Wayther Fog and visibility
The number nobody reads properly
Visibility is the one figure on a forecast where the step is more informative than the level. Ten kilometres to two is the difference between a drive and a crawl, and both of them are still called “visible” in the ordinary sense. It is also the figure people skip, because it is not a temperature and it does not feel like a headline — and it is the one that decides whether you slow down.
The mechanics
Treating “fog” as one thing is the first mistake. It comes in two forms with opposite answers — one is on a clock, one is not — and it is also the loudest cause of poor visibility rather than the only one. All four are below.
On a clear, still night the ground radiates its heat away to the sky and cools, and the air immediately above it cools with it. Once that air reaches its dew point, condensation forms. It is thickest just before sunrise — the coldest, stillest moment of the day — and it usually clears within a couple of hours of the sun coming back up, as the ground starts absorbing heat again. Which means an hour of timing removes it completely.
Warm, moist air moving across a cold surface is cooled below its dew point and fogs, and nothing about that surface is warming up during the day. This is the fog of a warm front arriving over a cold sea or a cold plain, and it can sit through the entire day. If your rule of thumb is that fog burns off by mid-morning, this is the case that breaks it — and the case where changing the departure time is pointless.
Fog is the loudest cause of poor visibility and not the only one. Falling rain, blowing snow, spray off a wet road in front of you, and dust in dry summer wind all reduce it, and none of them behaves like fog: spray is a local problem that comes and goes with the vehicle in front, and dust arrives with a wind direction. Reading the cause tells you whether the number is about the sky or about the road.
The same figure is a different proposition at night. Visibility is a distance, and a distance you cannot light is much shorter than the one on the dashboard, so a stretch that is unremarkable in the afternoon can be a wall at four in the morning. It is arithmetic rather than weather, and it is why a fog forecast and a day-or-night split have to be read together.
The part you can act on
If the fog in the forecast is the overnight kind, the answer is boring and effective: go later. Move the departure until you are arriving after the sun has been up a while, and the stretch you were worried about is clear — not because the weather improved for you, but because it was never going to last.
If it is the kind with no schedule, the same move achieves nothing. Then the only real levers are a different road or a different day, and which one depends on where the cold surface is. Fog that is arriving over the sea is often avoidable by turning inland; fog lying in a valley or a basin is often not avoidable at all, because the road goes through the valley.
Both of those are things a forecast for your departure city cannot tell you, and both are visible in a forecast evaluated at each point of the route at the hour you reach it.
The same thing, at 05:00 tomorrow
The ferry goes at half past seven, so you are leaving at five, and there is fog on the route. The departure is fixed. That removes your best lever before you have read anything, so this decision starts somewhere different from the other three.
Two fogs with the same visibility figure have opposite answers, and the difference is decided by where you are rather than by how thick it is. Getting this wrong is what sends someone out at five into fog that was never going to lift, or cancels a trip that would have been fine.
This is the fog that forms as the ground cools overnight and burns off as the sun comes back up and the ground warms again. It is tied to sunrise, and sunrise is on the app. At five in the morning, with the sun not yet over the horizon, the honest reading is that the answer is usually leave — and the number being low right now is not a reason to wait.
This fog is warm, moist air arriving over a colder surface, and it is fed from the direction it is coming. Sunlight does not dissolve it, because it is not sitting in the cold air — it is arriving in it. No departure time fixes this one. Either the road changes or the day does, and that is worth knowing at five rather than at the coast road.
A visibility figure that improves as you go is a problem you are driving out of. One that gets worse towards your destination is one you are driving into, and it is the same journey with the opposite ending. The direction of the trend is the useful part; the number is only a measure of how much of it there is.
It is dark, or nearly, and dark plus low visibility is a different set of hazards from either of them alone — wildlife on the road, unlit stretches, and no margin for the ones you did not see. The summary tells you how much of the trip falls in the dark, and on an early ferry that number deserves reading before the fog number does.
Visibility FAQ
There is no single figure here, and the reason is worth more than a number would be: visibility is only meaningful against a speed you are travelling at. Ten kilometres is generous at motorway speed and a wall at a crawl through a village. What matters is whether you can see far enough, at the speed you are doing, to stop before whatever is in front of you. Where an authority publishes a threshold for your area, that is the figure to follow rather than one from a weather app.
Because on a clear, still night the ground radiates its heat away to the sky and cools, and the air immediately above it cools with it. Once that air reaches its dew point, condensation forms and you get fog. The coldest, stillest moment of the day is right around dawn, so the fog is thickest there, and once the sun comes up and the ground starts absorbing heat again it usually clears within a couple of hours.
It is worse for anyone riding a motorcycle, and for the same reason it is worse for anyone else: the driver has no windscreen wipers working at road speed, no housing, and a much narrower field of view through what is left of it. A rider also has no reference point from the road surface, because there is no bonnet and no pillars. Beyond that, fog does the same thing to everyone, which is make speed the wrong variable to fix.
Sometimes, and the two kinds of fog need different answers. Fog that forms overnight on a clear, still night is predictable and clears, so moving the departure forward usually puts you behind it. Fog that arrives as warm moist air over a cold surface has no schedule and can sit all day, so shifting the start does not help and the route or the date is what changes. Telling them apart is the useful part, not the number attached to either.
Visibility charted along the route and listed for every point, with the day-and-night split beside it, up to ten days ahead.
Forecasts come from Apple Weather and cover wherever Apple Weather is available, in metric or imperial units. Wayther plans a trip; it does not navigate and it does not carry official warnings. Keep your maps app for directions, and follow official weather warnings and road closures.
