Fog happens when the air near the ground cools enough that the water vapor in it turns into tiny droplets you can see. Those droplets float in the air like a cloud sitting on the ground.
Basic cause
Fog forms when the air temperature drops to (or very near) the dew point, meaning the air is holding as much moisture as it can and the extra condenses into visible droplets. This often happens at night or early morning when the ground cools quickly and chills the air just above it.
Common local reasons
Typical local triggers include:
- Clear, calm nights that let the ground and nearby air cool rapidly, creating radiation fog in the morning.
- Moist air moving over a cooler surface, like cold land or water, which cools the air to its dew point and makes advection fog.
Why it clears later
As the sun comes up and warms the ground and air, the air can hold more water vapor again, so the droplets evaporate and the fog thins or disappears. A bit more wind can also mix the air and break up the fog layer.
