As the heat radiates way from the mountains overnight, the resulting cool air spills into the valleys and condenses water vapor close to the ground. Note the flat top to the fog, which indicates a temperature inversion above the fog. This valley fog or radiation fog, which is common this time of year through winter, will eventually “burn off” from the bottom up as the sun heats the ground in the valley. That’s if the sun makes an appearance or perhaps it will warm up sufficiently today to evaporate the fog.