Exhaust contrails usually form at high altitudes; usually above 8,000 m (26,000 ft), where the air temperature is below −36.5 °C (−34 °F). They can also form closer to the ground when the air is cold and moist.

What conditions cause contrails?

Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and "see your breath".

What weather is associated with contrails?

In short, contrails are cloud-like streamers frequently observed to form behind aircraft flying in clear, cold, humid air. ... If the humidifying effect of this moisture addition overcomes the the heat of combustion, then exhaust trails will form.