What Is Fog?

What You'll Learn

• Fog is a cloud that forms close to the ground
• Fog happens when warm, moist air cools down and water vapor turns into tiny liquid droplets
• The tiny droplets are so small and close together that they block your view
• Fog usually forms at night or early morning when the air cools
• Evaporation and condensation are both parts of the water cycle

Words to Know

Fog

A cloud near the ground that makes it hard to see

"We couldn't see the road in the fog."

Water vapor

Invisible water in the air

"Water vapor condensed into fog droplets."

Condense

When water vapor cools and turns into liquid droplets

"Dew condenses on grass in the morning."

Moist

Slightly wet or full of water

"The moist air cooled and made fog."

Check What You Learned

Fun Facts

• Fog is a stratus cloud that forms at or near ground level

  Source: National Weather Service

• Fog forms when water vapor condenses into tiny liquid water droplets suspended in the air

  Source: NOAA

• It typically forms when moist air cools to its dew point, especially at night/early morning

  Source: NWS

• Common types include radiation fog (forms on clear calm nights), advection fog (warm moist air moves over cool surface), and sea fog

  Source: NWS

• Fog reduces visibility to less than 1 kilometer (0.62 miles)

  Source: World Meteorological Organization

• The tiny droplets are about 0.01 mm in diameter and there can be 1 million droplets per cubic meter of fog

  Source: NOAA