Fog forms when the air becomes saturated with water vapor, causing the water vapor to condense into tiny water droplets that hang in the air close to the ground. This saturation can happen in a couple of ways:
- Cooling the air: When warm, moist air cools, it can no longer hold as much moisture. This excess moisture condenses into fog. One way this happens is through radiation fog, which forms on clear nights as the ground cools and chills the air above it.
- Adding moisture to the air: If warm, moist air moves over a cooler surface, such as a lake or ocean, the air near the surface cools and can cause fog to form. This is known as advection fog. Another way moisture can be added to the air is through evaporation fog, where rain falling through cold air evaporates, adding moisture and raising the dew point, leading to fog formation.
Essentially, fog is a low-lying cloud that forms when the air becomes saturated with water vapor. The formation of fog depends on the presence of moisture in the air and a mechanism to cool the air to its dew point, the temperature at which it becomes saturated.
