Fog is caused by tiny water droplets that are suspended in the air. It is essentially a cloud that lies at or near ground level.
The particles, which range in size from about 0.01 mm to 0.1 mm, cause light to be reflected in different directions which is what makes visibility and driving so difficult.
Driving in foggy conditions means your visibility may be limited
Fog is officially defined when the precipitation means visibility is reduced to less than 1,000 m, but in public weather forecasts The Met Office will refer to visibility of less than 180 m.
Our 10 tips will help you stay safe when driving in fog.
1. Plan your journey
Before you set out on your journey check the weather forecast, and as in any other severe weather conditions only drive if you really need to. Early morning mist and fog usually clears as the day gets warmer so, if possible, delay your journey for a couple of hours until visibility improves.
If you’ve already set off and the weather takes a turn for the worse then consider taking a break. A roadside sign with the ‘Fog’ will indicate that you should be prepared for a thick bank of fog ahead, even if the road is currently clear.
Remember, only stop where it is safe to do so – this is even more critical when visibility is poor.
2. Know your symbols
All cars are fitted with rear fog lights as standard and many models also have front fog lights. Make sure you know where the switch is for turning on your fog lights. You’ll find it on the dashboard, the steering wheel stalk, or next to the dial where your control your other lights.
The symbol you’re looking for is a lamp pointing at 3 straight lines with a wavy line cutting through them. The lamp pointing to your left with the lines pointing downwards is for your front fog lights and the one to your right with horizontal lines is for your rear fog lights.
Know your left from your right and your front from you rear
The indicator for the front fog lights is usually green and for the rear fog lights it’s amber.
3. Know the law
You should only use your fog lights if visibility drops below 100 m (328 ft) which is about the length of a football pitch. They are designed to make you more visible to other drivers, not to help light your way.
Switching on your fog lights at any other times, such as at night or in the rain when there is good visibility, is an offence for which you could receive a fixed penalty notice.
If you’re involved in an accident when driving in fog and you weren’t using your fog lights your insurance could be invalidated.
4. Turn on your dipped beams
It can be tempting to turn on your full beams when driving in fog to try and see that little bit further, but this can actually make your vision even worse.
Full beams are angled high and in foggy conditions, as well as in rain and snow, their bright light will bounce off the precipitation suspended in the air, directly back at you.
Use your low beams instead which are angled downwards and to the left, combined with your fog lights for the best effect. Don’t rely on your lights to come on automatically, but control them manually.
5. Slow down
With limited visibility you’ll have less time to react to what other vehicles are doing or when conditions change on the road. If you hit a dense patch of fog don’t slam on your brakes. Check your mirrors and slow down gently, using your brake lights to warn other drivers and to give traffic behind you enough time to slow down too so they don’t run into the back of you.
As well as hanging in the air, fog precipitation will coat the surface of the road, making it wet and increasing stopping distances further.
6. Keep your distance
Instead of leaving the usual 2-second gap between your car and the one in front leave at least a 3-second gap. A good way to measure this is to watch out for when the car in front of you passes a marker in the road such as a street light or road sign, or it drives under a bridge, then count ‘one hundred, two hundred, three hundred’. If you pass the marker before you’ve finished counting, then you’re too close and need to drop further back.
Drive slowly and stay alert
If you feel a car is too close behind you, although this can be frustrating, The Highway Code says you must not accelerate to get away from it.
7. Don’t use another car’s lights to help you navigate
Using the car in front’s rear lights to help you see where you’re going probably means you’re too close. It also means you’re not concentrating on what else is going on around you.
If the driver makes a mistake or has to stop suddenly stop then hanging on their tail lights could prove very dangerous.
8. Clear your windows
Foggy weather can cause condensation to build up on both the inside and outside of your car’s windows. A fogged up windscreen will impede your visibility even further and could mean you’re driving illegally.
Use your windscreen wiper and demister to keep the windscreen clear, and turn on the car’s heater and air-conditioning to help keep the atmosphere inside the car dry
Read our detailed guide on how to keep your car windows clear.
9. Don't get confused
The lack of external reference points can fool you into thinking you’re driving more slowly than you actually are. Don’t mistake fog lights for brake lights on the car in front. Concentrate on everything that’s going around you and don’t be distracted by passengers – bribe your kids to stay quiet at least until the worst has passed.
10. Use your ears
If visibility is very poor, then turn off your music and wind down your window so you can listen out for oncoming traffic at junctions and crossroads. When driving on motorways using your ears will help you hear overtaking vehicles that you may not be able to see.
If the fog’s so thick that you’re having to rely on your hearing rather than your sight then it’s probably time to stop the car. Find a safe place where you can pull over and wait for the fog to lift.