Why do some contrails vanish at once while others linger?
– The moisture of the upper air decides
Look up, and after a plane has passed you may see a white streak (a contrail). On some days it fades away in moments. On others it hangs in the sky for hours, slowly spreading until it looks like a cloud. It is the same kind of trail, so why does it disappear so differently?
On a clear day, have you looked up at a plane and seen a straight white streak stretching out behind it? Some days it vanishes quickly. Other days it lingers, spreads out, and becomes a thin sheet of cloud.
The plane flies much the same way and the engine works much the same way. So why does the trail last so differently from day to day?
A jet engine's exhaust contains water vapour. In the cold air high above, it is thought to freeze at once into tiny ice particles.
If the surrounding air is already moist, the ice particles last. If it is dry, they evaporate quickly and vanish, or so it is thought.
Let's look step by step at how the moisture of the surrounding air decides the outcome.
What a contrail is: ice particles frozen from exhaust water vapour
A jet engine is said to release water vapour in its exhaust as it burns fuel. The air several thousand metres up, at around 10,000 m, is extremely cold, tens of degrees below freezing. When the exhaust vapour meets this cold air, it freezes at once into countless tiny ice particles (ice crystals). These are thought to be what we see as a white streak: the contrail.
Why ice particles high above sometimes vanish quickly
Even freshly formed ice particles turn straight back into gas (water vapour) if the surrounding air is dry, with little water vapour in it. This change, from solid directly to gas without passing through liquid, is called "sublimation". In dry air the ice particles sublimate rapidly, and the contrail disappears in no time.
It depends on how moist the air was at that height, at that moment.
Why some contrails last a long time
If the surrounding air is already moist (holding a lot of water vapour), things change. When the air is already close to full of water vapour, the ice particles struggle to turn back into gas and keep their form. More than that, water vapour from the air can attach to the ice particles, so the particles grow and the contrail becomes thicker and wider, or so it is thought. Over time the wind spreads such a contrail out, and it can turn into a thin, streaky cloud much like natural cirrus.
Contrails that last and spread out (contrail cirrus) have been pointed out as possibly acting like a blanket, making it harder for heat leaving the ground to escape into space. Researchers are studying whether this effect can be reduced by adjusting flight routes and altitudes.
Things you can check for yourself
- On a clear day when contrails are visible, watch and record how long the contrail stays in the sky
- Using that day's forecast, check the humidity, and whether rain or cloud is approaching
- Repeat this for several days, and compare how the weather differed between days when contrails lasted and days when they vanished quickly
There is an old saying that when contrails last a long time, the weather often turns bad afterwards. It is thought to be linked to moisture in the upper air.
Summary
Contrails are countless ice particles, formed when water vapour in exhaust freezes in the cold air high above. If the surrounding air is dry, the ice particles sublimate and vanish quickly. If the surrounding air is moist, the ice particles survive, take in water vapour from the air, and grow into a long trailing contrail, or so it is thought.
The length of a contrail is a record, left behind by the plane, of how moist the air was that day at that height.
For those who want to know more – terms, numbers and links to textbooksFrom middle-school science to topics under active research, with the level of each part marked
- Middle schoolCovered in Japanese middle-school science
- High schoolCovered in high-school Earth Science Basics
- High school+Advanced or sidebar material in high-school textbooks
- UniversitySpecialist university content (cloud physics) not taught in high school
- ResearchNot taught even at university as settled; researchers are still working on it
Middle schoolTerms: words around contrails
- Water vapour: Water in gas form. It cannot be seen.
- Sublimation: A solid changing directly into a gas, without passing through liquid (or the reverse).
- Humidity: The proportion showing how much water vapour the air contains.
