What is Rime Ice?

Rime icing is a fairly common sight in Iowa during the wintertime. Especially during a cold morning with fog. By definition, rime is a deposit of interlocking ice crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches.

What does this mean? This means the supercooled water droplets in the fog freeze on contact with objects below freezing. Turning it from a liquid state, to a solid state. This leaves behind the beautiful frosty trees and vegetation. Up close, the ice looks like needles. This type of icing is sometimes confused with hoar frost.

What is Hoar Frost?

Hoar frost is often confused with rime icing, however, the two are different. Unlike rime ice, hoar frost skips the liquid state, and goes from a gas state, straight to a solid state. Up close, this icing looks feathered like. This type of icing often develops on cold, clear nights from water vapor in the air.

Rime ice can come in different formations. Soft or hard rime. Soft rime forms during light fog or mist with calm or light winds. The fog freezes usually on the windward side of the object, and it appears similar to hoar frost. Hard rime looks somewhat less milky, and is more dense than soft rime. This type of icing occurs during high winds, and is often seen on trees atop of mountains or ridges.