Fog
Valley Fog Explained: Why Low Spots Fill With Fog Overnight
How valley fog forms, why cold air drains downhill, and why fog can be patchy and persistent in low terrain.
Quick answer: Valley fog forms when cool dense air settles into low areas and moisture condenses near the ground.
Cold air drainage
At night, slopes cool and dense air drains into valleys.
If moisture is present and winds are light, fog can form in the low spots.
Why it lingers
Valleys can stay shaded and stable longer than surrounding hills.
Fog may burn off late or persist when nearby higher ground is clear.
Travel hazards
Visibility can change quickly when roads dip into valleys.
Use low beams, slow down, and allow extra distance.