Contrails – Are caused by the engine exhaust of aircraft when water vapor is condensed and then freezes around exhaust particles. This makes ice crystals that form a cloud, usually seen as a line behind each engine of the aircraft. These contrails usually form above 24,000 feet, up to about 40,000 feet and where the air temperature is lower than -34º Fahrenheit. This is where a website like FlightRadar24 comes in handy. If you can verify there was an aircraft in the area of the contrail and the altitude was above 24,000 feet, you could be looking at a contrail. Figures 72 through 75 show contrails.
Figure 72 – A new and old contrail and a lens reflection.
Figure 73 – A contrail from an aircraft at high altitude.
Figure 74 – Crossing contrails and a lens reflection.
Figure 75 – Enlarging this photo will show a double contrail.