Hoarfrost that has formed at the edge of a pawprint track at the Montgomery County Fairgrounds, taken Dec. 7, 2018.
Hoarfrost that has formed at the edge of a pawprint track at the Montgomery County Fairgrounds, taken Dec. 7, 2018.
We’ve had some interesting weather phenomena the last couple of weeks in Montgomery County. You might have noticed freezing fog, “fancy” frost, or fuzzy snow. Each of these phenomena are caused by a specific set of conditions that we tend to see throughout winter in the Midwest. Here are the details on a few that I have seen recently and another that I look forward to seeing (hopefully) later this year!
Freezing fog is not all that different from fog during the warmer months. It forms when the air near the ground is very humid and calm and is often seen in the early morning (maybe during your commute to work). The water vapor in this humid air begins to condense and form the ground-level cloud we call fog. The major difference, however, between freezing fog and “warm” fog is that the water within freezing fog becomes supercooled, and will instantly freeze to any cold surface that it touches, which produces the next phenomenon…
Rime is what we see when water vapor (usually found within freezing fog) condenses to liquid and then freezes to a surface. It’s the frozen cousin of dew, along with hoarfrost. Rime typically appears as shaggy white spikes clinging to tree branches, and is often found on clear mornings after freezing fog has disappeared. Sometimes, light winds can sculpt rime into interesting shapes before it freezes completely solid.
Hoarfrost is similar to rime, but forms when water vapor goes straight from gas to solid as it touches cold surfaces. Hoarfrost most commonly occurs on cold, calm nights in which the water vapor can freeze into delicate, intricate patterns (“fancy” frost!). Check for hoarfrost after cold, calm nights after a day where the temperature has hovered around 32F (sort of like the conditions we saw last Thursday and Friday). Sometimes, hoarfrost can form directly on the surface of snow. If you look out the window on a clear morning and see snow that looks fuzzy, check for the delicate, fernlike ice patterns formed by hoarfrost.
Diamond dust is unlike the last few phenomena because it forms during very cold, clear conditions. This is one of the most surprising types of frozen winter weather because it is considered a kind of “clear weather precipitation,” popping up in patches under bright sunlight. I remember the first time that I saw diamond dust I was driving on 231 over the CSX rail line south of Linden. Ahead of the car, you could see a cloud of brilliantly sparkling crystals tumbling through the air. I couldn’t see all the way through it either – it was like a little like fog, but dry (and a little spiky if you stuck your hand out the window as you drove through it). This particular day was very cold, during one of those weeks last year where we experienced temps below 0 for many days, and had otherwise been a crystal clear kind of afternoon. If you’ve ever seen a sun halo (a.k.a. sun dog), you’ve seen a condition created by diamond dust!
These weather phenomena are just a few of the things that can make Midwest winters beautiful! While winter driving and snow removal can be a real drag, I hope that the chance of seeing these kinds of weather phenomena can put a little more wonder in your winter this year.
Ashley Adair is the Extension Educator for Agriculture & Natural Resources at Purdue Extension – Montgomery County. She can be reached at (765)-364-6363 or holmes9@purdue.edu.