Why does pitot tube freeze at an altitude whose temperature is way below the freezing point? 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."
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.