Types of Frost

The other day I titled the post of an ice fisherman on Lake Dubay as “Hoar Frost in Central Wisconsin”.  I have since corrected it to be “Rime Frost in Central Wisconsin”.  Let me explain.

Frost is the deposition of ice crystals on cold surfaces caused by the freezing of water vapor.  There are three general types of frost: fern, hoar and rime.

Fern frost is what you notice on your window on a cold winter day. Dew first begins to form on the cold glass and then turns to ice.  More dew accumulates and freezes and the cycle continues forming what looks like feathery fingers. Patterns of ice crystals then occur often forming a fern like appearance.

Delicate hoar frost

Delicate hoar frost

Hoar frost can be seen as very delicate crystals deposited on vegetation. It typically occurs when the  night is clear causing a drop in air temperature to around  32°F.  The ground temperature needs to be much colder.  As the temperature drops, the air holds less water and becomes nearly saturated with water vapor. Water vapor in the air then touches very cold surfaces and freezes on it instantly. Hoar crystals look like spiky fingers.

Rime frost occurs on very cold mornings and represents ice deposition that occurs quickly, often under conditions of heavily saturated air and windy conditions. Typically an ice fog is responsible for the deposition. It really isn’t a frost since it is due to freezing of supercooled water droplets.  Frost is due to water vapor freezing. Rime has an icy solid appearance to it.

The image of the ice fisherman was obtained on a bitterly cold morning with an ice fog, perfect conditions for rime deposition.

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One Response to “Types of Frost”

  1. arnoldtesa Says:

    I have grown up with the terms without ever appreciating the distinctions. Terry

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