What is rime ice?

It’s a type of ice formed when tiny water droplets are cooled and freeze on the first surface they come in contact with. Their small size and very cold temperature are making them freeze almost instantly, which leads to a formation of ice particles with air trapped inside.

Rime may appear similar to frost, but there’s a certain difference. Rime is formed by condensing water droplets like fog or mist, which is then coming into contact with a hard surface. On the other hand, frost is created when water vapor is directly decomposing from vapor to ice.

Types of rime ice

There are two main types of rime ice: soft rime and hard rime. As the name suggests, the soft rime is quite soft and fragile – it can be easily shaken off the surface it has formed on, while hard rime is rather hard to get rid of. Soft rime also forms into feather-like spiky crystals similar to needles (they can be as long as 4 inches or around 10 cm) – and hard rime is rounder and comb-like.

Soft rime ice is mostly formed when the water droplets are very small, a temperature is very low and the water accretions slowly. The opposite is required to the creation of the hard rime: bigger droplets, higher temperature (although it still has to be low enough to ensure that the liquid will eventually freeze) and faster accretion of water.

Both hard and soft rime is milky white, because of the air trapped inside the ice. This makes differencing it from another similar type of ice – glaze – easier. Glaze ice is smooth and transparent, while it’s almost impossible to see through rime ice. Even so, the rime is usually less damaging, as it clings less and is, therefore, easier to remove, for example from an aircraft on which it could affect aerodynamic characteristics of the wings.