About the Peculiar Frost We Saw in Southeastern Wisconsin

How to identify this frost when it appears in our area.

Jack Frost had fun designing his latest Milwaukee mural Monday and Tuesday. He used a unique kind of frost to cover the tree branches, facades and lamp posts. 

The frost, known as rime ice, only forms in specific conditions and it has its own classification because of this. Water droplets collect on surfaces in foggy areas and take the shape of a familiar crystalline winter wonder: frost. The water droplets from the fog freeze when they touch surfaces at or below freezing temperatures.

The two kinds of rime ice are hard rime and soft rime. Hard rime forms with a heavy wind and the rime is harder and more compact when compared to soft rime. Soft rime is far more delicate and fuzzier than hard rime. There are other ways for frost to form, like hoarfrost and glaze. Rime ice is denser than hoarfrost, but it is less heavy. Glaze is even more dense than rime ice.



Glaze forms in similar conditions to rime ice. Both form in the fog, and when waterdrops touch a surface at or below freezing temperature. Glaze builds up in a harder and more transparent sheet while rime ice forms into visible crystals. Sometimes air bubbles form under the surface creating small imperfections.

Hoarfrost forms under separate conditions entirely. It accumulates in clear weather and the water droplets in the air skip turning into a liquid and move right to a solid state when they touch a freezing surface. Despite only forming in clear conditions the air still needs to have a lot of water for hoarfrost to thrive.

Fern frost forms in the same way as hoarfrost, in moist air under clear conditions where the droplets freeze instantly on a surface. We all know this type of frost from our windows on chilly days and the windshields of our cars. Scraping off that bottom layer of ice early in the morning started off as fern frost. What is unique about this formation is the shape. Each design is unique to the glass and surface it finds its home upon. Variations and imperfections in the surface such as scratches, cracks, dust, and more change how the water droplets freeze in the fern-like pattern.

While at times frost is be pesky, it can also be beautiful.