Fog lamps provide a wide, bar-shaped beam of light with a sharp cutoff at the top, and are generally aimed and mounted low. They may produce white or selective yellow light, and were designed for use at low speed to increase the illumination directed towards the road surface and verges in conditions of poor visibility due to rain, fog, dust or snow.
Moreover, they are sometimes used in place of dipped-beam headlamps, reducing the glare-back from fog or falling snow, although the legality varies by jurisdiction of using front fog lamps without low beam headlamps.
In most countries, weather conditions rarely necessitate the use of fog lamps, and there is no legal requirement for them, so their primary purpose is frequently cosmetic. They are often available as optional extras or only on higher trim levels of many cars. An SAE study has shown that in the United States more people inappropriately use their fog lamps in dry weather than use them properly in poor weather. Because of this, use of the fog lamps when visibility is not seriously reduced is often prohibited in most jurisdictions; for example, in New South Wales, Australia:
"The driver of a vehicle must not use any fog light fitted to the vehicle unless the driver is driving in fog, mist or under other atmospheric conditions that restrict visibility."