A weather app can show you a sun icon and still leave you creeping along a familiar road at walking pace, nose practically against the windscreen. Fog is the quietest hazard in the forecast. It rarely makes headlines, it arrives in patches, and it turns places you know well into somewhere you don't recognise.

The good news is that fog is readable. Three figures do most of the work — visibility, probability and dew point — and once you know what each one is telling you, a glance at the hourly forecast becomes genuinely useful rather than vaguely reassuring.

What the word "fog" actually means

For forecasters, fog is a visibility measurement rather than a mood. In the UK, it is generally called fog when you can see less than about 1,000 metres; between roughly 1,000 and 2,000 metres, it is mist. That is why an app can call a morning misty at 6am and foggy an hour later without anything dramatic having happened — the same grey soup has simply thickened past a threshold.

The distinction matters for one practical reason: mist is a nuisance, fog is a hazard. It also explains why two apps can appear to disagree. Some label anything below 2,000 metres as fog; others save the word for the thick stuff. If the icon looks ambiguous, dig out the numbers behind it.

Visibility: the figure to look for first

Visibility is usually given in metres or kilometres, and it is far more useful than any symbol. A rough translation:

• 2,000 metres and above: hazy. Distant hills lose their detail, but driving is normal.
• 1,000 metres: fog by definition. You can still see a fair way down a straight road, but identifying landmarks becomes guesswork.
• 500 metres: The queue of traffic ahead of you disappears. Headlights start earning their place in daylight.
• 200 metres: Serious. You cannot see a bend before you are in it, and junctions become a gamble.
• Under 50 metres: Dense fog. Many drivers find this harder than heavy rain. If the journey is optional, wait.

Below about 100 metres, that is also roughly the point at which fog lights do their job. Switch them off as soon as visibility improves, because they dazzle other drivers and can mask your brake lights.

Probability: a measure of confidence, not thickness

A forecast saying a 40% chance of fog does not mean the fog will be 40% thick, or that it will cover 40% of the county. It means fog is expected to form somewhere in the area at some point in the period, roughly four times out of ten.

So treat it as a confidence figure rather than a severity figure. A 20% chance on a cold, clear, still October night is still worth taking seriously if you are leaving at 6am. Low-probability events happen, and because mist is patchy, that 20% could easily be your valley and not the next one along.

Two things make probability more useful. Pair it with a time window — "fog from 3am to 8am" tells you when to be careful, whereas a whole-day 40% tells you almost nothing. And check the pattern: fog shown at 5am, 6am and 7am is a trend, while a single blip is often just a model quirk. If your app offers a separate figure for freezing fog, look twice. Ice on the road plus poor visibility is a different sort of journey.

Dew point: the number most people skip

Temperature tells you how warm it is. Dew point tells you how close the air is to giving up its moisture. When air cools to its dew point, water condenses — as dew on the grass, mist over a field, or fog in a valley.

The practical rule is the gap between the two. If the overnight minimum is forecast to sit within a degree or two of the dew point, the air is close to saturation and fog or mist becomes much more likely. If the temperature is heading for 4°C and the dew point is 3°C, expect something grey by morning. If the dew point is 0°C and the minimum is 6°C, the air has room to cool before anything condenses.

Relative humidity is the figure people reach for instead, and at night it is less helpful. Humidity of 95% at 3pm is not the same as 95% at 3am, because the number shifts with temperature. Dew point travels with the air mass and stays put as the thermometer moves. If your app shows it, use it.

Reading the hourly forecast like a local

Fog forecasts are really forecasts about conditions, and conditions are stubbornly local. The classic recipe for radiation fog — the kind that forms overnight and burns off mid-morning — is a clear sky, light winds, a long night and damp ground. A clear sky lets the ground lose heat, a light breeze stirs just enough air to spread the cooling, and a long night gives it time to work. Add a river valley or a hollow and you have somewhere cold air can pool.

So check the ingredients as well as the label:

• Wind: flat calm can leave fog sitting as a shallow layer, while anything brisk tends to mix it away.
• Cloud: thick cloud overnight usually rules out radiation fog altogether.
• Season: autumn and winter, with the still, damp mornings of November doing much of the damage.
• Place: valleys, low fields, and coasts in spring and summer, where sea fog arrives with a change of wind direction and can linger for days.

Why your app sometimes gets it wrong

Fog is small. Weather models divide the country into boxes several kilometres wide, and fog can form in one corner of a box and not the other. Your forecast point might sit on a hilltop while the road you actually drive runs through the valley beneath it. The nearest observation the app can pull might be an airport ten miles away on flat ground — excellent for aviation, not much use for your back lane.

None of that makes the app useless. It means you should read it as a careful guess about your area rather than a promise about your street.

A five-minute fog check before you set off

1. Find the lowest visibility figure in the next few hours, not the current one.
2. Note the fog window. When does it start, and when is it forecast to clear?
3. Compare overnight temperature with dew point. A gap of a degree or two is your warning sign.
4. Glance at wind and cloud. Clear and calm means fog is more plausible, not less.
5. Check the real world. A look at the streetlight outside tells you plenty — a visible halo means moisture in the air, and if the light itself is a soft blob, you already have fog.
6. Adjust the plan. Allow extra time, use dipped headlights, leave a bigger gap than usual, and remember that fog often lifts as the ground warms. Leaving an hour later can turn a stressful drive into an ordinary one.

Read those three figures together — visibility, probability, dew point — and you will rarely be caught out by a morning that the app described as bright.

Photo: Tima Miroshnichenko / Pexels