Condensation trails or contrails belong to the group of the specials clouds. They are formed in the wake of aircraft and the most common are the ones induced by exhaust gases of the engines. They form when water vapor within exhaust gasses mix with and saturate the air in the wake of an aircraft. Whether or not the wake reaches saturation depends on the ratio of water vapor to heat in the exhaust gas as well as on the pressure, temperature, and relative humidity of the surrounding environment.

Calculations of formation of contrails can be done easy when temperature and relative humidity at the pressure levels are known. However if no accurate data is available, this table gives an estimation.

 

Contrail probability

Pressure(hPa)	95%	90%	75%	50%	25%	10%	5%
150	-60.5C	-59.3C	-57.1°C	-55.5°C	-53.6°C	-51.5°C	-50.7°C
175	-58.8°C	-57.4°C	-55.3°C	-53.1°C	-51.4°C	-49.6°C	-48.5°C
200	-58.5°C	-56.6°C	-54.8°C	-53.1°C	-51.0°C	-48.5°C	-47.0°C
250	-58.1°C	-56.3°C	-53.8°C	-52.2°C	-50.1°C	-47.1°C	-45.3°C
300	-55.5°C	-54.0°C	-52.0°C	-50.7°C	-49.1°C	-46.3°C	-44.3°C
350	N/A	-49.9°C	-49.4°C	-49.0°C	-48.0°C	-45.9°C	-43.6°C

The picture shows a short lived contrail produced by the four engines of an Airbus A340 aircraft crossing the Moon from the observers point of view. The disk isn't exactly a large object, so a bit of luck or some patience is needed to obtain these kind of shots.