full text of the classic FAA guide
Although icing at high altitude is not as common or extreme as at low altitudes, it can occur. It can form quickly on airfoils and exposed parts of jet engines. Structural icing at high altitudes usually is rime, although clear ice is possible.
High altitude icing generally forms in tops of tall cumulus buildups, anvils, and even in detached cirrus. Clouds over mountains are more likely to contain liquid water than those over more gently sloping terrain because of the added lift of the mountains. Therefore, icing is more likely to occur and to be more hazardous over mountainous areas.
Because ice generally accumulates slowly at high altitudes, anti-icing equipment usually eliminates any serious problems. However, anti-icing systems currently in use are not always adequate. If such is the case, avoid the icing problem by changing altitude or by varying course to remain clear of the clouds. Chapter 10 discusses aircraft icing in more detail.