What Are The Mechanisms That Cause Fog?

     Sometimes, fog can seem like it appears out of nowhere. It is not unusual for someone to be driving and have fog suddenly roll in, completely obscuring the driver’s vision. Thankfully, there are ways to predict where and when fog might show up. There are generally four types of fog that all have their own ways of forming. By knowing these mechanisms of fog formation, one can much more easily predict where fog might form. The types of fog are as follows:

1.   Radiation Fog        

This type of fog is the most common in many areas of the United States. All fog occurs as a result of air becoming saturated, which is a fancy word for saying it can’t hold any more water. Generally speaking, the cooler air is, the less water it can hold, so the easier it is to become saturated. Radiation fog happens at night when the air cools down because there is no direct radiation from the sun. When the air cools, it can hold less water, so if there’s already enough air for the water to become saturated, then it’s likely that radiation fog will form.

2.   Advection Fog  

This type of fog happens when warm, moist air rolls over a relatively cool surface. A good example of this type of fog is the fog that occurs under the Golden Gate Bridge. The water under the Bridge is usually cool, while the air the moves over it is typically warmer. The air that moves over the cool water cools down as a result, meaning it can’t hold as much water like in radiation fog. Then, the warm moist air becomes saturated from cooling, which forms the fog. One can expect advection fog near cool water bodies during warm months. Of course, this isn’t the only way advection fog can form, but this is the most common way by which this happens.

3.   Upslope Fog  

This type of fog happens when moist air moves up mountains and cools down. Since it gets colder and colder as you move up mountains, moist air that moves up mountains will cool down. So, like in radiation and advection fog, the air cools down, and it can’t hold as much water. When the air becomes saturated, it’s likely that fog will form. That is why it’s so common to see fog on mountains, especially at night. At night, there can be a combination of radiation and upslope fog, creating much more fog than elsewhere.

4.   Evaporation Fog  

Evaporation fog is unique to the other types of fog in that, instead of lowering the temperature of the air to create fog, it adds moisture to the air. This type of fog occurs when water vapor is added to an air mass that is already close to being full of water. That extra bit of water vapor is enough to push the air to become saturated, forming fog. This type of fog most often comes from cool air moving over a warm body of water, and is often mistaken for steam. One can expect this type of fog most often during autumn, because the water is still warm from the summer, but the air is getting cooler.

These are all the major types of fog that form. To help increase understanding of where fog most often forms, and what type of fogs form where, a map showing the average number of days with fog in the United States has been added below (credit: Mike Marston).

Meteorologists use these different classifications of fog and their causes when determining when and where fog might form. They utilize this information to create fog warnings that can help drivers prepare for dense fog. This information is available on the National Weather Service website, and on most weather apps. It’s always important to be wary of fog, especially when driving. By knowing how fog forms, one can be ready for it when it arrives. 

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© 2019 Weather Forecaster Cole Bristow