Where and when fog forms
Radiation fog forms on clear, still nights as the ground cools, and it collects in valleys, along rivers and over low, wet ground. It is thickest around sunrise and usually burns off by mid-morning once the sun gets to work — later in autumn and winter, when the sun is weak.
Hill fog is simply cloud sitting on high ground. A mountain pass can be in cloud while the valley is clear, and it can arrive in minutes. Coastal fog — the haar on the North Sea coast, for example — can blanket the coast while a few kilometres inland stays sunny.
Too thick to ride
Your stopping distance has to fit inside what you can see. At 200 m of visibility that already means well under normal road speeds, and below that it only gets worse. Fog also wets the road and your visor, so it takes grip and vision at the same time.
RideWindow vetoes any hour with visibility under 200 m (0.2 km) — the score is 0, No-Go. Above that, poor visibility still costs points, and anything under 1 km is shaded grey on the forecast chart, so a foggy morning is visible at a glance.
Waiting it out
Because valley fog usually lifts during the morning, the fix is often simply to leave later. The ride windows on a forecast will start after the fog clears if that is when the good hours begin. Check your own area or a city or pass — river towns such as Liège and Nijmegen see it often in autumn.