Contrails explained: why some planes leave white trails

6 min read · Updated October 11, 2026

What a contrail is

A contrail, short for "condensation trail", is a line-shaped cloud of ice crystals created by an aircraft. Burning jet fuel produces water vapour as well as carbon dioxide. When that hot, moist exhaust hits extremely cold air, the water vapour condenses onto tiny particles in the exhaust and freezes almost instantly, much like your breath fogging on a winter morning.

The key ingredients are:

• Very cold air, typically around -40 °C (-40 °F) or colder.
• Enough humidity at that altitude for the ice to form and, if it is high enough, to persist.
• Exhaust from an engine that adds water vapour and particles.

Those temperatures are usually found above roughly 26,000 ft in mid-latitudes, which is why contrails are mostly made by jets at cruise. In very cold weather they can form lower.

Why some planes make them and others don't

If you watch the sky for a while, you will notice one jet trailing a long white line while another at a similar height leaves nothing. The usual reasons:

• Different altitudes. Two planes that look side by side may be thousands of feet apart. The air at 38,000 ft might be cold and humid enough while the air at 31,000 ft is not. Checking their altitudes in the live sky tool often settles this immediately.
• Different air. Humidity at altitude varies in layers and patches, so one region of sky can be "contrail-friendly" while the next is not.
• Different engines. Engine efficiency affects exhaust temperature, which slightly changes the conditions needed. The effect is small compared with the weather.
• Low aircraft. Small planes, helicopters and jets below the cold layer almost never make contrails.

Short, persistent and spreading contrails

What happens after a contrail forms depends on the humidity of the surrounding air relative to ice:

Persistent contrails are made from the same stuff as natural cirrus clouds, and they behave the same way. When the upper air is humid enough for them to linger, the sky can fill with crisscrossing lines during busy periods.

What contrails tell you

About altitude

A plane leaving a contrail is almost certainly a jet at cruise altitude, roughly 26,000 to 41,000 ft. That is a useful clue when you are trying to identify a plane overhead: rule out anything below about 20,000 ft in the list.

About weather

Persistent, spreading contrails mean the upper atmosphere is moist. That often happens ahead of an approaching weather system, so old weather lore that "long trails mean rain is coming" has some basis, although it is far from a reliable forecast.

About wind

A lingering trail drifts and stretches with the winds aloft, which are often 50 to 150 knots and can exceed that in the jet stream. A trail that quickly becomes wavy or spreads shows wind shear at that altitude.

Other trails you might see

• Aerodynamic contrails: brief trails from wingtips or over the wing, caused by the pressure drop as air flows over the wing. They are often seen in humid air during approach or takeoff, and look like a flash of mist over the wings.
• Distrails: the reverse of a contrail, a clear lane cut through a thin cloud layer by an aircraft's heat and turbulence.
• Shadows: a contrail high above a cloud layer can cast a dark line on the cloud below, which sometimes looks like a black contrail.
• Smoke trails: aerobatic and airshow aircraft use smoke systems deliberately. These are low and short-lived.

What about "chemtrails"?

Some people believe long-lasting trails are chemicals sprayed deliberately. It is a widely shared idea, so it is worth addressing calmly. The evidence does not support it:

• The physics of contrail formation and persistence is well understood and was described long before modern jet airliners, including in studies of high-altitude bombers during World War II.
• Whether a trail lingers lines up with measured humidity at altitude, which is why trails come and go with the weather rather than with flight schedules.
• The planes making the trails are ordinary scheduled flights. You can check them yourself: look up the plane in the live tool and you will usually find an airline flight at cruise altitude heading for a normal destination.
• Scientists who surveyed atmospheric chemists and geochemists about the claims found that nearly all saw no evidence of a secret spraying program.

Contrails do matter for the environment, just not in the way the theory claims: persistent contrails trap some heat and are an active research topic in climate science, and some airlines are testing route changes to avoid contrail-forming air.

Why the same sky can look so different day to day

The number of flights over a city on a Tuesday is similar to the number on a Wednesday. If one day the sky is crisscrossed with lingering lines and the next it is clean, the difference is the upper-level humidity, not the traffic. Weather balloons launched twice a day measure temperature and humidity up through the cruise altitudes, and forecasters can use them to predict contrail-friendly days with reasonable skill.

Try it yourself

Next time you see a contrail, find the plane in the live sky tool. Note its altitude. Over a few days you will start to see the pattern: trails at 35,000 ft, none at 24,000 ft, and long-lasting lines on days when a front is approaching. For more on reading the altitude figures, see what altitude numbers mean.

Frequently asked questions

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