High schoolChecking with a formula: telling from humidity whether a contrail will last
Whether a contrail lasts depends on whether its ice particles grow or fade away. Let's calculate that dividing line from the humidity of the upper air.
| In symbols | RH_ice = (e ÷ e_ice) × 100 |
| In words | Humidity relative to ice = pressure of water vapour in the air ÷ pressure of water vapour in balance at the ice surface × 100 |
| Where it comes from | At an ice surface, water molecules leaving and returning are in balance. The pressure at that balance is the saturation vapour pressure, and how it changes with temperature is set by the Clausius–Clapeyron equation. Leaving ice takes more energy than leaving liquid water, so at the same temperature the balance pressure is lower over ice. |
| RH_ice | Humidity relative to ice. Unit: percent |
| e | Pressure of water vapour in the air. Unit: hectopascals |
| e_ice | Pressure of water vapour in balance at the ice surface. Unit: hectopascals |
| Temperature at flight height | About −50 ℃ |
| Ratio of the ice balance pressure to the water balance pressure at that temperature | About 0.62 |
| Dry air example (humidity relative to water, as in weather forecasts) | 40 % |
| Moist air example (humidity relative to water, likewise) | 70 % |
| Humidity relative to water at which air is exactly saturated for ice | 0.62 × 100 ＝ 62 |
| Humidity relative to ice, dry air | 40 ÷ 0.62 ≒ 64.5 |
| Humidity relative to ice, moist air | 70 ÷ 0.62 ≒ 112.9 |
③ In everyday terms: the dividing line is a humidity relative to water of 62%. In air at 40% humidity, the humidity relative to ice is only about 64.5%, so the ice particles sublimate and vanish. That is the contrail that disappears quickly. In air at 70%, it comes to about 112.9%, so the ice particles take in water vapour from the air and grow. That is the contrail that lasts tens of minutes and spreads out. The fun part of this calculation is that 70% doesn't feel very humid by ground-level standards, yet for ice it is more than enough.
※ The ratio of 0.62 is a rough figure near −50℃. The warmer it gets, the closer it moves to 1, and the dividing-line humidity rises too.
High school+A more detailed idea: "humidity relative to ice"
When looking closely at whether a contrail's ice particles survive, the humidity relative to ice (the proportion measured against the amount of water vapour in balance at an ice surface) is often used, rather than humidity relative to water. The higher the humidity relative to ice, the less easily the ice particles sublimate, and the more easily the contrail persists, or so it is thought.
UniversityIce crystal formation and growth in cloud physics
In cloud physics, researchers study how tiny particles in exhaust (such as soot) act as the seeds (condensation nuclei and ice nuclei) from which ice particles start to form. The number and size of the ice particles are thought to change with the properties and amount of these particles.
The condition for whether a contrail forms at all is called the Schmidt–Appleman criterion. It checks whether, as engine exhaust mixes into the outside air, the mixture ever passes saturation with respect to water. Its foundation is the Clausius–Clapeyron equation. Whether the cloud that forms then persists is decided by the humidity relative to ice (ice supersaturation) that we calculated above.
📖 Derivation of the formula, and further reading: Clausius–Clapeyron equation (Japanese Wikipedia) ／ Contrail (Japanese Wikipedia)
ResearchWhat is still unclear
- How much spread-out contrails (contrail cirrus) affect the global climate is a topic still being studied in climate science.
- How to choose flight routes and altitudes that reduce the climate effect is being studied from both aeronautical engineering and meteorology.
- Work also continues on technology to predict upper-air humidity more accurately, and so predict when contrails form and last.
Even a contrail drawn across the sky hides a rich topic, still under research, where cloud physics and climate science meet.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: changes of state of water | Basic terms: water vapour, sublimation, humidity |
| High school | Earth Science Basics: humidity | Calculating humidity relative to ice to tell when a trail lasts |
| High school+ | Earth Science: how clouds form (advanced) | The idea of humidity relative to ice |
| University | Cloud physics | Ice nucleation on exhaust particles |
| Research | Climate science and aeronautical engineering (under research) | Climate effect of contrail cirrus, flight route optimisation |
- Explanations of humidity and saturation vapour amount in Earth science textbooks.
- Explanations of the conditions for contrail (wake cloud) formation in meteorology materials.
- Research reviews on ice nucleation in the field of cloud physics.
- Research reviews on the radiative effects of contrail cirrus in the field of climate science.
- Explanations of research on reducing climate impact through flight route optimisation in aeronautical engineering materials.
※ The water vapour and humidity figures are rough values to help explain the mechanism. The actual duration of a contrail is said to vary with many conditions, including temperature, humidity and wind.
※This article is a general-audience science explainer. When watching the sky, take care never to look directly at the sun, and stay aware of your surroundings.