|Contrail Circumhorizon Arc
Taken in Schenectady, New York July '11 by Peter Barvoets.
The contrail is lit in part by the spectral colours of a circumhorizon arc. An apparently unconnected wisp of cirrus is similarly lit. The colours are the collective glints of millions of plate aspect hexagonal ice crystals.
Water vapour generated by aircraft fuel combustion is rapidly cooled by mixing with high altitude air to form tiny supercooled water droplets which scatter sunlight to appear as a white contrail. The droplets are metastable and in the presence of any ice crystals or suitable growth nuclei there is a transition to more ice crystals that grow rapidly.
Here, large crystals are precipitating out of the contrail in the form of long tendrils. Halos need crystals larger than ~0.05mm across to refract light cleanly without excessive blurring by diffraction. Those falling out of this contrail are much larger.