if in doubt, turn on the main headlights.
how much fog makes it officially foggy? at what point is it right to turn on your fog lights? all cars will have a rear fog light and the button shows a lamp with a wiggly line in front of it. some cars will also have front fog lights. as a general rule, the front fog light button will have the light pointing the same way as it does on the headlight switch.
the highway code states you should use your rear fog light if you cannot see more than 100m beyond your car. the best way to measure this is to use the 