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See this video of Birmingham strong crosswinds landings:

At 2.57, there is a 737-800 landing (screen shot below). One can clearly see some vapor (?) coming out of what seems to be two holes on both wings, from 3.03 to 3.10, were the plane lands.

This does not seems to be a 'classical" vapor trail, as asked in this question, as it clearly does not occur at the tip of the wing. enter image description here

What is this? What explains this? Something specific to the 737-800?

Edit: While it is clear to me that this is a vapor trail, the question is more about why there is a vapor trail at this precise location of the wing. Usually, these appear at the wing tip, and this is clearly not the case here. Maybe something with the flaps, as suggested ? Thus, while the question itself is not exactly a dupe, this answer (and the associated pictures) give a clear explanation.

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    $\begingroup$ It may be the vortices created by the flap tips just as it happens with the wing-tips $\endgroup$
    – ppinto
    Commented Feb 16, 2020 at 17:58
  • $\begingroup$ @ppinto Mmh, could be, indeed. But why only the 737 ? Some specific flaps geometry on that plane? $\endgroup$
    – kebs
    Commented Feb 16, 2020 at 19:14
  • $\begingroup$ @RonBeyer Thanks for suggestion, but not a dupe, IMHO. My question is more about why there is a vortex/vapor trail at that precise location of the wing (usually, they appear at wing tip). $\endgroup$
    – kebs
    Commented Feb 16, 2020 at 19:15
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    $\begingroup$ @kebs IMHO that is explained in Peter Kämpf's answer to the other question. The last photo in there shows exactly this phenomenon. What exactly is unclear? $\endgroup$
    – Bianfable
    Commented Feb 16, 2020 at 19:43
  • $\begingroup$ Oh, indeed. The linked question is not a dupe as itself but the answer you mention does indeed answer my question! $\endgroup$
    – kebs
    Commented Feb 16, 2020 at 22:51

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This are indeed the vortices a wing sheds where lift suddenly drops. That they occur at mid-span and not at the tips is due to flap deflection. They do indeed start at the outer edge of the Fowler flaps. @NilsNielsen explains well why you see a white line at the core of the vortex.

See this photo (source): This is the same thing happening. It is not specific to the 737-800, but to any aircraft with powerful flaps which end at mid-span to make space for the ailerons.

Tu-204 (?) with wake

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  • $\begingroup$ Great answer, thanks. $\endgroup$
    – kebs
    Commented Feb 16, 2020 at 22:45
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Those are tip vortices, which are propagated into the plane's wake from sharp corners and projecting tips along the trailing edge of the wing like the corners of the flaps (when extended) and the ailerons. You can imagine them as "sideways tornadoes"; much has been written about them and I recommend you search this site for more info.

The reason they appear as white vapor is that the pressure inside the vortex is lower than that surrounding it, and if temperature and humidity conditions are just right then the pressure and temperature drop inside the vortex will push water vapor out of solution and make the otherwise invisible vortex easy to see as a wiggling "rope" of water droplets.

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  • $\begingroup$ Ok, so if I get your point, this is just some optical illusion: they are generated at the wing tips but appear on the middle of the wing due to view angle ? Is that it ? $\endgroup$
    – kebs
    Commented Feb 16, 2020 at 19:13
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    $\begingroup$ no, they originate at the outboard corners of the trailing edges of the flaps. In your original image, the flaps were not directly visible. $\endgroup$
    – niels nielsen
    Commented Feb 16, 2020 at 21:43

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