A contrail is a thin white cloud that forms behind an aircraft when hot, humid exhaust from its engines mixes with extremely cold air at high altitude. The process is essentially the same thing that happens when you exhale on a cold day and see your breath, just scaled up to the size of a jet engine flying at 30,000 to 40,000 feet.
How Contrails Form
Jet engines burn fuel and produce exhaust that contains a large amount of water vapor, along with tiny soot particles. This exhaust is both hot and humid. At cruising altitude, the surrounding air is extremely cold (often colder than minus 40°F) and holds very little moisture. When the hot exhaust mixes with this frigid air, the temperature of the mixture drops rapidly. At some point during that mixing, the air becomes saturated, meaning it can no longer hold all its water vapor as invisible gas.
That excess moisture needs something to condense onto. This is where the soot particles from the engine come in. The water vapor condenses onto these tiny particles, forming liquid droplets that almost instantly freeze into ice crystals. Billions of these microscopic ice crystals together create the visible white line trailing behind the aircraft. The turbulence generated by the engine exhaust drives the mixing process, which is why contrails typically appear right behind each engine rather than from the aircraft as a whole.
Why Some Contrails Disappear and Others Don’t
Not all contrails behave the same way. How long one lasts depends almost entirely on how much moisture is already in the surrounding air.
If the air at cruising altitude is dry (below 100% humidity relative to ice), the ice crystals in the contrail quickly evaporate back into invisible water vapor. These short-lived contrails vanish within seconds to a few minutes, often disappearing not far behind the aircraft. If you’ve ever watched a plane leave a trail that seems to erase itself as the plane moves forward, this is what’s happening.
If the surrounding air is already nearly saturated with moisture (at or above 100% humidity relative to ice), the ice crystals have no reason to evaporate. In fact, they can grow by pulling in additional moisture from the surrounding air. These persistent contrails can last anywhere from several hours to more than a day. Some hold their shape as a thin white line across the sky. Others spread out over time, becoming wider and fuzzier until they look indistinguishable from natural high-altitude cirrus clouds. At that point, they’ve essentially become human-made clouds.
Research confirms that for a contrail to survive more than roughly 10 minutes and spread into a broader cloud, the surrounding air generally needs to be at or near ice-saturation. Long-lived contrails and the cirrus clouds they evolve into have been observed at temperatures ranging from about minus 45°F to minus 90°F.
Where in the Atmosphere They Appear
Contrails form at the altitudes where commercial aircraft cruise, typically between 30,000 and 40,000 feet. A Boeing 737, for example, generally flies in this range. The air at these altitudes is cold enough and thin enough for the exhaust-mixing process to produce ice crystals.
NASA research has found that between 30,000 and 35,000 feet, the likelihood of persistent contrails increases with altitude. This trend reverses above 40,000 feet. Between 40,000 and 50,000 feet, the atmosphere becomes so dry that persistent contrails become less common as altitude increases. The sweet spot for long-lasting contrails tends to be in that middle band where temperatures are very low but enough moisture exists for ice crystals to survive.
Geography and season matter too. Cold, moist upper-atmosphere conditions are more common at higher latitudes and during winter months, which is why you’re more likely to see persistent contrails in those settings.
How Contrails Evolve Into Cirrus Clouds
When a persistent contrail sticks around long enough, it can transform into something much larger. Wind shear at altitude stretches and spreads the contrail horizontally. Meanwhile, internal gravity waves (natural oscillations in the atmosphere) cause the ice crystals to cycle through periods of rapid growth and partial evaporation. Over time, this process selects for larger, more durable ice crystals that can remain aloft for extended periods.
The result is contrail cirrus: thin, wispy cloud cover that can span hundreds of square miles and persist for many hours. These clouds are visually and physically similar to natural cirrus clouds, and once they’ve fully spread, it’s often impossible to tell them apart from clouds that formed on their own.
Why Contrails Matter for Climate
Contrails are more than a visual curiosity. The cirrus clouds they create trap heat radiating from Earth’s surface, producing a warming effect. This warming effect is surprisingly large. The current best estimate of the warming caused by contrail cirrus is roughly three times greater than the warming caused by all of aviation’s cumulative carbon dioxide emissions. Put another way, CO₂ accounts for about one-third of aviation’s total warming impact, while contrails and other non-CO₂ effects make up the remaining two-thirds.
That said, the exact numbers are still being refined. A 2024 study in Atmospheric Chemistry and Physics calculated a global contrail warming effect for 2019 that was 44% lower than previous best estimates. The uncertainty reflects how difficult it is to track contrails globally and model their interaction with natural clouds. What isn’t in dispute is that contrails represent a significant portion of aviation’s climate footprint, one that researchers and airlines are actively working to reduce by testing flight path adjustments that avoid the most contrail-prone patches of atmosphere.
Why Contrails Vary From Day to Day
If you’ve noticed that some days the sky fills with contrails while other days you see none at all, even with similar air traffic, the explanation is straightforward. The upper atmosphere’s temperature and humidity change constantly. On a day when the air at cruising altitude is cold and moist, every flight passing through that zone will leave a long-lasting trail. On a day when the upper atmosphere is dry, those same flights will produce contrails that vanish almost immediately, or no visible trail at all.
Even along a single flight path, conditions can change. You’ll sometimes see a contrail appear, disappear, and reappear as the aircraft passes through patches of air with different moisture levels. The plane’s engines are doing the same thing the entire time. It’s the atmosphere that’s different from one mile to the next.