All About Fog - KCBD NewsChannel 11 Lubbock

All About Fog

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What is fog?

  • In the simplest of terms, fog is a cloud at the earth's surface. Technically, fog is a suspension of small water droplets in the air, reducing horizontal visibility at the earth's surface.
  • Fog is classified into different types, depending on how it forms:
    • radiation fog
    • advection fog
    • steam fog
    • upslope fog
    • precipitation fog

How does fog form?

  • 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 into the air
    • 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.
    • Radiation fog is common over land in late fall and winter.
    • 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, and 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)
  • 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.
    • 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.
    • 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.
  • 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 fogforms 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

Where does fog commonly occur?

  • In the U.S., heavy fog is more prevalent in coastal regions than in the center of the continent. Three major regions stand out as having the most days with heavy fog:
    • Pacific Coast
    • Appalachian Highlands
    • New England

How does fog dissipate?

  • Meteorologists often say that the sun will "burn off" fog as the day progresses. While the sun does not actually burn the fog off it does play a part in getting rid of the fog by disrupting the fog forming process:
    • Some sunlight penetrates the layer of fog and warms the ground.
    • The air near the ground is warmed as well.
    • The relative humidity of the warmed air is lowered (as warm air can hold more moisture than cold air.)
    • The warmer, drier air mixes upwards with the cooler, moister and and the fog is dissipated.
    • It will either dissipate totally or lift into a stratus cloud.

What are some of the hazards associated with fog?

  • Fog often causes poor visibility which can result in:
    • Traffic hazards
    • Aviation hazards - airplanes landing and taking off
    • Boating hazards
    • Interference with outside work, sports, and activities
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