Why Do Aircraft Sometimes Leave Vapor Trails?
Vapor trails form in the sky because the water in the exhaust of aircraft engines condenses when it meets the cool air. Vapor trails, or condensation trails, are artificial clouds of a roughly tubular shape, formed of liquid drops of ice crystals. Sometimes short-lived trails may be seen near the wing tips or propeller tips of an aircraft because water vapor in the air is expanded rapidly at these points and cools quickly.
For each gram of fuel burned in the engine about 1.4 grams of water vapor and about 10,000 calories of heat are produced and exhausted into the wake of the aircraft. For a vapor trail to form, the aircraft must be flying in air colder than about -60° C (-76° F). In warmer air the heat will overcome the humidification and no trail will form. Nowadays most jet-propelled, pressurized aircraft operate in the cold upper layers of the atmosphere so the narrow wakes of the vapor trails can be seen more often.
Actually, there are three types of trails an aircraft can leave in the sky:
- Condensation of water vapor in the exhaust gas: This needs the very cold, very dry air at higher altitudes between 6000m and 12000m. These trails can form condensation cores for humidity already present, so the trail grows over time and stays in place for hours. They are called condensation trails or “contrails”.
- Smoke from partially burnt fuel: This was prevalent in early jets and allowed to spot them from a distance.
- Oil mist or smoke flares: When oil is injected into the hot exhaust stream which forms a dense mist. This can be done with jets and piston engines, and gliders or parachutists use smoke flares. Once they are ignited (in most cases electrically) they burn down and cannot be switched off.
There are many more things that can be sprayed from an aircraft (fuel, obviously, when it has to be dumped in an emergency, fertilizer or insecticides, silver iodide to seed clouds), but these are special cases.
If you’ve ever seen an air show, you may have seen airplanes creating messages in the sky with what look like clouds. Are these contrails?
Not exactly…Skywriters use small airplanes equipped with special smoke machines to fly in special patterns to create written messages visible from the ground. The smoke machines usually consist of pressurized containers full of oil. At a pilot’s command, the machines spray oil onto the plane’s hot exhaust system, where it burns quickly and creates clouds of dense white smoke.
Pilots have to learn how to fly special patterns and work the smoke machine carefully to be able to create their unique messages in the sky. From advertisements to marriage proposals, the messages can be practical, personal or just plain silly. Skywriting goes way back. There are reports of successful skywriters before World War I, possibly as early as 1915. Today, skywriters take advantage of satellite navigation to program messages before flight, thereby increasing accuracy.