ASK THE METEOROLOGIST
“Is Fog Formed From Clouds?”
Samantha G.
It’s the time of the year where morning fog is becoming a staple in the forecast. Fog has many different types, but essentially forms because of a process known as condensation. This is the same process that clouds form by. So technically, fog is a cloud. Fog is a cloud that touches the ground or comes into contact with something on the Earth’s surface.
Air has two temperatures. One is the actual temperature of the air itself, but there is also a dew point temperature. The dew point is the temperature that the air must cool to for it to become saturated and form clouds. Fog needs not only temperatures to fluctuate, but it also needs a moisture source nearby. Clouds can be seen much further from their moisture source because of how cold it is where they are in the atmosphere. This allows them to sustain themselves for further distances.
Fog must have a moisture source nearby, because temperatures fluctuate more at the surface and therefore the further you get from the moisture source, the less fog can contain itself. There are many types of fog. A few of which are common here in Southern Oregon and Northern California. Here are some of the many types of fog:
Advection fog is when fog forms over a body of water and then is moved around by winds. This is the type of fog that San Francisco and Seattle see quite often.
Valley fog is common in the Rogue and Shasta valleys. It is formed when cold air (which is heavier than warm air) sinks into the valley and the warm air overhead traps it in. When the temperatures reach the dew point, fog forms it sits there because the air becomes stagnant and winds can not move in to mix out the fog. This type of fog can last for weeks or months if the right weather patterns occur.
Radiational fog is one of the most common types. This forms on clear nights with calm or light winds. The Earth emits some of the radiation, it receives from the sun, back out into the atmosphere at night. When clouds are present overhead, this radiation gets trapped and doesn’t allow for the radiation to escape as much. This leaves temperatures a little warmer than what they could reach. However, on clear nights, the radiation is free to escape the surface and temperatures fall faster and lower. If they fall down to the dew point, the air condenses and forms fog. The fog will hang around until the sun comes out and heats up the temperatures, or the winds mix the fog out.
Up-slope or Orographic fog is common in the mountains. As air moves up a mountain, it cools and condenses. It forms the fog as the air travels upward. When the fog starts moving down the other side of the mountain, the air warms and evaporates and the fog disappears.
No matter what type of fog that forms, even a little fog can make roadways very difficult to drive on. The best thing to do is to slow down, and keep your low beams/fog lights on. No matter how thick the fog is, do not use your high beams or brights. The fog will actually send the light back into your eyes and can cause further visibility issues.
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