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If outside the aircraft is pretty cold and inside the cabin the air temperature is different, why don't the windows become fogged up on the inside?

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    $\begingroup$ Very low humidity really helps. $\endgroup$ – fooot Mar 14 '16 at 20:51
  • $\begingroup$ You didn't explicitly say it, but I assume you're asking about condensation on the inside? Condensation forms very commonly on the outside, when airliners descend from cold air into warm, moist air close to the ground. $\endgroup$ – Pondlife Mar 14 '16 at 21:15
  • $\begingroup$ @Pondlife yes, condensation inside the cabin. $\endgroup$ – kepler22b Mar 15 '16 at 0:24
  • $\begingroup$ Who says it doesn't? I've seen it, though rarely. I suspect, though, that the windows are double paned, for insulation, and condensation only happens if moisture somehow gets between the panes. $\endgroup$ – jamesqf Mar 15 '16 at 4:10
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The humidity at the altitude airliners fly at is VERY low. However, a tiny bit of moisture does build up - if you look closely you'll see little ice crystals on the window (it's also very cold up there - in the negative degrees F usually). On top of that - airliners have multiple air conditioning "packs" that dehumidify the cabin air so there would be indeed by very little moisture to condense on the inside of the windows. Also, if you look at the windows - they are often two or three layers with quite a bit of space separating the outside of the window from the inside (often multiple inches). Some of this is for pressurization and some of this is for thermal insulation. The VERY cold outdoor air is not in direct contact with the inside cabin pane.

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  • $\begingroup$ The OP is asking about condensation on the inside of the windows $\endgroup$ – TomMcW Mar 15 '16 at 6:46
  • $\begingroup$ I have no answer other than that there are air conditioning packs in the plane that run constantly and that dehumidifies the air, thus internal condensation would be limited in the same way that it is on your house windows. $\endgroup$ – Pugz Mar 15 '16 at 6:48
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    $\begingroup$ "Condensation would be limited in the same way that it is on your house windows": The conditions are not the same, a small cabin volume for several hundredths of breathing people can't be compared with a house volume for, say, four persons. When the aircraft is in clouds, air should be very damp. Packs seem to extract water from air (4. Pack operation and 5. Water extractor). $\endgroup$ – mins Mar 15 '16 at 10:06
  • $\begingroup$ @mins: The packs handle bleed air only. The very first picture of the linked answer shows that at some point, air from packs and from "recirculation fans" is mixed. $\endgroup$ – sweber Mar 15 '16 at 14:28
  • $\begingroup$ @sweber: You are right, only bleed air, which could be damp air from a cloud unless bleed air from the compressor section is dehydrated before reaching the pack. In addition water from breathing is collected in the cabin and recirculated. That's my understanding. $\endgroup$ – mins Mar 15 '16 at 15:09
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Double glazing. The gap in between the panes has very little moisture in it, so there is nothing to condense on the outside pane. There is no condensation on the inside pane because of the temperature gradient between the panes.

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Outer window panel is pneumatically isolated from the outsife, it is a pressure window , the second and third inboard windows are primarily there for thermal and acoustic insulation. Condensation (frost) forms on the inside of the outer pressure window because there is a pneumatic pathway to the cabin and passenger breath contains water vapour which becomes attached to this surface during the flight.

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