Yes, it is about the dew point and the temperature and the wet bulb thermometer that we probably don't carry or use...

But in the summer in Maine, along the "Downeast Coast" and places like Penobscot Bay -- I've observed that if you have a wind that is off the cold water (especially to the east and southeast, the fog will come crashing down and won't easily lift, even if it's clear "ashore" and the sun is shining.

A west wind or north wind generally won't yield that persistent fog in the summers-- especially on warmish days. I've had situations where we were sailing or motoring in fog not to far from shores where they were in "clear blue skies".