Fog happens when the air near the ground cools enough, and has enough moisture, that water vapor turns into tiny suspended droplets, making a low cloud you can see.

What fog actually is

Fog is basically a cloud at ground level made of countless tiny liquid water droplets (or, in very cold conditions, ice crystals) floating in the air.

It reduces visibility because those droplets scatter light in all directions, which is why distant objects look faded or disappear completely.

The key ingredients

Fog usually needs two things to form.

  • Air that is moist enough (high humidity, lots of water vapor present).
  • Cooling of that air until it reaches its “dew point,” the temperature where water vapor condenses into droplets.

When the air temperature and dew point are very close (typically within about 2–3 °C), condensation starts and fog can form.

How the air gets cooled

There are several common ways nature cools air enough to make fog.

  • Clear, calm nights: The ground radiates heat into space after sunset, cooling the air just above it and creating “radiation fog,” common in autumn and winter mornings.
  • Warm air moving over a cold surface: Along coasts, warm, moist air drifting over cooler land or water cools down and makes “advection fog.”
  • Cold air in valleys: Heavy, cool air can settle into low-lying valleys, trapping moisture and creating “valley fog” that can linger.
  • Air pushed up slopes: When moist air is forced up hills or mountains, it cools as it rises, producing “upslope fog.”
  • Cold air over warmer water: Over lakes or ponds on chilly days, evaporation from the warmer water adds moisture to cold air until fog forms, sometimes called “evaporation fog” or “steam fog.”

Why fog appears and disappears

Fog is most common late at night and around sunrise because that is when the ground and the air just above it are coolest.

Once the sun climbs higher and warms the ground, the air temperature rises above the dew point, droplets evaporate back into invisible water vapor, and the fog “burns off.”

Wind also matters: a gentle breeze can help form fog by mixing moist layers, but strong winds can break it up or turn it into low cloud instead.