Contrails typically form at an altitude of approximately 9 km (30,000 feet), where low temperatures ensure rapid condensation and freezing.
The relative humidity of the air also plays a big role. After water vapor from the exhaust condenses and freezes to form small crystals, the crystals grow as water vapor from the surrounding air condenses and freezes on the newly formed crystals. Higher levels of relative humidity in the air means more water vapor is available to contribute to the contrail, which makes it last longer.
What is skywriting?
Skywriting typically happens at an altitude of approximately 3 km (10,000 feet) to keep the letters visible for ground-level audiences. But since this altitude is too low for contrails to form reliably, skywriting involves a different process that creates smoke, instead of clouds.
Heating oils with relatively low viscosity, which means they flow easily, can create the
desired effect. Traditionally, skywriters used mineral oil, which you can buy at a drugstore. Canola oil can also be used, and it’s considered more environmentally friendly than mineral oil.
Skywriters carry many gallons of oil in pressurized canisters near the engine. When pilots are ready to make some smoke, they flick a switch, injecting oil into the exhaust manifold, which is the chamber where the gases from the combustion of fuel in the engine collect.