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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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