In which situation is it illegal to use fog lights?

Using fog lights is specifically intended for conditions where visibility is significantly reduced due to atmospheric conditions, such as fog, heavy rain, or snow. When there is no fog, rain, or snow, employing fog lights can create glare for other drivers and be deemed illegal. Fog lights emit a wide, bar-shaped beam that is low to the ground, minimizing the amount of light that is reflected off of moisture in the air — which is why they are effective in fog or similar conditions. However, if the weather is clear, using fog lights can cause unnecessary brightness that may distract or impair the vision of other road users. Therefore, using fog lights in the absence of these specific weather conditions is not only unnecessary but also against the rules governing their use.