, and even turn into ring-like forms. When this effect is visible, it is a sign of the action of the Crow Instability.

The Crow instability is caused by the sharp edge of the aircraft's wing slicing through the air. When conditions are correct, the wing's "wake" turns into a whirling vortex of air behind each wing. Where these vortices meet, they spin the contrail into the twisted, ring-like forms that are visible even from the ground. The contrail may seem to crinkle, twist, or even break into separate circular forms under the force of this rotating wake.

Although these whirling vortices are largely an interesting, harmless phenomenon when they happen high in the sky, they can be dangerous near airports when many aircraft are taking off and landing through one another's wakes. A vortex of air here can be enough to cause a bumpy landing or even an accident, and for this reason, the Crow instability is of great interest to physicists, meteorologists, and other experts, as well as those who simply want to know why some contrails end up visibly twirled and kinked like a telephone cord.

(Photo taken by Mila Zinkova, used under GNU Free Documentation License.)