When the air is full of moisture and the temperature drops, fog or mist may occur. Fog or mist is simply cloud at ground level. It consists of countless tiny droplets of water suspended in the air. Light passing through fog scatters in every direction, making it translucent, like tracing paper. In thick fog, visibility can be reduced to less than a few yards. Travel in these conditions is treacherous, and accidents on the roads, at sea, or in the air are common. Not much can be done to reduce the danger, but foghorns or radar can locate ships and airplanes, and lighthouses and traffic signals can help to guide them to safety. Fog costs airlines millions of dollars each year through airport shutdowns. When fog combines with smoke, a thick and dangerous mixture, called smog may form. Fog and mist is also caused by a change in humidity*. 

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