This week got off to "a foggy start for many", in the Met Office's words, with more fog patches forecast overnight. Fog is most common in autumn and winter, and it can roll in and clear again surprisingly quickly. Here's what the rules say, and how to stay safe when you can't see far ahead.
When does mist become fog?
The Met Office counts it as fog when you can see less than 1,000 metres, and dense fog when visibility drops below 100 metres. It issues warnings for dense fog when visibility is expected to fall below 200 metres. Fog tends to form on long, clear, calm nights, when moist air cools close to the ground. Some clears soon after sunrise, but on cold, still days it can hang around into the afternoon.
The 100-metre rule
The Highway Code says you must use headlights when visibility is seriously reduced – generally when you can't see more than 100 metres, about the length of a football pitch. That's also when you may switch on your fog lights. But fog lights must not be used when visibility is better than that, because they dazzle other drivers and can hide your brake lights. Switch them off as soon as the fog clears.
Why 100 metres? Because at 70mph a typical car needs 96 metres to stop on a dry road – and much more on a wet one.
8 tips for driving in fog
- Use dipped headlights, not full beam. Full beam reflects off the fog and creates a "white wall" effect.
- Check your lights are actually on. Automatic headlights and daytime running lights may not light up the back of the car in daytime fog, so switch your lights on manually.
- Slow down. Fog can make you feel as though you're travelling more slowly than you are, so keep an eye on the speedometer.
- Leave a much bigger gap. Don't follow the tail lights of the car in front – they give a false sense of security.
- Keep your windows clear. Use your wipers and demister, and give the windscreen a proper clean before you set off.
- Take care at junctions. Where visibility is limited, stop, open your window and listen for traffic before pulling out.
- Watch out for freezing fog. Its droplets freeze on contact with the road and can quickly form a layer of ice.
- Fog lights off when it clears. Fog is often patchy, so switch them off as soon as you can see further than 100 metres – and back on if it thickens again.
If fog is forecast to be thick, the simplest advice from the Met Office is to delay your journey if you can.
Fog catches people out because it's so patchy – you can be in bright sunshine one minute and down to 50 metres the next. If you manage drivers, a quick reminder this week costs nothing: check that every bulb works, including the fog lights, and that windscreens and wipers are in good shape before winter really sets in. Our full winter driving guide is coming later this month.
Sources: Met Office – advice for travelling in fog, Met Office – fog warnings explained, Met Office forecast, The Highway Code (GOV.UK)