Have you ever noticed the cloudlike vapor trails that sometimes form behind an aircraft and wondered what they were called or how they are formed?
Contrails (short for “condensation trails”) or vapor trails, are long thin artificial clouds (sometimes called cirrus aviaticus) that are seen behind aircraft. They usually are formed at very high altitudes (above 26,000 feet) where the air temperatures are -34°F and relative humidity is 60% but they can also occur at low altitudes when the air is cool, humid, and there is a sudden decrease in pressure (like that seen around aircraft wingtips, flaps, or engine intakes).
What causes them to form? There are two main products created by burning fuel to produce power; carbon dioxide and water vapor. When the water vapor exits an engine’s exhaust it can temporarily raise the relative humidity past the saturation point where the water condenses into water droplets (or ice crystals if the air temperature is below freezing) resulting in a long thin artificial cloud!
In contrast, at lower altitudes on humid and cool days, vapor trails that might be seen are usually caused by condensation as a result of a decrease in air pressure. An aircraft wing, propeller, flap, or rotor creates lift by increasing the airflow over the top of the airfoil relative to that flowing past the underside which results in two things: decreased pressure and temperature (Bernoulli’s principle). If the temperature drop is great enough a fog like condensation trail will result. This most commonly is seen when an aircraft is low and slow on approach to an airport but it also can happen when a jet goes extremely fast, or in the case of jet engines, the fan blades reach transonic speeds.