Icing

Historically, aviators have been concerned with icing that occurs when super-cooled liquid water freezes upon contact with an airframe at temperatures below 0C. Generally at very cold temperatures, the amount of super-cooled water present in a cloud decreases. At temperatures below -40C, it is generally accepted that super-cooled water does not exist in clouds, and that at temperatures below         -20C, the threat of icing is minimal as at that temperature, although some super-cooled water may exist, there would be inadequate amounts to present a serious icing threat.  However, in recent years,engine power -loss incidents related to aircraft flying through areas of ice crystals at very cold temperatures, indicate that these ice crystals may present a problem affecting the performance of jet engines. This issue has been covered in Aero, a Boeing publication.

http://www.boeing.com/commercial/aeromagazine/articles/qtr_01_10/5/