It's not 90% of the time, but i get you.
For me it is more annoying than mist in mistlands.
When I see fog in plains I just quit my exploration plans and try again the next day.

If you want to know the weather in Valheim, try searching Valheim Weather Tool on google.
You just need to know which day it is and you will be able to see forecast for all biomes and even wind direction and speed. Amazing tool for explorers.