It might spoil some cherished childhood memories of singing his song, but no — Frosty the Snowman probably wasn’t frosty. Snow is a bit different than the branching, spiky ice crystals of hoar frost, though the two are often seen together.
Frost vs. Snow
The main difference between the two is how they form. Frost forms like dew, in that it appears on a surface of the ground or the hood of a car. When a surface is colder than the dewpoint of the air around it, moisture collects on that surface. When that temperature is below freezing, that moisture takes the form of ice crystals. By contrast, snow condenses around particles in clouds and falls to earth as a precipitate.
Hoarfrost, Surface Hoar, and Other Varieties
There are several different ways that ice crystals can grow, including:
- Windows develop ice crystals when they are exposed to freezing temperatures outside, and warmer, the moister air inside. The condensation collects on the surface of the window in lacy, fern-like patterns.
- Hoarfrost develops when there is a particularly ample source of humid air. Hoar frost crystals are particularly large and relatively common near bodies of water or other places where there’s a lot of moisture.
- Surface hoar is easy to confuse with snow, as it often forms atop snowdrifts. As snow evaporates during the day, it releases humidity into the layer of air right atop the drift. When temperatures drop at night, this humidity condenses on the re-frozen surface of the snow bank as small, glittery ice crystals.
- Crevasse Hoar frost develops in glacial crevasses. There, water vapor builds up from sublimating ice, and the freezing temperatures and calm air allow for an ideal environment for hoarfrost to grow.
When Warnings Occur
Guidelines for frost warnings or advisories state that they should be issued when ice crystals are predicted to form at three or more observational sites, typically in late spring to early autumn. They are of particular interest to farmers and gardeners, as they can affect planting and harvesting times or kill plants. The first widespread freeze usually signals the end of the growing season, and meteorologists typically stop issuing warnings until the following spring.
If you’re not a weather frost expert, it can sometimes be difficult to tell surface Hoar frost from snow. One look at the lacy patterns on windows or the large, pointy crystals of hoar frost shows that they wouldn’t be very good building material for a snowman, however magical he might be! To learn more about snow and ice removal from the snow, ice and frost experts, contact our certified snow professionals today for a consultation.