- Fog forms when the air near the ground becomes saturated
and condensation occurs on tiny particles suspended in the air.
- Fog forms in stable air with light winds, high relative
humidity and conditions which bring the air to its dew point.
There are basically three ways to bring the air to its dew
point:
- by cooling the air
- by evaporating water in to the air and
- by mixing of air parcels.
Fog Formed by Cooling the air:
Radiation Fog is produced at night or in the early
morning when radiational cooling at the earth's surface cools the air to the dew point
temperature. The resulting layer of fog is normally only a few hundred
feet thick.
Ground fog (layer of fog is less than 20 feet)
and valley fog (that forms in low lying areas) are types of radiation fog.
The ideal conditions for radiation fog to develop are:
- clear skies
- light winds ( 2 to 12 knots)
- winds less than 2 knots will result in dew or frost ( if the
surface is below freezing) forming on the ground
- winds greater than 12 kts will result in mixing and will
prevent the fog from forming
- a shallow layer of moist air near the ground, and
- long nights (allowing more time for cooling)
Radiation fog is common over land in late fall and winter.
Advection Fog forms when wind moves warm, moist air over a cooler
surface. The cold surface cools the warmer air and lowers it to its dew point.
Condensation occurs, resulting in the formation of a layer of fog.
- The layer of fog will deepen as wind speed increases up to
about 15 knots.
- With winds stronger than 15 kts the fog will lift as
stratus cloud.
This type of fog persists unless there is a change in
the air mass or the wind direction.
Advection fog is common along the Pacific Coast since
surface water along the coast is much colder than the surface water farther
offshore. As warm, moist air from the Pacific Ocean moves over the colder
coastal waters it is chilled from below , lowering it to its dew point forming fog.
Upslope Fog is formed as moist air flows up an
elevated plain, hill or mountain. As the air moves up the slope it is cooled by
expansion. When it is cooled to its dew point temperature condensation occurs and
fog forms.
- Upslope fog often forms with moderate winds and it can
persist for several days until there is a change to a drier air mass or the wind direction
changes.
- Upslope fog commonly forms in winter and spring on the east
side of the Rockies and in the Appalachian and Adirondak mountains.
Fog Formed by Evaporation or Mixing:
Steam Fog is formed when cool air moves over
warm water. Moisture from the warm water evaporates into the cooler air. Since
the air is so much cooler than the water the air quickly reaches its dew point
(becomes saturated) and the added water vapor condenses into fog. Since
the layer of fog is less dense and warmer than the surrounding air, it rises,
resembling steam.
- steam fog often forms when cold air moves over heated
water in an outside swimming pool. Water evaporates from the pool into the colder
air and the air becomes saturated and then condenses. The colder air is heated from below
and rises as steam.
- Steam fog often forms over lakes in autumn or winter or above
a wet surface on a sunny day (after a rain shower).
Evaporation or mixing fog forms by the mixing of two unsaturated air masses.
If the mixing of warm, moist air with cooler air results in saturation
then condensation occurs and fog will form.
- This type of fog often forms ahead of a warm front when
the Relative Humidity of the cold air is raised to saturation by warm rain falling through
it (sometimes called precipitation fog).
- Another example of this process occurs when moist
air from your mouth meets the cold air of the environment and condenses to form a
"cloud".
- Mixing fogs are common in ski areas during times of
rainfall. As rain falls onto the snow it begins to melt. The melting process extracts heat
from the surrounding environment, including the air close to the ground. Fog readily forms
in the cool, rain-saturated air
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All About.......Fog