Condensation trails or contrails are clouds which form in the wake of aircraft when the atmosphere at flying level is sufficiently cold and humid. They are formed in three ways: the most common ones are due to the exhaust of airplane engines, less common the aerodynamic ones due pressure changes and quite rare are the ones due to instability provoked by aircraft flying through an undisturbed layer.

Depicted is an aerodynamic contrail due to sudden lowering of the pressure field on the upper side of wings and wingtips. This military plane of the Royal Air Force, a Boeing C-17 Globemaster III, flew just below a thin Cirrus layer accentuating its typical silhouette and was briefly at the right angle of the sun to produce this rainbow alike effect named iridescence. This is caused by sunlight deflection in the water droplets and usually the closer to sun the better this effect becomes visible.