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  • Sara Thompson

Seeing Fog More Clearly

By Sara Thompson

Special to the Enterprise

the South tower of the Golden Gate Bridge in San Francisco, CA, emerging from fog at sunset.
The South tower of the Golden Gate Bridge in San Francisco, CA, emerging from fog at sunset. Photo by Brocken Inaglory

Recently our area has been enveloped in fog for most of the mornings and sometimes into the afternoon. Fog is comprised of water droplets suspended in the air, basically a cloud at ground level. Although common, especially in our area in wintertime, what are the exact conditions that cause this low hanging cloud?

Several conditions can cause fog, but the two main factors that cause it to form are temperature and dew point. The dew point is the temperature air needs to be cooled to for the water in the air to condense and form dew. When the difference between the air temperature and the dew point is less than 4oF, fog will have the best conditions to form. Fog also often forms when relative humidity, which is the amount of evaporated water in the air, is near 100%. Fog can still form at lower relative humidity, but the more moisture in the air means more water to condense with the dew point. If conditions are right, a fog can drizzle like a rain cloud if the fog is cooled or compressed by surrounding air. The water droplets in the fog will combine into larger drops and fall like rain.

There are several main types of fog, that are distinguished by how and where they form, thickness, and temperature. Advection fog is when moist air passes over a much cooler surface and is cooled down, most common on coasts where warm air connects with cooler water temperature, such as around San Francisco. Evaporation fog, also known as steam fog, forms over smaller bodies of water, such as lakes, and can become very dense. Freezing fog is in areas where the air temperature drops to freezing or below and can cause fog droplets to freeze to surfaces. A frontal or precipitation fog can form in front of a stormfront where the falling rain is relatively warm and evaporates as it falls towards the cooler ground air, a frontal fog can also be a low hanging stratus cloud. Ground fog forms close to the ground and can be measured in a few inches in some areas, ground fog doesn’t obscure the sky as much as others. Hail fog can form in places of large hail accumulations due to the lower temperatures, but higher moisture. Ice fog occurs in very low temperatures and can create halos in the sky. Radiation fog is the most common type of fog, formed by the cooling of the ground after the sun sets and causes the air temperature to drop and reach the dew point. Upslope fog forms when moist air is traveling up a slope and condenses into a fog.

Common in winter when air temperature is cold during the day as well as the night, fog usually dissipates when the air temperature warms up with the sunrise. When in a fog, take caution as visibility is often reduced for you and to all of those around you.


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