Is this frost or frozen bacteria in clouds? 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?



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