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Contrails Are Condensation, But Not Like Your Breath

The trails that aircraft leave in the sky are called “contrails”, which is short for “condensation trails”. They are formed by the condensation of the water vapor in the aircraft exhaust.

SWA Contrail

When you breath out on a cold day, you see a little cloud of condensation form from your breath. This is the same kind of thing, your damp warm lungs add moisture to the air, and when you breath out, you get condensation.

But the condensation from your breath quickly evaporates, usually in less than a second. Condensation trails from a jet can last for many minutes, even for hours sometimes. So why is there this difference? Why do jet contrails sometime persist, but your breath condensation quickly evaporates?

The difference is because a contrail freezes.

It’s really that simple. Contrails form at -40 degrees Fahrenheit (which is also -40 Celsius), or colder. At that temperature the tiny drops of condensed water will instantly freeze. Once frozen they can not evaporate. They also can’t melt, as it’s -40. They can however fade away through a process known as “sublimation” – where a solid turns into a gas.

You’ve seen sublimation before. Dry Ice is frozen carbon dioxide. It does not melt, it just sublimes directly into the gas. If you take a bit of dry ice, and just leave it in the sun, it will just kind of fade away. That’s what happens to the ice in a contrail.

Ice will only sublime if the humidity (at that altitude) is lower than around 60% to 70%. So if it’s a bit higher then the contrail can last for a long time, just like clouds do sometimes. If the humidity is low, then the sublimation happens very fast, and the contrail only lasts a minute or so. If the humidity is high (above 70%) then you get reverse sublimation (also called desublimation or deposition, where water vapor turns directly to ice, but only when in contact with ice), and even more ice will form on the frozen condensation, the ice crystals will get bigger, and sink faster, causing the trail to spread out as it sinks through altitudes with different wind speeds.

So, the difference can be summed up as:

Contrail = Condensation + Freezing + Sublimation
Breath = Condensation + Evaporation

This isn’t really a property of your breath though, it’s a property of temperature. It you breath out at -40 degrees or colder, then your breath will freeze, and it will not evaporate. Instead of a little cloud that quickly evaporates, your breath at -40 degrees will look like smoke. Like these guys in Siberia, at -52C, you can’t tell the difference between their breath, and cigarette smoke.


Note: when we say a contrail freezes we are generally talking about an exhaust contrail – from the engine. There’s another type of contrail that’s the aerodynamic contrail you sometimes see when a plane is landing, streaming from the wing tips or flaps. That’s actually liquid water condensation, like your breath, and that’s why it quickly vanishes.

10 thoughts on “Contrails Are Condensation, But Not Like Your Breath

  1. lamentira says:

    Hi Mick. I translate this for my blog. Good job, like always.

  2. la mentira says:

    Wow! Thank you Mick!

  3. SR1419 says:

    Here is a great example of hot water injected into very cold air and freezing instantly:


  4. ashley says:

    Lies this whole site is full of misinformation.
    Go to US Academy of Science if yo want to learn search for geoengineering.

  5. Jay Reynolds says:

    This site must have been quite a wake-up for you. Now you know what the chemtrail sites aren’t telling you. I see you haven’t found anything specific to complain about, which is good. Maybe you need to reassess what you have been told in light of this new information. Go to your chemtrail believer friends and ask them how to explain away what is shown on this site, because you know this is the high bar you will have to cross to maintain your belief.

  6. siegmund says:

    Hello Mick,
    The ice crystals of contrails are produced by resublimation because:
    “The transition of a solid material to gas is called sublimation, the reverse process is known as resublimation”.
    English wikipedia is wrong whereas German is correct.

  7. Jay Reynolds says:

    The english wikipedia page for sublimation states:
    “Sublimation is the transition of a substance directly from the solid phase to the gas phase without passing through an intermediate liquid phase. The reverse process of sublimation is deposition. The formation of frost is an example of meteorological deposition.”

    The english wikipedia page for “deposition” states:
    “Deposition is a process in which gas transforms into solid (also known as desublimation). The reverse of deposition is sublimation.”

  8. Siegmund, I’m not sure what point you are making. Do you think there’s an error in the article? Need more clarity somewhere. I did try to make it as simple as possible, and the sequence in the image omits possible deposition for simplicity.

    “Resublimation” in english is the wrong word to use for the process, as “re-” indicateds a back and forth in a process, a repetition of one state or action, rather than the reverse.

    “Refreeze” means to “freeze again”. Like in the sequence: Freeze then Melt then Refreeze.

    Defreeze (or the more common defrost) means to melt.

  9. Nick says:

    Chemtrail believers are the most hypocrytical people. They talk about the government hiding information from us and talk about looking at real evidence. However, they refuse to look at actual evidence and on sites like Youtube they filter comments and block users that have different ideas than them. They sound a lot like the government they’re complaining about.

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