We woke up to the coldest temperatures so far this season Friday morning in Anchorage. Along with the cold came freezing fog. Visibility dropped below a quarter-mile in areas, making for a slow commute. But as the fog lifted, it left behind something rather magical — rime ice. 

Anchorage from Glenn Alps Friday morning. The city blanketed in fog.

Small amounts of water vapor in the air condensed as temperatures dropped overnight. The condensing water vapor is what forms fog or a cloud on the ground. With temperatures below freezing, those little water droplets are super-cooled, meaning they are colder than 32 degrees, but not frozen. This happens because the water droplets are so pure. Once those pure, super-cooled droplets come in contact with something, they freeze instantly. 

These tiny, frozen water droplets stick to everything: your windshield, railings, roads and of course — trees!

This coating on a lot of the trees in Anchorage is known as rime ice. Although it looks similar to hoarfrost, it is actually different. Hoarfrost is essentially frozen dew. It happens when water vapor condenses onto objects and freezes.

On the other hand, rime ice forms from super-cooled condensed water droplets that freeze onto surfaces. 

Simply put, rime ice is the by-product of freezing fog, while hoarfrost forms without the presence of fog.

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