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Name: Benjie
Status: other
Grade: 12+
Location: SC
Country: USA

I would like to know why wind frost, or advection frost, forms against the direction of the wind and what are the physical processes behind this formation?

I am not quite clear what you mean when you write: “…frost, forms against the direction of the wind…”

I feel like I am in a fog (or possibly a frosty situation). Condensation (fog) occurs when water from a gaseous state cools to a liquid state. Freezing (or frost) occurs when water in a liquid state cools (freezes) to a solid state.

There are a number of ways that fog can form, yet if the ambient temperature is below zero C, then frost can form. Deposition (frost) occurs when water in a gaseous state cools (freezes) to a solid state. Fog and frost form through heat and moisture exchanges between the ground (or water body) and overlying air. When the relative humidity of a parcel of air reaches 100%, then fog (or frost) may form. Cooling air, or adding moisture to air, will increase the relative humidity.

Here are some explanations of how fog may form through cooling or adding moisture to a parcel of air.

Radiation Fog:

Earth cools rapidly at night. The air cools, relative humidity reaches 100%, and fog forms. Common over marshes (high moisture content).

Advection Fog:

Advection means horizontal movement of air, where warm moist air is blown over a cool surface. The air cools, relative humidity reaches 100%, and fog forms. Common in early spring as air moves over snow covered land or, cold ocean, or lake.

Upslope Fog:

Air undergoes expansional cooling associated with physical upslope movement. The air cools, relative humidity reaches 100%, and fog forms. Common with humid air on hillsides or mountain slopes.

Evaporation Fog:

Also known as steam fog or Arctic sea smoke. Cold dry air moves over warm water, thus increasing the vapor content of the air mass. Moisture is added, relative humidity reaches 100%, and fog forms. Common in lakes, heated swimming pools, and highways.

Frontal Fog:

Lower density, warm and moist air, is lifted over colder air, rain (moisture) falls into the colder layer increasing the vapor content of the air mass. Moisture is added, relative humidity reaches 100%, and fog forms. Common when warm fronts approach.

When you state that “…frost, forms against the direction of the wind…” you might consider that the frosty surface was cold enough to cool the temperature of the wind. The air cools, relative humidity reaches 100%, and frost forms.

I hope this helps. You might be interested in learning more about the role of relative humidity, energy transfer, and change of state.


N.B.: Sublimation is the opposite of deposition (and is quite common with ice in a freezer).

Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences
Johnson State College


The formation processes of all frost, aside from rime, are essentially the same. Water vapor in the air freezes on a cold surface to produce crystals of ice, called frost. Rime can more properly be thought of as icing, as it usually deposits as a liquid (from water droplets) and then freezes on the cold surface.

Advection frost and radiation frost form in essentially the same way. However, the way in which the air is cooled to produce the conditions needed for the frost to form are different. Radiation frost occurs when, with little or no wind, the surface on which the frost forms is cooled to below the frostpoint by radiative cooling. Advection (or wind) frost occurs when cold air moves into or drains into an area and results in the temperature of surfaces being lowered to below the frostpoint.

Both radiation and advection frosts exhibit crystalline structure, including ice spikes or needles. Radiation frost is often referred to as hoar frost, but both radiation and advection frosts can look the same.

In the case of advection frost, the crystals of ice may extend towards the direction of the cold air blowing into the area, just as any crystal begins at its base and extends outward from it to the source of the "mineral" (in this case water vapor) that is forming the crystal.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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