Is this mostly useful to aviation? Or does the weather person go to the top floor and look for all the pre-marked landmarks around the area?
Hmmm, looking at the Wikipedia page on visibility, visibility is defined as how far humans can discern various objects but does not say how it actually is determined on any given day. I did however happen to remember that Secchi disks are used to determine actual water visibility.
When they give the visibility from the observatory on top of Mt Washington in NH the furthest distance they give is 130 miles, which is the distance to Mt Whiteface in the Adirondacks. It’s the furthest you can easily see with the naked eye and represents the maximum visibility from that point.
I’m pretty sure that a specific type of sensor is used, especially at aitports. Here’s one:
https://www.vaisala.com/en/products/instruments-sensors-and-other-measurement-devices/weather-stations-and-sensors/fs11
And another:
There are different technical definitions of “visibility” for aviation use in various jurisdictions. The traditional US definition in daylight is the farthest distance an unlighted prominent object can be seen over one half of the horizon circle. At night it’s the same idea but correcting for how brightly lit whatever you’re sighting towards is.
Traditionally manned control towers or weather observing platforms had premade diagrams of nearby hills, buildings, towers, etc. in all various directions with their pre-surveyed distances. In actual use, once an hour the weather observer would look out the window to see which of the landmarks they could and couldn’t see. And they’d take special observations if the clouds or precipitation were changing enough to warrant.
As linked just above, nowadays calibrated light or laser systems measure visibility over a much shorter baseline and essentially continuously in time. At busy airports prone to low vis there are sensors called RVR located at 1 to 4 points along the length of each low-weather runway. Which provide very very localized info about exactly what’s likely to be visible to an airplane using that runway. The overall airport “prevailing” visibility will be reported in addition to the RVR readings for the runways(s) in use.
That’s observations. Which same concepts would also be useful to ships in foggy harbors, although I can’t vouch for what formal standards are used. In the US, NOAA being responsible for them all would suggest their the same.
Forecasts are a different matter. Ultimately the same idea applies though. Make an educated guess / calculation of how intense the precipitation is likely to be or how thick any ground hugging clouds might generally be in the area in question and you’re in business.
There’s an absolute treasure trove of educational info on weather observation & forecasting at the link below. It’s aimed at the professional weather forecaster audience, so I mostly look at the pictures & get the gist.
I assume the conditions listed in the local weather report were for residents. Today it dropped from 10 miles to 2.3 in a hour or so.
If you’re in the US, then your local NWS office is responsible for observations, forecasts, and their dissemination. If you start here
https://www.weather.gov/
and put your location into the upper left search box among other things it’ll tell you which observing station is being reported as your local weather. In my case it happens to be the nearest general aviation airport 5 miles away. Not all airports have weather stations, automated or otherwise, and not all weather stations are at airports. But there’s a vast overlap of the two.
Note the specific terminology: an observation or report is what was seen to be actually happening at a specific time & place. A forecast is a prediction about the future for some time interval across some geographical area. it’s common in casual speech to call both “the weather forecast”, but that’s not good usage.