How hoarfrost forms

One of the most beautiful times during winter is when frost builds up on tree branches, blades of grass and fences.

The whole world takes on a bright, shimmering white hue, and it makes the bitter temperatures worth it. You might even call it a winter wonderland.

Photo of hoarfrost

This type of frost is called hoarfrost and can look like white feathers growing on a variety of surfaces.

It starts with water vapor or fog — the gaseous version of water — hovering over the ground the is at a maximum of 32 degrees. Because water freezes at 32 degrees or below, the water molecules in the air will come into contact with the freezing ground, a freezing branch, a freezing blade of big bluestem, a freezing metal arbor and will immediately turn from gas into a solid state as an ice crystal.

(Are all snowflakes unique?)

The more moisture that is in the air, the more the ice crystals will interlock on freezing surfaces and will build larger and larger into those feather-like frosty structures that we see in the beautiful outdoors. These large accumulations of frost is what is called hoarfrost.

Photo of hoarfrost

If there is not enough moisture in the air, hoarfrost will not occur.

Do you know why it’s called hoarfrost?

Hoar comes from an Old English word meaning gray, venerable or old, and it was put with this kind of frost because it apparently made people think of an old man’s beard. Hoary is still used as a term for something that is gray or white with age. Other things in nature are also called hoary, like hoary vervain. This purple flower has a leaf that is covered in whitish hairs.

(Butterflies and moths love vervain!)

Photo of hoarfrost


1 Comment

  1. Michele on December 11, 2018 at 1:36 pm

    Thank you! It is so beautiful, and now I know the rest of the story. It is a great trivia question and answer.

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