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It's supposed that Air France 447 crashed into the Atlantic because the pilots stalled the airplane when the frozen pitot tubes forced them to handfly it while cruising through the thunderstorm. But how could the pitot tubes freeze in that high altitude where the temperature was way below the freezing point? Isn't it that ice only forms on the aircraft between slightly above 0 and below -20 Celsius degree?

Edit) Wikipedia: "Below −20 °C (−4 °F), icing is rare because clouds at these temperatures usually consist of ice particles rather than supercooled water droplets. Below −48 °C (−54.4 °F), supercooled water cannot exist, therefore icing is impossible."

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    $\begingroup$ Water can freeze at any temperature below the triple point. Water can also exist as a liquid at temperatures down to about -50c when conditions are right. $\endgroup$ – Simon Jul 4 '17 at 19:02
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    $\begingroup$ @J.Hougaard I've editted the post. Thank you. $\endgroup$ – lemonincider Jul 4 '17 at 19:02
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    $\begingroup$ Ah, your edit coincided with my comment. The key word in all of that is "rare", not impossible and frequent enough to affect many flights a year. $\endgroup$ – Simon Jul 4 '17 at 19:04
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    $\begingroup$ @Simon Yes I got that wrong but still the quote says icing is impossible below 48 Celsius degree $\endgroup$ – lemonincider Jul 4 '17 at 19:08
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    $\begingroup$ the atmospherical conditions above a thunderstorm are quite different from the "standard atmosphere" the wikipedia article is based on. for exact details on the differences, I would suggest earthscience.SE $\endgroup$ – Federico Jul 4 '17 at 19:15
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First, AF 447 was flying into the top of a tropical storm. The air was much more humid and warmer than that of the standard atmosphere.

Next, in the absence of seed crystals or nuclei, water vapour or liquid droplets stay in the air down to temperatures of -48.3°C at sea level pressure without freezing. From the linked article:

Droplets of supercooled water often exist in stratiform and cumulus clouds. An aircraft flying through such a cloud sees an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft's wings or blockage of its instruments and probes, unless the aircraft is equipped with an appropriate de-icing system. Freezing rain is also caused by supercooled droplets.

So it was the impact with the probe which caused the supercooled droplets to freeze.

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    $\begingroup$ For us who live in climates with winters below zero, a cool experiment is to leave a half full bottle of water in your car overnight. In the morning, if conditions are right, you can gently pick up the bottle to see it is still liquid. Give it a shake and it freezes instantly. Same principle, super cooled liquid is introduced to a nucleation point causing freezing. $\endgroup$ – Ron Beyer Jul 5 '17 at 3:35

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