What Is The "Frost Point"?

Weather Talk

POSTED: Monday, December 23, 2013 - 11:18am

UPDATED: Monday, December 23, 2013 - 11:53am

Humidity, Temperature and Air Pressure All Play A Role In Determining the Frost Point

The atmospheric conditions were right for some frost to form in parts of east El Paso and the Borderland late Friday into Saturday. There were even some early morning snow flurries detected at the airport and in East El Paso. Some spots in Hudspeth and Culbertson counties got 1” to 2” of the white stuff and the southern Sacramento Mountains received 3” to 12” higher up.

I have always loved the beauty of frost covering the grass, a field or your car. Although it is not so fun when it is thick enough to damage crops or cover a road.

I like many, originally thought “frost” was frozen dew. According to the American Heritage Dictionary, “Frost is a deposit of minute ice crystals formed when water vapor condenses at a temperature below freezing”.

Most of the time temperatures must be below freezing for water vapor to develop into frost instead of dew. The frost point is a point when the air becomes completely saturated with water vapor and it begins to precipitate out as frost.

Humidity, temperature, and air pressure all play a role in the determination of the frost point. The frost point is closely related to the dew point, where tiny drops of water or dew begin to form because the air is completely full of water vapor.

Cooler air is less capable of holding evaporated water. When humidity levels are high and temperatures start to drop, the water vapor in the air will start to condense into dew. As long as conditions are below freezing, the condensed water vapor will develop in the form of frost, rather than dew. In some cases the temperature of the surrounding air can be above freezing but there can be pockets of air at or below the freezing mark, allowing frost to form in some isolated spots.

When the frost point is reached, at locations where saturated air comes into contact with objects, a thin layer of frost will form. Ideal weather conditions for frost to form are calm, clear, cold nights when the temperature drops to near or below freezing. Frost typically forms on low-to-the ground objects like grass or shrubs and cold objects such as car roofs and windows because the temperature of those objects is actually colder than the surrounding air.
A light frost may not damage grass or crops, but a heavy frost will kill even the heartiest of plants. Plants near a building or under an eave are less likely to frost over than the same plant out in the open. The frost crystals will remain intact until temperatures start to rise, melting them.

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX
www.twitter.com/ Chuck DeBroder NC9 @wxchuckNC9

(Photo-Chuck DeBroder-KTSM)

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