Are you allowed to use the fog lights when driving in heavy snow?

Yes, fog lights are primarily intended to improve visibility in foggy conditions but they can also be suitable during heavy rain or snowfall.

In the event of fog, heavy rain or snowfall it is usually both more efficient and safer to use fog lights or dipped headlights than full beam headlights. This is because the light is so powerful and is directed at such an angle that it is reflected back at you from water particles.