Cirrus clouds and contrails. Vertical panorama of high cirrus clouds in a deep blue sky with aeroplane contrails (vapour trails) and rays. Cirrus clouds are found at high altitudes, typically above 5000 metres. They consist of tiny ice crystals, which are blown by the wind into characteristically wispy shapes. Contrails form when, at high altitudes, the water vapour emitted as a product of a jet engine's combustion raises the relative humidity of the surrounding air past saturation point and the resulting water droplets freeze into visible ice crystals.