Wind or Advection Frost Formation
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.
Earth cools rapidly at night. The air cools, relative humidity reaches
100%, and fog forms. Common over marshes (high moisture content).
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.
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.
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.
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
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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Update: June 2012