http://contrailscience.com/aerodynam...bow-contrails/Very pretty. But what is it? It’s clearly not a regular exhaust contrail, as the trail seems to start actually ON the wing, and it has a weird rainbow effect you don’t find in exhaust contrails.
It’s actually an aerodynamic contrail. It’s formed by the reduction of pressure in the air as it moves over the wing. When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator). The reduced temperature cause small drops of water to condense, which then may freeze. The (frozen) drops get larger as more water condenses on them. The different sized drops (or ice crystals) have different optical properties, which affect different wavelengths of light, which accounts for the “rainbow” effect.