What Causes Fog? Types, Formation, and When to Expect It

What Is Fog?

Fog is a cloud that forms at or near ground level, reducing visibility to less than 1 kilometre. It consists of tiny water droplets (or ice crystals in extreme cold) suspended in the air.

Types of Fog

Radiation Fog

Forms on clear, calm nights when the ground radiates heat and cools the air above it to its dew point. Common in valleys and low-lying areas. Burns off after sunrise.

Advection Fog

Forms when warm, moist air moves over a cold surface. Classic example: San Francisco fog, where warm Pacific air meets the cold California Current. Can persist for days.

Upslope Fog

Forms when moist air is pushed up a hillside or mountain slope, cooling as it rises until it reaches saturation.

Evaporation Fog (Steam Fog)

Forms when cool air passes over warm water. The water evaporates into the cold air, immediately condensing. Looks like steam rising from a lake on a cold morning.

Foggiest Places on Earth

Fog and Safety

• Driving: use low-beam headlights (high beams reflect back), reduce speed, increase following distance
• Aviation: fog is the leading weather cause of airport delays and closures
• Maritime: fog horns and radar are essential — historically, fog caused many shipwrecks

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