Originally Posted by camachinist

PIREPS = pilot reports

* Soft rime is less dense than hard rime and is milky and crystalline, like sugar. Soft rime appears similar to hoar frost.
* Hard rime is somewhat less milky, especially if it is not heavy.
* Clear ice is transparent and homogeneous and resembles ice-cube ice in appearance. Its amorphous, dense structure helps it cling tenaciously to any surface on which it forms.

Both rime types are less dense than clear ice and cling less tenaciously, therefore damage due to rime is generally minor compared to clear ice. Glaze ice is similar in appearance to clear ice but it is the result of a completely different process, occurring during freezing rain or drizzle.

These three types occur also when ice forms on the surface of an aircraft when it flies through a cloud made of supercooled water liquid droplets. Rime ice is the less dense, milky ice is intermediate and clear ice is the most dense. Dense ice can spoil lift and may have a catastrophic effect on an airborne aircraft.