What Causes Fog?
QUICK ANSWER
Fog forms when air cools to its dew point, causing water vapor to condense into tiny suspended droplets at ground level. The main types are radiation fog (overnight ground cooling), advection fog (warm air over cool surfaces), and steam fog (cold air over warm water). Fog is essentially a low cloud.
Fog is one of the most common atmospheric phenomena, occurring almost everywhere on Earth at one time or another. It's essentially the same as a cloud, but at ground level where it affects visibility and travel. The mechanisms that produce fog are well understood and explain why some regions are notably foggy while others rarely see fog. Understanding fog reveals connections between temperature, humidity, and weather.
What is fog actually made of?
Fog is made of tiny water droplets suspended in air, the same composition as low clouds. The droplets form when water vapor condenses around microscopic particles like dust, salt, or pollution. Fog droplets are typically 1-20 micrometers across, smaller than most cloud droplets and far smaller than raindrops. There can be hundreds of thousands of droplets per cubic foot in dense fog. The droplets scatter light in all directions, which is why fog reduces visibility: the light from distant objects is scattered before reaching your eyes.
What is radiation fog?
Radiation fog is the most common type, forming overnight when the ground cools by radiating heat to space. As the ground cools, it cools the air directly above it. If the air is humid enough, it can cool below its dew point, causing water vapor to condense as fog. Radiation fog typically forms in valleys and low areas where cool air pools. It's most common on clear, calm nights when radiation cooling is strongest. It usually burns off in the morning as the sun heats the ground and the air.
What is advection fog?
Advection fog forms when warm moist air moves horizontally over a cooler surface. The cooler surface chills the air from below, eventually cooling it below its dew point and producing fog. Famous examples include the fog that frequently blankets San Francisco when warm Pacific air flows over the cold California current, and the fog that forms over the Grand Banks where warm Gulf Stream air meets cold Labrador current waters. Advection fog can persist for hours or days as long as warm moist air keeps flowing over the cool surface.
Why is some fog dangerous?
Fog is dangerous primarily because it reduces visibility on roads, in airports, and at sea. Dense fog can reduce visibility to a few feet, making safe driving and aviation nearly impossible. Multi-car pileups in sudden fog banks have killed hundreds of people in various incidents. Maritime accidents from fog are also common. Coastal regions, river valleys, and areas with frequent advection fog have specific weather warning systems and traffic management protocols. Some fog (freezing fog) deposits ice on surfaces, adding additional hazards beyond visibility reduction.
Fog forms when air cools to its dew point, causing water vapor to condense into suspended droplets at ground level. The main types include radiation fog (from overnight cooling), advection fog (warm air over cool surfaces), and steam fog (cold air over warm water). Essentially a ground-level cloud, fog reduces visibility and can be dangerous for travel.
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