1. ki_cz

    ki_cz Member

    I've been trying to find the scientific reason behind contrails 'breaking up' thin layers of cloud cover and so far haven't managed to find anything. It's something that I don't see too often, but attached is an example, I don't want to speculate regarding the cause, but have some ideas. If anybody can post some actual information about the cause of this it would be greatly appreciated:

    Attached Files:

  2. Trailblazer

    Trailblazer Moderator Staff Member

    They're called distrails.


    They can form in one of two ways: either the heat of the engine causes water droplets in relatively low clouds to evaporate, or by triggering glaciation (freezing) of supercooled droplets, which then fall out of the cloud. Your picture looks to me like the former type, but I could be wrong.



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  3. Hama Neggs

    Hama Neggs Senior Member

    Would this mean that there are areas of atmosphere which are devoid of condensation nuclei?
  4. Trailblazer

    Trailblazer Moderator Staff Member

    Freezing nuclei aren't the same as condensation nuclei. There are condensation nuclei everywhere in the atmosphere, but not ice nuclei. That's why you get ice-supersaturated regions, where RHi is above 100#, which are where persistent contrails form.
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  5. Hama Neggs

    Hama Neggs Senior Member

    I'm not sure I fully understand that. Could you elaborate?
  6. Spectrar Ghost

    Spectrar Ghost Senior Member

    Freezing nuclei need the right structure to act as seeds for ice crystals. CCN simply need a surface for water to condense into and a small enough size that their terminal velocity is low and they don't just fall out of the atmosphere.
  7. Trailblazer

    Trailblazer Moderator Staff Member

    Yes, vapour will condense onto just about any particles in the air if the humidity is above 100% relative to water. But if the humidity is below 100% relative to water but above 100% relative to ice (usually about 60-65% relative to water at the altitudes we are talking about) then it will only form ice crystals if there are suitable freezing nuclei available.

    In very simple terms, it's harder for ice crystals to form, because they have to form in a certain alignment (hexagonal crystals), whereas water droplets can form in any random configuration.

    If there are such freezing nuclei present, you get cirrus clouds. If there aren't, you get areas that are supersaturated relative to ice, just waiting for some suitable nuclei, such as those produced by aircraft exhaust.
  8. Hama Neggs

    Hama Neggs Senior Member

    OK, but what has to be present for natural cirrus clouds to form? Where does the initial nucleus come from?
  9. Trailblazer

    Trailblazer Moderator Staff Member

    Mineral dust, for the most part. It seems to be not totally understood as yet.


    A recent study found that ice nuclei are mostly mineral dust and metallic particles. It's not clear from the abstract where the metallic particles come from, but I would assume human activity, and probably jet exhaust (which contains metal particles due to engine wear).

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  10. Hama Neggs

    Hama Neggs Senior Member

    Interesting. Thanks.

    This part caught my eye:

    I found that quite a surprise. So that means organic particles, or am I misunderstanding something?
  11. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    Sea salt, compounds of ammonia, sulphur dioxide etc.
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