Contrails (condensation trails) are those white lines you see behind planes or jets high in the sky.
They form from water vapor condensing and freezing on particles produced from the plane’s exhaust.
The water vapor that creates contrails comes from the atmosphere and from the airplane’s turbine. This happens because water vapor is one of the by-products of burning petroleum-based fuels (like jet fuel).
The condensed water droplets quickly freeze because planes fly high up in the atmosphere where it is VERY cold (less than -40°F)!
Contrails can be used to tell how dry the air is. A longer contrail indicates more humid air since there is more water vapor to condense and freeze to make the trail…and a shorter trail indicates drier air!
When there are no contrails then the air does not have enough moisture available to produce them – which indicates a very low relative humidity.