How is visibility measured in meteorology? Is it based on particles in the air or on actual visible distance measurements? During recent days when smoke from the wildfires was in our area, I saw several mentions of visibility in different cities.

MIKE MOSS SAYS: Glenn, For many years, human observers estimated visibility based on how well a series of objects or lights of known distance could be seen, and related these distances to a reference map of the area that showed how far away these objects (towers, buildings, hilltops, bridges, etc) were known to be. Most airports now rely on automated weather systems that include a "forward scatter sensor." This instrument produces pulsed flashes of blue light, some of which is scattered at an angle toward a nearby detector. Visibility is estimated from the intensity of scattered light, which increases when there are more fog droplets, smoke or haze particles, raindrops or snowflakes in the beam. These sensors report a visibility based on one minute samples averaged over the past ten minutes leading up to each observation.