Where I grew up in Co Kerry they used to say: "If you can see the Blaskets, it is going to rain; if you can't see them - it is raining." And, of course, similar aphorisms with a local landmark occur in nearly every region of the world where rain is common.
Meteorologists, however, when considering such matters, think in terms of a concept called "the visibility", which paradoxically, has little to do with how far a body sees. To take an obvious, example, on a very dark night you may be able to distinguish nothing more than five yards in front of you, but a meteorological observer might still record the visibility as being 20 miles. For meteorological purposes, the visibility is a hypothetical concept carefully defined as "the furthest horizontal distance at which a person of normal sight can distinguish and identify an object for what it is known to be, in normal conditions of daylight illumination".
The definition is framed to exclude extraneous matters from the reckoning - irrelevant variables like the eyesight of the observer, the amount of light (if any) that may be present at the time, and the degree of contrast that may exist between an object and its background. What it does try to isolate as the most important factor is the "turbidity", or transparency, of the atmosphere.
Others, however, have different priorities. Airline pilots, for example, have little interest in the concept of turbidity; they want to know how far down the runway they will see when they come to land an aeroplane. So when landing conditions at an airport are poor, instead of being told the visibility, the pilot is given the Runway Visual Range - the distance he or she would actually see looking down the runway in the conditions pertaining at the time.
Runway Visual Range - or RVR - used to be measured by a very simple method: someone would stand at the end of the runway and count the number of runway lights he could see. This figure was easily translated into distance, which was then communicated to the pilot. Nowadays at most major airports, a transmissometer is positioned beside each runway, which measures the transparency of the air, and also senses the background illumination by means of a photoelectric cell. The system is connected to the switch which controls the runway lights, so that it can take into account whether the lights are on or not - and whether they are at full strength or at a lower intensity. All this information is fed into a computer - and out comes a figure for the RVR.