What is Pictured? Hoarfrost or Rime Ice?

While winter may cause people in the north to endure exceptionally tough conditions at times, this season also provides an untouchable beauty. After a night of temperatures that reached below 0F, I quickly fed some frosty animals on my farm and tried to ignore the biting sting in my fingers; then, I stopped in awe! I hadn’t noticed the astonishing wonderland that surrounded me. Throughout the night, crystals had formed all over the farm. The trees were caked in sparkling beauty and there were fan-shaped crystals covering sticks and objects on the ground below. As painful as my fingers were, I took out my camera and took some pictures of these crystals and the stunning white paradise.

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How do these crystals form and what causes them?

I figured these crystals were some kind of frost but I had to do some research to be sure!

From what I read, the crystals pictured are either hoarfrost (a type of frost) or rime ice.

Hoarfrost (also known as radiation frost) forms when the water in the air freezes when it contacts cold objects. The water exists as a gas in the air and as it freezes it changes from a gas to a solid (also known as desublimation).

Rime ice is formed when liquid water in the air freezes upon contact with other objects. Usually this is in the form of fog and is the water changing from a liquid to a solid. The water in air needs to be warmer than 32F and usually one cannot clearly see the crystalline structure.

Now that you have a little background of the differences between hoarfrost and rime ice, I will provide the conditions surrounding the formation of the crystals pictured. Take your best guess if these are hoarfrost or rime ice!

During the day before these crystals formed, the temperature reached 35F. That night and early the next morning temperatures fell to -1F. There was hardly any wind (2 – 5 mph). No fog was observed.

Are you ready to take your best guess at which formation this is?

Drumroll please….

I believe it is….

Hoarfrost!

The crystals are most likely hoarfrost. Water that was in the air as a gas cooled so quickly that it skipped condensing into a liquid and froze directly from the air (desublimation). This created the appearance that the ice crystals grew and fanned outwardly. In addition, no fog was observed and the temperature was too cold for fog to be present which means that the water that formed the crystals needed to be in a gaseous state before freezing.

This is my best guess but I would love to hear your thoughts / ideas! Please comment!

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