Visibility is considerably reduced. When are you allowed to switch on fog headlamps?

You answered this question correctly. In Germany, fog headlamps (Nebelscheinwerfer) can be used not only in fog but also when visibility is significantly reduced due to rain or falling snow. Reduced visibility means you cannot clearly see more than about 50 meters ahead. The rule is that you may use fog headlamps in all these cases—heavy rain, falling snow, or fog—to improve your view of the road. Remember, their use is only allowed when visibility is actually much reduced; otherwise, you must keep them turned off to avoid blinding other drivers.