I would suggest temperature change may be affecting the cable housing dimensions, which in turn affects the cable tension and therefore the dérailleur settings. I have been noticing this happening to myself this fall, especially switching to a bike with full length housing and 10 speeds on the rear dérailleur. With the large daily temperature changes in fall I find myself adjusting the rear dérailleur tension by a quarter to half-a-turn in either direction depending on how the temperature has changed. (Luckily the bike has inline adjusters near the bar, which makes this a trivial exercise).
Metal will expand or contract with temperature, and therefore the dimensions of the cable housing (which has metal strands) and the cable itself can change slightly with temperature changes.
For what its worth I never noticed this issue on older bikes with 9 speeds or lower (with or without full length housing). I have however started noticing this issue with full length housing and higher speed drive train (i.e., 10, and 11 speed).
If you are handy at adjusting derailleurs, then you can test this hypothesis and potential fix the problem at the same time. The next time the shifting problem emerges try adjusting the derailleur.
- If it happens in the fog, you will likely have to add tension/elongate the housing by loosening the rear derailleur adjuster. If the shifting improves, then half the hypothesis has been verified (H1: cable housing contraction in colder weather).
- If after (1) the weather warms up and the shifting goes off again, try tightening the rear derailleur adjuster (effectively shortening the cable housing). If this fixes the shifting then consider the second half of hypothesis verified (H2: cable housing expansion in warmer weather).
Bike Geek Aside
As an aside, the expansion and contraction of the housing and cables will counter act each other to some degree (e.g., contracting housing will lessen cable tension, but contracting cable will increase tension). Each component will also expand and contract at different rates. My anecdotal experience seems to suggest the expansion and contraction of the cable housing has the biggest effect. Another big effect I have found was adjusting the cable tension inside, where it is warm, the riding it outside where it is cold would quickly throw all my setting off.