When conditions are right, fog can develop. Where the fog forms can be an indicator of what type of weather follows.

What You Need To Know

  • A hollow is defined as a depressed or low part of a surface

  • A hollow can also be a river valley

  • On calm and cool nights, fog can form in hollows

Fog is a cloud near or on the ground and typically needs calm conditions to form. Fog will form when the air temperature reaches the dew point temperature. On a side note, a great experiment on how to make a cloud or fog can be found here.

When fog forms in the hollows, it is an indication of high pressure controlling our weather. High pressure means the air is sinking. Sinking air suppresses cloud formation and brings dry weather.

So, next time you wake up early and see patches of fog in the hollow or fog along the river valley, you can say "fog in the hollow, fair weather to follow."

Other kinds of fog can help you understand the weather that's happening.