I’ve always assumed that skywriting was like water vapor in the sky, but it turns out it’s actually canola oil. The water vapor trails that we usually see from planes are created from the plane’s combustion when oxygen and fuel react, creating carbon dioxide and water vapor. Unfortunately, the vapor is not visible by itself, so the water particles need to hit very cold air to condense into droplets that form the contrails. Unfortunately, this happens too high up in the sky for people to easily read a written message from the ground.
Skywriting planes, therefore, need to fly lower, where it’s not as cold. To create visible smoke clouds, they use a different strategy and carry canisters of mineral or canola oil. The pilot then flicks a switch to release the oil into the plane’s exhaust, and the heat boils it. When the vaporized oil is released into the less cold air, it condenses into visible smoke clouds.
If you want a more environmentally friendly option, though, you could just talk to each other instead of riding in the sky. That’s just an idea.