Ice on windows

By Marcus Wilson 12/08/2010

 No, not ice on car windows this time, but ice on aeroplane windows. John Fouhy has sent this question to me, and I don’t know the answer. I have a couple of ideas, but what do you think? Below I reproduce John’s question in full, along with the picture.  Airpoints Gold Card holders, help us out here…





 I recently flew into Auckland from Singapore. It was a night flight, so we only opened the windows as we prepared to descend. When we did, I looked outside and noticed ice on the outside window. Well, fair enough, it’s cold outside. But if you look at the photo I attached, there is only a small patch of ice. It’s in the shape of an annulus, centred on a small metal pin. The pin appears to be attached to neither the inner nor the outer surface of the window (I imagine aeroplane windows have many layers).

It didn’t take long for the ice to melt in the morning sun. Drops of water remained on the window for a while after, in the same place. So it is possible (from the absence of water elsewhere) that ice did not form elsewhere.   [Marcus – I’ve seen this before too – is it something to do with the manufacture process?]

As far as I could see, by craning my neck, the next window up from me had the same feature.

So my questions, if you have time: Why would ice form only near this metal pin? Why would ice _not_ form even closer to the pin?

0 Responses to “Ice on windows”

  • Isn’t the “pin” actually a narrow hole through an internal pane? Movement of air & condensation within the window might then explain what you’re seeing. (Even though they are sealed, I imagine some moisture gets inside the window frame.)

    FWIW, I’ve seen freezing this way on aircraft windows, too, but also elsewhere on the window interior.

  • The “pin” is probably a pressure balancing hole between layers of the window “glass”. The air coming through the pinhole may be a wee bit warmer than the air where the ice is forming. This area could very well be the coldest spot inside the window. But…If the area where the ice is very close to the temperature of the pinhole is, then some interesting things might happen. Condensation may have formed but the fractionally warm air would come through the hole may warm the area around the hole and thus prevent condensation. So it could be just a temperature difference. However, If the area around the pinhole was clean, then condensation may have occurred and reamined as liquid a wee bit longer. It remained as supercooled water. Water may have condensed in the area where the ice is as well. But if there were some nucleation sites there then ice would preferentially form. Once the ice is formed another interesting thing occurs. The vapour pressure of water above ice is lower than water. So then the supercooled water now begins to evaporates (because the air close to the water is no longer saturated) and recondenses on the ice. I too have noticed the ice on aircraft windows. Have a close look next time and where there are single ice crystals, notice that the area around the each crystal is devoid of condensation/ ice. Once that crystal is formed then it sucks up the surrounding moisture. Fascinating!

    We see this on our Dew Point Mirrors here in the Humidity Standards, Measurement Standards Laboratory, Industrial Reseach Ltd. Users need to know if the condensation on the mirror is ice or liquid because the mirror temperature measured by the dew point meter does not know the difference!. There can be up to nearly 2 deg C error at -10 deg C (as measured on the DPM) in vapour pressure calculations if you do not know what phase is on the mirror.