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Fog Defined: Low-level clouds can cause major impacts

Weather Glossary

Fog is as simple as a cloud at the surface, yet it can rapidly reduce visibility and cause accidents. Frozen fog can build on vehicles and surfaces. We get fog as the air becomes completely saturated at the surface. There are several ways this can happen. Have you seen fog recently? Do you have any pictures? Feel free to share them by tagging our News 8 meteorologists on twitter.

Advection Fog: Warm, moist air moves over a cold surface. This is most commonly seen around San Francisco as warmer air moves over the cold waters of the bay. In that case it can also be called steam fog. This fog can cause delays of flights in and out of the San Francisco airport.

Courtesy: NWS

Radiation Fog: This happens from radiational cooling, a phenomena that happens under clear skies, light winds, and can get a boost from a light snow pack. Temperatures fall until they hit the dew point, water droplets condense and fog is formed. This is one of the more common ones we see on a cool Rochester morning.

Courtesy: NWS

Steam Fog: This happens when very cold air moves over a warmer body of water and water vapor from that warm source in the air condenses and forms fog. This can often happen over the Finger Lakes in fall and early winter.

Upslope Fog: Cooler air can pool at the base of valleys and drop the temperature down to the saturation or dew point. If there is enough water vapor already in place, fog can form. This can create difficult driving conditions as visibility can rapidly deteriorate over a short distance.

Dense Fog Advisory: This is issued by the local NWS Office (in Rochester’s case this would be NWS Buffalo office). It will be issued when there is dense fog over a relatively large area that will reduce visibility down to a quarter mile or less. That will inherently cause concern when driving.

Freezing Fog Advisory: This is issued when there is fog in place and temperatures are below freezing. The fog can freeze on contact and cause slick conditions as well as the already reduced visibility.

  • Meteorologist James Gilbert

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