What Causes Fog?

    What is Fog?

    Fog is considered a cloud at the ground level but forms differently compared to a cloud. Radiation fog is a very common type of fog in the United States. Fog usually occurs in the fall and winter months.

    When and How Does Fog Occur?

    Fog usually occurs in the fall and winter months. Fog forms generally overnight as the air near the ground cools and stabilizes. This cooling causes the air to reach saturation which results in fog formation. Fog starts to form at or near the surface of the earth while thickening as the air temperature continues to cool down. The layer of fog starts to deepen overnight as the air temperature above the first layer of fog begins to cool down. While the cooling of the air continues, the fog will extend upward in the air.

    Where Does Fog Occur?

    Fog generally is patchy and likes to stay in one area while dissipating under sunlight. Thicker fog can occur in areas like valleys or calm bodies of water such as a lake or river. Wind flow can disrupt the formation of radiation fog.

    Fog in Winter:

    In the winter months, Freezing Fog can create very dangerous driving conditions. Freezing fog occurs when the water droplets remain in a liquid state until they come into contact with a surface upon which they freeze. This creates a very slippery and icy coating.

    How The Public is Alerted:

    When fog is expected to affect areas in the United States, the National Weather Service will issue an alert depending on the scenario. A Dense Fog Advisory is issued when widespread dense fog develops. Visibility can drop to a quarter of a mile or less which creates difficult travel. In the winter months, a Freezing Fog Advisory can be issued when fog is expected with below-freezing temperatures. When the National Weather Service issues these alerts, the public can be informed by local media, weather apps, or websites such as Weather Buzz. We have a page dedicated to the latest alerts issued by the National Weather Service, click here to visit our alerts page.

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