Definition
Fog formed when moist, stable air is forced upward along sloping terrain. As the air rises, it cools by adiabatic expansion until it reaches its dew point, at which point the water vapor condenses into a layer of fog that clings to the rising ground.
Plain English
Fog that forms when damp air is pushed up a hill or mountainside. As the air rises it cools, and once it cools enough, the moisture in it turns into fog along the slope.
Context Anchor
Seen in weather discussions for hilly or mountainous areas, especially when wind is blowing moist air toward rising terrain.
Derivation
Upslope' simply means 'up the slope.' The name describes exactly how the fog forms — air moving up a slope cools as it rises, and the cooling produces the fog.
Why Pilots Care
It can appear suddenly in mountainous areas, sharply reducing visibility and forcing unplanned instrument flight or route changes where terrain limits options.
Grounding Statement
Picture moist wind blowing across the plains and running into the foothills — as it climbs, it cools, and a low gray fog forms right along the rising ground.
Intuition Check
Upslope fog is not just any fog near a hill. It specifically forms because air is being forced uphill and cooling as it rises.
Example Sentence 1
The pilot delayed departure when upslope fog reduced visibility along the eastern face of the mountains.
Example Sentence 2
With moist air flowing toward the mountains, upslope fog reduced visibility to less than a mile on the approach into the valley airport.