I am writing a pre-scientific work about the chemtrail conspiracy theory. A part of that is the exact process of contrail formation. There is one more puzzle piece missing. Hopefully the answer to this question doesn't make the puzzle bigger.

But here's the question: According to chemtrail conspiracy theorists, the difference between contrails and chemtrails are their persistence. "Normal" contrails dissolve after a few seconds, but "chemtrails" persist longer, minutes or even hours.

Contrail and "Chemtrail"


And now to their argument: They say that for a contrail to be persistent, the relative humidity of the ambient air has to be over 100%, hence oversaturated, because then the water vapor resublimates into ice and the contrail grows and stays persistent.

And then they come to their argument: The air in altitudes that planes fly at (~FL340, ~250 hPa pressure altitude (standard atmosphere)) is very rarely oversaturated, but persistent contrails can be seen pretty often.

Now I need to know, is it true that resublimation is only possible in oversaturated ambient air? If yes, is resublimation even needed for a contrail to stay persistent? Because I found a source that stated that only ~70% relative humidity is needed for a contrail to stay persistent.

Or maybe I've just been unfortunate with all the sounding data I've viewed so far, and the air up there is pretty often oversaturated, although I don't think so.

Please help me understand the last puzzle piece I need in order to finish my pre-scientific work and prove the conspiracy theorists wrong. Thanks a lot!

  • $\begingroup$ Cannot be a contrail made of simple water droplet freezing to minute ice crystal? Why these ice crystals should dissolve given that we speak of km altitudes? Aren't contrails formed by water in the exhaust? I don't see were "resublimation of water vapors (??) into ice ....." comes into play. Higher the rel humidity can surely help the growth of ice but the important thing seems to be the initial water vapour should condense "in the trail not in the atmosphere". But I am not a specialist of clouds and meteo. $\endgroup$
    – Alchimista
    Feb 12, 2018 at 12:33
  • $\begingroup$ @Alchimista Contrails are made of frozen water droplets and when they dissolve, those ice crystals sublime away into water vapour again. I want to know when this process of sublimation gets slower, so the contrail stays persistent, or even reverses into resublimation, so it even grows and stays visible for a long time. $\endgroup$ Feb 12, 2018 at 13:39
  • $\begingroup$ It suffices that they stay long enough for a trail to be permanent. Says, at minus 10 or minus 20 °C why a ice crystal should be so short living?obviously higher the rel hum easier the condensation. But I mostly see it as a problem of T. If the T is low enough you can condense a vapour stream into water and ice independent of rel hum. The water is provided by the exhaust. We are speaking contrails not snow or rain $\endgroup$
    – Alchimista
    Feb 12, 2018 at 13:52
  • $\begingroup$ @Alchimista Hm, I don't know if understand right, can you explain it a bit more? $\endgroup$ Feb 12, 2018 at 13:59
  • $\begingroup$ I am not a specialist of clouds formation. At minus 40 and athmosoericS pressure water is ice. I need 100% rel hum to make ice means I need water to makes ice cubes. In the freezer hum is very low. Oversaturated air will mean that more denser contrails form, but it is not a requisite for their formation and persistance. Less they sublimate longer they stay, in this the complotionists are right. Resublimation in your q is cryptic. The process is water cond - freezing - more or less slow (or fast) sublimation of ice. As for humidity in atmosphere I am taking the fact that clouds can form. $\endgroup$
    – Alchimista
    Feb 12, 2018 at 14:41

2 Answers 2


They're wrong about the relative humidity not reaching high enough levels. Here's a map of the continental US from instantweathermaps.com, showing today's relative humidity at a pressure altitude of 300 hPa (300 mb):

enter image description here

So, across most of the US, the relative humidity is greater than 90%, which is enough for persistent contrails to form. The intuition here is that the air in the upper troposphere is quite cold, so it can't hold much water vapor in the first place. Even though there isn't much water vapor up there, the presence of even a tiny amount quickly pushes air toward saturation.


If the air at altitude were not supportive of persistent ice crystals, we would not so frequently see high altitude (ice crystal) clouds, or fallstrokes, naturally.

Since those exist quite frequently, and persist, it is not clear why any disturbance that forms the same structure (e.g., contrails) ought to require soemthing more special.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.