• A break down of the key classifications of fog and mist
  • Answers to the most searched questions about driving in fog
  • When to turn your fog lights on, and when to turn them off

Fog: the facts

Fog is the accumulation of water droplets in the air, when water vapour has condensed back into a liquid – kind of like a cloud at ground level [1].  Foggy weather significantly affects your vision of the road ahead, reducing the time you have to react to potential hazards on the road.

There are three classifications of fog used by weather reporters, which give drivers an idea of the conditions on the roads:

  • Aviation fog, when visibility is 1,000m or less
  • Thick fog, when visibility drops to 200m or less
  • Dense fog, when visibility is only at 50m or less [2] 

A car travelling at 30mph will cover 50m in less than four seconds – that's roughly the equivalent of 13 car lengths [3]. 

If you can see equal to or further than 1,000m, but there are still water droplets in the air, this is classified as mist [4]. 

Fog is a common phenomenon during the autumn and winter months, when there is also a high chance of rain and frost. Be extra careful when there has been a frost or rainfall, which could affect your braking distance and the control you have over your vehicle.

Be especially careful in freezing fog, when air temperature drops below 0°C and the water droplets suspended in the air start to freeze. Freezing fog can cause unseen slippery patches on the roads and obscure your windscreen [5]. 

1. When should I turn my fog lights on?

The Highway Code (rule 226) says that you can turn your fog lights on when visibility on the road has dropped below 100m. 

How can you tell when? If you're travelling 30mph, you should cover 100m in roughly 8 seconds. To judge the distance, choose a fixed point on the road ahead of you just as it emerges from the fog, such as a sign, and count the seconds as you approach it. If you get any higher than eight, it's time to turn your fog lights off.

2. Is driving WITHOUT fog lights illegal?

It's never illegal to drive without your fog lights on, but you must switch your headlights on if visibility drops below 100m.

Fog lights can dazzle other road users, while rear fog lights can obscure your own brake lights, preventing the vehicle behind you from seeing when you are braking.

Make sure you switch your fog lights on as soon as visibility improves, and try to remember to turn them off as you approach other vehicles going in the opposite direction.

3. Is driving WITH fog lights illegal?

Driving with your fog lights on when you can see further than 100m ahead of you is illegal. Make sure that you switch them off if the fog clears, or switch to headlights if there's still thick fog on the road.

4. How can I prepare for a drive in fog?

Before driving in fog, make sure that your mirrors are clean and that you have demisted your front and rear windscreen. Fill up your windscreen wash so that you can use your wipers to keep your screen clear.

Make sure you take a high-vis jacket and a warning triangle with you in the car in case you have an accident.

When you come to fog on the road, check your mirrors and slow down when it is safe to do so.

A close up of somebody about to press their fog light button on their car

5. What's the difference between fog lights, driving lights and daytime running lights?

Drivers who often have to cope with difficult driving conditions install driving lights on their vehicle, but you are unlikely to need them on UK roads. Driving lights are strong lamps that emit a focused white beam [6].

Driving lights are stronger than the standard fog lights that should be installed as standard on any roadworthy UK video(?). Unlike driving lights, fog lights have a wide, flat beam meant to illuminate the road and its surroundings ahead.

Daytime running lights are automatic headlights that turn on when the vehicle turns on. Newer cars often have daytime running lights installed over the standard headlights as they can make it easier for other drivers to see the car during the daytime.

Daytime running lights are too bright for night-time usage, however, as they can dazzle other drivers, so they turn off automatically when headlights are turned on.

6. What if my car doesn't have fog lights?

If you are struggling to find your car's fog lights, they may be labelled as something else.

Some models, such as a number of the Volkswagen Group's latest designs, have the equivalent of fog lights integrated into their headlamps and rear lamps. 

Read your car manual to find what your vehicle has instead of fog lights, and then turn them on while parked to see how bright they are. They may be called 'all-weather lights' or something similar [7]. If your manual doesn't have the information, ask at the car manufacturer's local dealership.

If your car definitely doesn't have fog lights, judge whether your head and rear lights will be strong enough in foggy conditions.

7. Will I be okay if my car has automatic headlights?

If your car is relatively new and has headlights that switch on as it gets darker, don't rely on them in foggy conditions. You should also switch them on when you go into darkness abruptly, such as when driving through tunnels.

Extra steps you can take to stay safe while driving in fog

Rule 235 of The Highway Code states the following, which is good advice for drivers in fog:

  • Keep a safe distance behind the vehicle in front. Rear lights can give a false sense of security 
  • Be able to pull up well within the distance you can see clearly. This is particularly important on motorways and dual carriageways, as vehicles are travelling faster 
  • Beware of other drivers not using headlights 
  • Not accelerate to get away from a vehicle which is too close behind you 
  • Check your mirrors before you slow down. Then use your brakes so that your brake lights warn drivers behind you that you are slowing down 
  • Stop in the correct position at a junction with limited visibility and listen for traffic. When you are sure it is safe to emerge, do so positively and do not hesitate in a position that puts you directly in the path of approaching vehicles.

It's also worth checking typical stopping distances in The Highway Code. This can also reduce when the roads are icy. 

Fog can drift over the UK at any point of the year, so be careful on the roads and check out weather reports before taking a drive. If you do your checks beforehand, you're familiar with your lights, and you go at a steady speed, you'll be well prepared if you do have to venture out.

Sources:

  1. http://www.metoffice.gov.uk/learning/fog
  2. http://www.metoffice.gov.uk/learning/fog/dense-fog-infographic
  3. http://www.drivingtestsuccess.com/pages/stopping-distances-and-the-theory-test
  4. http://www.metoffice.gov.uk/learning/fog/difference-mist-and-fog
  5. http://www.metoffice.gov.uk/learning/fog/dense-fog-infographic 
  6. http://www.ebay.co.uk/gds/What-s-the-Difference-Between-Driving-Lights-and-Fog-Lights-/10000000177633614/g.html 
  7. http://www.honestjohn.co.uk/our-cars/audi-a6/the-front-fog-lights-mystery/