Dense fog forms when very moist air near the ground cools enough that water vapor condenses into many tiny droplets, making visibility very low.
Basic requirement
Fog develops when the air temperature drops to (or very near) the dew point, so relative humidity is close to 100% and water vapor starts condensing into suspended droplets. Dense fog simply means this cloud at ground level is thick enough that visibility falls below about 1,000 meters, often much less.
Main causes of dense fog
- Strong cooling of the ground at night (on clear, calm nights) chills the air just above it, producing “radiation fog,” which can become very dense in valleys and low-lying areas. Long autumn and winter nights allow more cooling time, often leading to thicker fogs.
- Warm, moist air moving over a colder surface (land, ocean, snow, or ice) produces “advection fog,” which can blanket large regions and become quite dense, especially along coasts.
- Extra moisture added near the surface—such as from evaporation off warmer water into cold air, light rain falling into cool air, or urban pollution providing many particles—can quickly saturate the air and thicken existing fog.
Conditions that make it thicker or longer-lasting
- Light winds help fog spread out into a deeper, uniform layer, while stronger winds tend to mix the air and reduce fog.
- Temperature inversions (cold air trapped near the ground under warmer air aloft) can trap moisture and pollutants, keeping dense fog in place for many hours or days, especially in valleys.
- High soil or surface moisture, wetlands, and nearby water bodies all supply extra humidity to the lowest layers of air, making dense fog more likely.
