Most everyone has heard of or has seen some type of fog, but not many know how it forms, or much less that there are different types. In fact, there are seven different types of fog, with major differences in the way they form. For those who don’t know what fog is, it’s a thick cloud of tiny water droplets suspended in the atmosphere near the earth’s surface, restricting visibility. It can be influenced by a variety of reasons including topography, temperature, saturation levels/precipitation patterns, etc. Fog is most commonly seen during the early morning or evening hours but can also be seen overnight or during the middle of the day depending upon the atmospheric conditions at hand.
Ground fog, also known as radiation fog, is the most common type of fog that people experience. This fog is formed generally after sunset, when the Earth is beginning the cool down process and starting to emit the radiation that was absorbed during the day from the Sun. As this process begins, the air layer immediately above the surface cools rapidly and falls below the dewpoint temperature, which means the air has reached its maximum amount of water vapor that it can hold, and the water vapor is now able to be physically seen. This fog will typically dissipate after sunrise when the earth begins absorbing radiation again.
Valley fog is similar to ground fog except the duration of it is much longer. Valley fog forms in a valley as the air cools above the ground beneath it. However, what makes this last so much longer than regular ground fog is that the air gets trapped by the less dense layer above it-- Potentially extending its lifespan to last for days or even weeks.
Evaporation fog is the type typically seen over water and occurs when the cool air right above the surface is heated by the water below it, thus causing that layer of air to be warmer than the air further above it. As the layer of air warms, it begins to rise and mixes with the cooler air above it and, in turn, causes the water vapor in the air to condense to form the fog.
Upslope fog is formed when a warm moist air mass is blown up the windward side of the mountain and begins to cool as it gets higher in elevation. As it begins to cool, it will form fog as the water vapor condenses.
Advection fog is typically seen in areas that border an ocean. This type of fog forms when warm tropical air is blown over the cooler surface of the water and causes condensation by advection. This phenomenon is mostly common on the Pacific Coast of the United States due to the chilly southerly-flowing California Current.
Freezing fog, which was seen in north Mississippi in the fall/early winter of 2018, is formed when the water vapor inside the fog become supercooled due to freezing temperatures and freeze on contact with the surface below it. This kind of fog can be dangerous for reasons other than visibility because it can cause black ice on roadways.
Finally, frozen fog is formed when the air is cooled to temperatures of -40°F or below causing ice crystals to be suspended in the air.
Fog is a very interesting topic. Most people have different schema when they hear about fog. Some relate it with graveyards, some with early morning commutes to work, and even some relate it to lakes. Across the country, the weather patterns that people experience will vary and not everyone will experience every weather pattern, but everyone will experience some sort of fog in their lifetime.
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©2019 Meteorologist Ashley Lennard