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Hoar Frost

12-4-14  hoar frost on black-eyed susan 048Frost is to dew as snowflakes are to raindrops. When water vapor condenses into liquid water, you get dew and raindrops. When water vapor condenses directly into ice, then you get frost and snowflakes. Frost is not frozen dew and likewise, snowflakes are not frozen raindrops.

When frost forms as minute ice crystals covering the ground, we just call it frost. But sometimes the frost grains grow larger and are called hoar frost crystals. Under clear frosty nights in winter, especially when there is a source of water vapor nearby, such as an unfrozen lake or stream, soft ice crystals might form on vegetation or any object that has been chilled below the freezing point by radiation cooling. These deposits of ice crystals are hoar frost. The interlocking ice crystals become attached to branches of trees and shrubs, as well as vegetation on the ground and any other object below freezing temperature that is exposed to supersaturated air (the relative humidity is greater than 100%). Hoar frost often vanishes once the sun has risen and warmed the surface of branches, grasses, etc., so it’s most easily observed in the early morning. (Photo: hoar frost on black-eyed Susan seedhead.)

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16 responses

  1. Al

    Much like what you see around holes in the ground in moist areas, or if there is an animal inside (such as around the vent hole at the top of an active beaver lodge).

    December 9, 2014 at 1:41 pm

    • Exactly, Al. Warm, humid breath produces exactly what you describe (to say nothing of the evaporating water from a beaver’s coat).

      December 9, 2014 at 4:01 pm

  2. Kathie Fiveash

    A hoar-frosted cattail marsh backlit by the rising sun is one of nature’s most beautiful gifts.

    December 9, 2014 at 1:45 pm

  3. Bill On The Hill...

    I have seen this unusual ice or frost condition 100’s if not 1000’s of times living here in New England my entire life.
    Very well explained as to what is actually occurring.

    December 9, 2014 at 1:48 pm

  4. Elizabeth

    How does hoar frost differ from rime ice?

    December 9, 2014 at 2:56 pm

  5. Susan Holland

    I love this photograph!!!! We get a lot of hoar frost at the island in Ontario and it always seems magical. (Except when it’s on the rim of the outhouse seat!)

    December 9, 2014 at 3:57 pm

  6. Jenny Sawyer


    December 9, 2014 at 4:31 pm

    • You are so very, very welcome, Jenny. Thank you for your support!

      December 9, 2014 at 5:00 pm

  7. “Hoar” comes from old Germanic languages meaning gray, venerable, old. Thought that might help prevent misunderstandings! The word “herr”, used for addressing men, is from that same root.

    December 9, 2014 at 5:00 pm

    • Thanks for the clarification, Helen! My mind is so narrowly focused on natural history the double meaning never occurred to me!

      December 9, 2014 at 5:01 pm

  8. Middle English rim, from Old English hrīm; akin to Old Norse hrīm frost

    (Can’t help myself; love language AND nature.)

    December 9, 2014 at 5:08 pm

  9. Forgot to give due credit.

    December 9, 2014 at 5:09 pm

  10. Ben Freeman

    Thank you for all you do to inspire curiosity and wonder. I took this photo and shared with my students yesterday, then laughed out loud when I read your post this morning!

    Thank you. Keep up the wonderful work.

    Ben Freeman Burr and Burton Academy Mountain Campus Director 802-824-4846

    Mountain Campus blog:

    December 9, 2014 at 8:17 pm

    • Thanks so much, Ben. Unfortunately, WordPress doesn’t allow photos to come through. If you have the time to send it to me at, I’d love to see it. Thank you so much.

      December 9, 2014 at 8:35 pm

  11. Well-explained, Mary. Hoarfrost is a delightful sight. Like walking through a sparkling, crystal fairyland!

    December 11, 2014 at 2:36 am

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