by Skip Via
skip@westvalleynaturalists.org

Editor’s Note: A followup to this article, with more accurate information on how different varieties of frost form, is available here: Hoar Frost, Rime Ice, or Something Else?

A few days ago, driving around West Valley, I remarked on the beauty of the hoarfrost that had coated some Ponderosa pines along the roadside. That got me wondering about the nature of hoarfrost and how it forms. It turns out that some of what I have been calling hoarfrost is, in fact, not hoarfrost, but rather rime ice. The difference is not important for the casual observer, but it can be important meteorologically.

We all remember the three states of water–vapor, liquid, solid–from elementary science classes (although it turns out there may be a fourth state…). Hoarfrost forms when water vapor in the air condenses on cold surfaces on clear, windless nights. The hoarfrost crystals freeze directly from the vapor state and do not turn into liquid water in the process (deposition). This accounts for hoarfrost’s delicate needle-like crystals. (The opposite of this process—when ice evaporates directly to vapor without passing through a liquid state—is called sublimation.)

Hoarfrost is easily dislodged from the surface by wind or by touching the branch. It requires a surface colder than the surrounding air in order to form, which is why we see it more typically on thin branches, leaves, and needles rather than on trunks or rocks–the former’s large surface areas relative to their volumes causes them to cool quickly relative to the surrounding air.

Hoarfrost does not from from fog or low clouds. Fog consists of liquid water droplets, not vapor. When liquid water condenses and freezes on a cold surface, we get rime ice.

Rime ice forms in foggy or cloudy conditions when the water droplets in the air freeze on cold surfaces. While in the right conditions its crystals can resemble hoarfrost, most occurrences of rime ice will be thicker and will coat the entire surface more evenly. Rime ice crystals tend to resemble feathery snow crystals rather than needles, and, unlike hoarfrost, can build up over time as long as the foggy and cold conditions persist. Hoarfrost doesn’t weigh down the structure that becomes its host–rime ice can build up enough to break tree limbs or down power lines. (Rime ice is not the same phenomenon as freezing rain. See this article for more information on that.)

We very often encounter rime ice along heavily-traveled roadways. This is because moisture droplets from auto exhaust can add to the condensation of rime ice on tree limbs and needles.

Simply because “hoarfrost” sounds more idyllic than “rime ice,” it’s likely that I’ll keep referring to any ice accumulation on branches and needles as the former. But it’s worth knowing, next time you encounter such a phenomenon, how it formed.

Here are a few images for you to think about. Hoarfrost or rime ice?