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How does fog form?

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Quick answer

Fog forms when air near the ground cools below its dew point, the temperature at which invisible water vapour turns into liquid. Tiny droplets then condense onto dust or salt particles and stay suspended, scattering light and cutting visibility to under one kilometre.

Fog forms when air near the ground cools to its dew point, the temperature at which water vapour condenses into tiny droplets. These droplets stay suspended in the air, reducing visibility to less than one kilometre.

Step by step

  1. Moist air sits close to the ground, holding invisible water vapour.
  2. The ground and the air above it cool, often overnight or over a cold surface.
  3. The air reaches its dew point, the temperature at which it cannot hold all its vapour.
  4. Vapour condenses onto tiny dust, salt or smoke particles called condensation nuclei.
  5. Millions of microscopic droplets form and remain suspended in the still air.
  6. The droplets scatter light, and visibility drops below one kilometre: this is fog.

Key things to understand

  1. Air contains invisible water vapour, and warmer air can hold more of it than cooler air.
  2. When the air cools to its dew point, the vapour can no longer stay gaseous and begins to condense.
  3. Condensation needs tiny particles called condensation nuclei, such as dust, salt or smoke.
  4. Water molecules gather on these nuclei and form microscopic droplets that float in the air.
  5. If the air stays cool and calm, these droplets remain suspended, and we see them as fog.

Labelled diagram

  1. Moist air

    Air near the ground holds invisible water vapour.

  2. Cooling

    The ground loses heat, cooling the air above it.

  3. Dew point

    Air reaches the temperature where vapour must condense.

  4. Nuclei

    Dust, salt or smoke particles give droplets a surface.

  5. Droplets

    Microscopic water droplets form and stay suspended.

  6. Fog

    Droplets scatter light, cutting visibility below one kilometre.

How fog forms near the ground

Real-world examples

  • San Francisco's summer fog forms when warm, moist Pacific air moves over cold coastal water.
  • In Delhi, winter fog often mixes with smoke and dust, creating dense smog that delays flights.
  • The Grand Banks off Newfoundland is one of the foggiest places on Earth, where the cold Labrador Current meets warm Gulf Stream air.
  • Valley fog appears in mountain basins such as California's Central Valley when cold air sinks and traps moisture overnight.

Common mistake

✗ Myth: Fog is smoke or a cloud that has fallen to the ground.

✓ Fact: Fog is a cloud that forms at ground level, made of water droplets, not smoke. It does not fall; it condenses in place when air near the surface cools to its dew point.

People also ask

What causes fog?
Fog is caused by air near the ground cooling to its dew point, the temperature at which water vapour condenses into droplets. The droplets need tiny particles such as dust or salt to form on. If the air stays calm, these droplets remain suspended and reduce visibility.
Why is fog more common in winter?
Winter nights are long and the ground cools quickly, so air near the surface often reaches its dew point before morning. Cold air also holds less water vapour than warm air. These conditions make condensation near the ground more likely, which is why winter mornings are often foggy.
How does fog disappear?
Fog disappears when the air warms above its dew point or when the droplets evaporate. Sunlight heating the ground warms the air from below, turning droplets back into invisible vapour. Wind can also mix drier air into the foggy layer and clear it.
What is the difference between fog and mist?
Fog and mist are the same phenomenon, separated only by how far you can see through them. Meteorologists call it fog when visibility drops below one kilometre, and mist when visibility is between one and two kilometres.
Why does fog often form at night or early morning?
The ground loses heat to space after sunset, so the air just above it cools fastest. If that air reaches its dew point before sunrise, condensation begins and fog appears. Sunlight later warms the ground and usually burns the fog away.
Can fog form without cold air?
Yes. Warm, moist air can also become fog when it moves over a cold surface, such as a chilly sea or lake. The surface cools the air from below until it reaches its dew point, producing what is called advection fog.
Why is fog sometimes thick enough to stop traffic?
Each droplet scatters light in many directions, so distant objects blur and fade. When millions of droplets fill a shallow layer of air, visibility can fall to a few dozen metres, making driving and flying dangerous.

Part of the guide: Water & Weather Phenomena →