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I was recently flying back from Palma de Mallorca Airport (PMI) to Birmingham Airport (BHX/EGBB) and I had the luxury of sitting at a window seat behind the wing. I think the plane was a Boeing 737-800 via Jet2.

Halfway though the flight I was looking out the window and noticed little specks on the outer window and was trying to work out if it was frozen water or frozen bacteria from the clouds?

Photo midflight

Does anyone know what this is or how it forms? Does this happen to the pilot windows also?

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    $\begingroup$ What kind of bacteria is large enough to be observed with the naked eye? As far as I know there is only one species (Thiomargarita namibiensis), so what are the odds of this being this specific bacteria? $\endgroup$
    – AEhere supports Monica
    Jul 24, 2019 at 13:25
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    $\begingroup$ I was thinking more like clumps or specs of frozen dust. Water made the most sense but I remember reading a white paper on how bacteria can travel in the cloud layer. $\endgroup$
    – Zac Grierson
    Jul 24, 2019 at 13:28
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    $\begingroup$ Are those specs really on the outside of the aircraft and not in between the outer and inner window panels? $\endgroup$
    – AEhere supports Monica
    Jul 24, 2019 at 13:30
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    $\begingroup$ @AEhere From what I could tell but my eyes can only do so much. $\endgroup$
    – Zac Grierson
    Jul 24, 2019 at 13:33
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    $\begingroup$ Is that coastline still Mallorca behind the trailing edge of the wing? I don’t have much comparisons, if it’s not frost I would assume usual cracks (looking there different): aviation.stackexchange.com/a/57359/1084. Question, I wonder about your usage of term „specs“ (here neither glasses, specifications nor speculation). $\endgroup$
    – Peter
    Jul 24, 2019 at 20:40

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If it disappeared after you descended, it was frost specks on the inside surface of the structural* outer panel, forming on dirt particles on the plexi, from the small amount of humidity in the cabin air that migrates through the little vent hole in the inner panel that is part of the interior fascia.

It's not a problem on cockpit windows because they are heated.

*I say structural, because the outer pane of quarter inch thick acrylic has about the same tensile strength as the adjacent much thinner aluminum skin and is part of the pressure hull.

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It is unclear from your photograph, but it can be either frost, dirt or surface damage on the plexiglass layer.

As far as I know there is only one species of bacteria visible with the naked eye (Thiomargarita namibiensis), so without a workable theory on how it got up to FL300+ in such numbers, that hypothesis is not plausible.

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    $\begingroup$ Apparently those bacteria live in the coastal seafloor off Namibia and are rather immobile, so any theory is promising to be somewhat entertaining. I believe that bacteria colonies could in theory be visible even for smaller bacteria, though. And also, I would be very surprised if it wasn’t simply frost. $\endgroup$
    – Cpt Reynolds
    Jul 24, 2019 at 13:55
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    $\begingroup$ It's not bacteria, but why would you assume he's talking about observing a single large bacterium instead of a colony? Have you ever seen a petri dish? Bacteria are easily visible to the naked eye. $\endgroup$
    – John Wiseman
    Jul 24, 2019 at 18:35
  • $\begingroup$ @JohnWiseman I have, in fact! And it was the fact that it looks nothing like a typical colony and lacks a growth medium that made the idea seem even more far fetched. $\endgroup$
    – AEhere supports Monica
    Jul 24, 2019 at 19:20
  • $\begingroup$ Just looks like crazing in the plexiglass to me. $\endgroup$
    – TomMcW
    Jul 24, 2019 at 23:42

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