Fog need one thing - humidity, and lots of it, and the change from warm humid air to cold dry air to "release" some of that humidity.
But there are different type of fogs. There's the "frontal" fog which is usually associated with a warm front coming in from the Atlantic. It's where low stratus cloud become so low that it's base covers hills and high ground and becomes as "hill fog".
That type of fog can occur at any time of the year since we get most of our weather in from the Atlantic anyway.
Then we have the "advection fog" type where warm moist air rolls over a cold sea surface. That causes condensation to take place and becomes as fog, which then roll inland in an onshore breeze. But usually as that fog dig further inland that fog dies out as it hits drier and warm air.
That type of fog is common in the late winter and Spring season as that's when the sea is at it's coldest while at the same time the sun is giving off more strength as it rises higher above the horizon.
But the type of fog you really are after is the "Radiation" type. They are spotted in "frost hollows", valleys and low lying areas being the most favoured places for that kind of thing.
As any moist air that has been heated up by the sun cools rapidly in the evening under clear skies, as well as that air sinking (as you know, warm air rises and cold air being denser sinks) towards lower ground, that sinking air then condenses in the process until eventually it all collects into one particular area and shows up as low-lying mist (that's why it feels so chilly cold in it, but get away from it and the air feels warmer).
For that to form, you need a warm and wet day, then for skies to suddenly clear in the evening thus allowing temperatures to fall and most importantly for winds to be calm so that no mixing occurs.
Radiation fog are at it's most common in the Spring and Autumn time as the variation in day time and night time temperatures are at it's most greatest.
What also would help is knowing the lay of the land so that you have an idea where those lovely low lying radiation-type fog are likely to show up (and they will always show up in the same favoured place anyway).
Hope that helps.
