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Fog Formation 101

We've been having some foggy mornings lately that have made for some interesting morning drives. Have you ever wondered what causes this fog to form?

Radiation fog on the trail at Gilbertson- the tiny droplets in fog scatter sunlight.


The most common type of fog we experience in Northeast Iowa is called radiation fog. Radiation, in this case, simply means the cooling process near the ground. This type of fog is most prolific in low-lying areas because cold air is denser than warm air and often drops down to the lowest levels due to gravity. The conditions needed to produce the best radiation fog formation include: light winds, wet ground, high relative humidity in the atmosphere and mostly clear skies.


Winds help circulate and mix the atmosphere. When they are calm, dew (or frost) will often form. Light winds will mix water vapor just a few feet above the surface while also keeping the dew point and temperature together. Too high of wind and there is too much mixing of moisture and the dryer air higher aloft in the sky-diminishing the likelihood of fog formation.


High relative humidity (90% or above) is formed when temperatures and dewpoints remain very close together. High relative humidity helps the water vapor condense quicker into liquid droplets and later fog/clouds.


Clear skies allow for large amounts or radiation to be lost from earth into space which means temperatures cool off the most because clouds aren't there to trap the underlying heat coming off the earth's surface. Clearer skies also means cooling temperatures which have a better chance of meeting a similar dew point.


Look for high ridgelines to get the most sweeping views of the fog coming out of the valleys or head down into Echo Valley State Park on a foggy morning to experience the beauty of sunbeams filtering through the fog with the sound of water rushing over the rocks.

Morning radiation fog coming out of low-lying valleys







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