Valley Fog
iWitness Weather
Valley fog near Idaho City, Idaho, on Oct. 15, 2011. (Credit: iWitness Weather/Booisenana)
Fog forms when air near the ground cools to its dewpoint, allowing cloud droplets to form at ground level.
On clear, calm nights, the coolest air tends to sink into the lowest terrain. Where terrain is hilly or mountainous, this can lead to pronounced differences between valley and hilltop temperatures.
Sometimes, that's all it takes for the valley floor to be cold enough for fog while surrounding higher terrain stays in the clear. In addition, valleys often have rivers or lakes that can contribute just enough moisture to create a fog in the valley while surrounding higher, warmer ground stays clear.
Either way, the results are visually striking, especially when viewed from above. This type of fog often forms in the fall months in the narrow, branched valleys of the Appalachian corridor and the Ozarks, and is frequently visible even on satellite imagery.
(PHOTOS/VIDEO: iWitnessWeather cloud gallery)
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