The extreme case is dense fog at night when light is scattered directly back at your car headlights. At automatic stations, such as those used on light vessels, there are various ways of measuring this scattering effect. First, there are forward scatter instruments. These emit a beam of light. Sensors a little way off the direct, narrow beam, measure only light that has been scattered.
Poor visibility leads to more scattering and, therefore, more light at the sensors.
The next technique, known as back scattering, is in use on the M25 where observant drivers will have seen sensors alongside the carriageway. These measure light scattered directly back to the source and, again, the greater the received light the poorer is the visibility. This technique is particularly useful for fog and the sensors are used to activate fog warning signs. A third system, using instruments known as transmissometers, measures the light loss, due to scattering, by putting the sensors in the direct beam. Here the less light received by the sensor then the poorer is the visibility.