Visibility obstacle is a term used to describe any environmental conditions that reduce visibility. Common examples of visibility obstacles include MIST, FOG, SMOG, and HAZE. MIST is a collection of tiny water droplets suspended in the air, while FOG is a thicker collection of water droplets that can reduce visibility to less than 1 km. SMOG is a combination of smoke and FOG, and is often caused by industrial pollution. HAZE is a combination of dust, smoke, and other pollutants that can reduce visibility to less than 10 km. All of these visibility obstacles can be dangerous for drivers, pilots, and other travelers.