Why Does Valley Fog Occur?

River Valley Fog

Fog in the river valleys is a common occurence across the region, but moreso during the late summer and fall months. Nights get longer, temperatures get cooler, and warm rivers exchange some moisture with the colder air right above it. The fog can be spectacular when viewed from above as fog "fills" the valleys while ridge-tops remain clear and sunny. These higher points will stick out like islands in this sea of white fog.

The recipe for a widespread valley fog event, or one that spreads the fog into even the most subtle river valleys, consists of:

Light winds through the lowest few thousand feet above the ground during the overnight hours.
Recent rainfall to enhance moisture available for fog making.
Clear skies overnight.
Soil moisture that is near normal or above normal

Below is a satellite picture of the region taken at 7:32 am August 23rd, 2009. Note the fog (the brighter grey) had developed into the tributaries of the Wisconsin and Mississippi River Valleys. This gives the fog a "vein"-like appearance from the satellite perspective. A large fog bank was located over central Wisconsin from Medford to Wautoma, and southeast toward Madison.

satellite fog image

Below is a time lapse animated image from a fog event in September of 2009, showing just how quickly valley fog can dissipate. The time span is only 1 hour. Click image to make larger.