Photo: Hobart William-Smith

Ever walk to your car on a cool but humid late summer morning and realize that you can’t even see the end of your driveway? Dense, heavy fog smothers everything around you making it even difficult to make out your feet. You might stand in amazement for a couple of seconds, pondering how you will make it to work. Well, as many Rochesterians know fog isn’t just reserved for the summer; in fact different varieties of dense heavy fog can make travel difficult at any time throughout the year.

Before we delve into the numerous types of fog, understanding the basics of this phenomenon is a necessity. Fog is essentially a collection of liquid water droplets or ice crystals suspended in the air just above Earth’s surface. When the air temperature cools to equal the dew point temperature, the air becomes saturated condensing into droplets and creating fog. This is the same process as cloud formation, thus it is fair to say that fog is essentially clouds at the surface. However, there are some differences between clouds and fog, mainly in the ways they are formed. In the upper atmosphere, the air is cooled as it rises, forming a cloud. At the surface the air is cooled in a multitude of ways, creating the many types of fog.

Anyone who lives on or near any of Upstate New York’s lakes knows that throughout the winter, steam sometimes appears to come off the lake. During the early morning, very cool air will tend to move over a warmer, moist body of water. When the cool air mixes with the warm moist air directly over the water, the moist air cools until it becomes saturated and fog forms. The following is a video of steam fog over Lake Ontario during the winter of 2005. Fog like this is common over Lake Ontario and many of the Finger Lakes throughout the winter.

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As late summer approaches and fall is imminent, another type of fog called radiation fog is pretty common in Western NY. Radiation fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is released into the atmosphere. As the earth’s surface continues to cool, the air will then become saturated and dense fog will form if enough moisture is present. You might see this fog in the early morning before the sun heats the surface.

Sometimes water droplets that compose fog are supercooled, or in a liquid form at temperatures below freeing.  This fog is termed “freezing fog”.  These water droplets remain in the liquid state until they come into contact with a surface upon which they freeze. As a result, any object the freezing fog comes into contact with will become coated with ice.

Rochesterians should always be on the lookout for different types of fog as upstate NY is a unique region where this phenomenon is prevalent.