Condensation clouds:
The clouds appear for the same reason that clouds always form, namely, that the air has cooled to the point that the ambient water vapor condenses. Flows around bodies and wings always change the temperature and pressure of the fluid. It is well known that lift is caused by pressure differences on top or bottom of a wing or body so that it ought to be obvious that the pressure varies from point to point in a flow around an object. The fact that the temperature changes can be seen by noting that most fluid flows and nearly every aerodynamic flow are frictionless. In the language of thermodynamics, the flow is said to be reversible or loss-free. Read More
(via boingboing)
