Your vehicle has front fog lights. When must you turn them on?

Correct answer: When there is fog or it is raining or snowing heavily.

Front fog lights are for fog, heavy rain, snowfall, smoke or dust that greatly reduces visibility. Without those conditions, they are allowed only on narrow sections with many signposted curves. A tunnel or an additional lane does not justify using them. In the image, the oncoming passenger car in the rain has its front fog lights on.

Reglamento General de Circulación (Real Decreto 1428/2003), texto consolidado