Fog happens when the air near the ground cools enough, and/or gets moist enough, that invisible water vapor turns into millions of tiny liquid water droplets floating in the air.
Basic idea
Fog is basically a cloud on the ground: it forms when air temperature gets very close to the dew point, so water vapor condenses into tiny droplets that hang in the air and block visibility. This usually happens when the air is almost fully saturated with moisture, meaning it cannot hold much more water vapor without some of it turning to liquid.
How fog forms
There are two main ways to reach that foggy state:
- Cooling the air: if warm, moist air is cooled (for example at night when the ground loses heat, or when warm air moves over a colder surface), the temperature can drop to the dew point and fog appears.
- Adding moisture: if the air is already humid and extra water vapor is added (for example from lakes, wet ground, or melting snow), the air can become saturated and fog forms.
Common situations
Typical situations that make fog include:
- Clear, calm nights when the ground cools quickly and chills the air just above it (radiation fog).
- Warm, moist air blowing over colder land or water, often near coasts (advection fog).
- Air being forced up hills or mountains and cooling as it rises (upslope fog).
- Cold air over warmer water so extra vapor is added and condenses, sometimes called steam or evaporation fog.
What makes fog go away
Fog usually thins or disappears when:
- Sunlight warms the ground and the air, raising the temperature above the dew point so droplets evaporate back into invisible vapor.
- Drier or stronger winds mix the air, reducing humidity and breaking up the fog layer.
