KMSP - A Fox9 viewer captures time lapse footage of fog moving around just before sunrise Thursday. Christine from Buffalo captures radiation fog waxing and waning with each passing breath of wind that edges it in one direction or another.
(picture courtesy of Environment Canada)
Radiation fog happens when the Earth radiates enough heat that the air near the surface cools enough to reach the dewpoint. The laws of the atmosphere don’t allow the temperature to drop below the dewpoint, which is the point at which dew forms. So, in order for the temperature to continue to cool, moisture has to be released out of the air. This will form dew on grassy surfaces AND can cause fog to form under the right conditions; clear skies and very light winds. The fact that this happened over a body of water is very common because you need plenty of moisture around for this to occur.