Why Do Planes Leave White Trails in the Sky?
A plane's white trail is a cloud of ice crystals formed when engine water vapor freezes in the frigid air at cruising altitude. Humidity decides how long it lasts.
The white trail a plane leaves in the sky is a narrow cloud of ice crystals called a contrail, short for condensation trail. Hot water vapor from the engines meets the frigid air at cruising altitude, condenses and freezes almost instantly. It is not smoke or a chemical spray: it is a cloud created by the plane's passage.
Where does the water in a contrail come from?
From the fuel itself. When jet fuel burns, its two main products are carbon dioxide and water vapor. According to a fact sheet prepared by experts at the Federal Aviation Administration (FAA), the Environmental Protection Agency (EPA) and NOAA, jet exhaust is about 71% carbon dioxide and 28% water vapor. Less than 1% is other products, such as sulfur and nitrogen oxides, carbon monoxide, unburned hydrocarbons and soot.
Several miles up, the air can be as cold as -70 °F (about -57 °C). When the hot, moist exhaust mixes with that air, the vapor condenses around tiny particles, forms droplets, and those droplets freeze into ice crystals. It is the same idea as seeing your breath on a cold morning, just under far more extreme conditions.
A contrail looks white for the same reason clouds do: the crystals are too big for the kind of scattering that makes the sky blue, so they redirect all colors of light about equally. They are cousins of the ice crystals at the top of storm clouds, where they help separate the charges that produce lightning.
Why do some contrails vanish fast while others last for hours?
It all comes down to humidity up there, which can be very different from what you feel on the ground. If the air is dry, the ice crystals turn back into vapor quickly. If it is humid, they survive and can even grow. The FAA fact sheet sorts contrails into three types:
- Short-lived: they form when the air holds little moisture and disappear within seconds or minutes, sometimes right behind the plane.
- Persistent, non-spreading: they form in very humid air and last from a few minutes to more than a day, keeping a thin line shape for hours when there is little wind shear.
- Persistent, spreading: also in humid air, they last hours or days and, where winds change speed and direction with height, spread across the sky into clouds that resemble cirrus.
If the air is too warm or too dry, no contrail forms at all. That is why two planes crossing the sky on the same day can leave very different marks: they only need to be flying at altitudes with different humidity. In some busy flight corridors, the FAA estimates that cloud cover has increased by as much as 20% because of persistent contrails.
What about so-called chemtrails?
It is understandable that a sky crisscrossed with lines that spread out and linger all afternoon can look odd, and the idea that these trails are chemicals released on purpose, so-called chemtrails, circulates widely. The question deserves a serious answer, and there is one: the differences between one trail and another are explained by humidity and wind at flight altitude.
The FAA fact sheet states that persistent contrails do not indicate that an aircraft released harmful chemicals, and that the U.S. government is not conducting weather modification or releasing harmful agents from aircraft. It also notes that jet fuel contains no metal-based compounds, because dissolved metals would threaten engine operation.
In 2016, a study in the journal Environmental Research Letters surveyed 77 scientists from two fields: atmospheric chemists who specialize in contrails and geochemists who study how dust and pollution settle out of the air. Of those, 76 said they had never encountered evidence of a secret, large-scale spraying program. The authors concluded that the data offered as proof can be explained by the well-understood physics and chemistry of contrails and aerosols.
So do contrails have no effect at all?
They do, and that is where the real concern lies. Like any thin, high cloud, persistent contrails can trap some of the heat Earth gives off, so they count toward aviation's impact on climate. The exact size of that effect is still being researched, and the FAA itself says more study is needed.
Next time a plane draws a line across the sky, watch how long it holds. If it fades right behind the tail, the air up there is dry; if it lingers and widens, there is a cold, moist layer several miles above you that was invisible until that moment.
