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Fog - Windows to the Universe

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This photo shows fog over the ocean and small islands in the San Juan Islands in Washington. The fog formed during cool temperatures in the early morning and is beginning to disappear.
Click on image for full size
Image Courtesy of Amy Hatheway

Fog

Fog is a cloud that touches the ground. Fog usually forms when moist air travels over cold land or water. The moist air cools down and the water vapor condenses and forms a cloud near the Earth's surface. Over land, this happens on clear nights with calm winds. Once the Sun comes up in the morning it warms the top part of the cloud and the fog evaporates. This is called ground fog.

Here are some other ways that fog can form:

  • Valley fog: Fog forms in valleys during the winter when cool air flows down the sides of hills or mountains. During the day, the Sun isn't strong enough to evaporate the fog completely.
  • Advection fog: This type of fog forms when moist air flows over cold ground or water. Once the water vapor in the air condenses, it forms fog.
  • Precipitation fog: This type of fog forms when rain or snow falls into drier air below the cloud and evaporates into water vapor. The water vapor cools the air and adds more moisture to the air, which allows fog to form.
  • Upslope fog: Upslope fog forms when winds blow up the side of a hill or mountain, which cools the air and creates fog. This type of fog happens along large hills and mountains, including the plains east of the Rocky Mountains where the land slopes up towards the mountains.
  • Steam fog: Steam fog forms when cold air blows over warmer water. The water evaporates into the cold air and condenses to form fog. Steam fog often forms in the fall.
Last modified September 18, 2008 by Becca Hatheway.

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