Counting fog days in the lowlands
A fog day here means visibility under one kilometre at 07:00 local time, measured against the same three landmarks every morning.
The definition matters more than the instrument. Choosing fixed landmarks at known distances gives a record that stays comparable across a decade, which an automatic sensor replaced twice would not.
Conditions that produce it
Radiation fog in this basin needs three things together: clear skies overnight, wind under two metres per second, and soil still damp from rain within the previous four days. Remove any one and the morning comes up merely hazy.
The strongest single predictor is not humidity but the previous afternoon's cloud cover. Below two oktas after 15:00, fog forms on roughly seven mornings in ten.
The downward trend
The count has fallen from 78 fog days in the first full year to 51 in the most recent. Part of that is warming, but part is drainage: two of the wet meadows upstream were tiled during the same period, and the soil no longer holds water into the autumn the way it did.
Separating the two is not possible from this record alone, which is worth stating plainly rather than implying a cause the data cannot support.