This morning I heard that visibility dropped from 2 miles to 1/10 mile as fog moved in. How do they measure this? Are there specific objects with known distances identified as landmarks from the observation station?
In general, your local weather observation will come from a nearby airport. That said, visibility is measured with two instruments. Primarily, an instrument called a transmissometer is used. A transmissometer consists of a laser transmitter, and a receiver. They are separated by a certain distance (maybe 10m). The transmitter sends a laser beam through open air toward the receiver. The instrument calculates how much the beam is attenuated by the atmospheric phenomenon and calculates visibility. Some weather station installations will also have an instrument called a forward scatter visibility sensor. It works in a similar fashion to the transmissometer, except it uses infrared light rather than a visible laser. The transmitter and receiver are close to each other (half a meter-ish) and are offset at an angle. If there is a visibility-reducing phenomenon present, the infrared light will be scattered by particulate suspended in the air. The offset angle allows the receiver to measure how much of the light has been scattered, and from that measurement it can calculate the visibility. Transmissometers are more accurate in lower visibility conditions, and forward scatter visibility sensors are better for accurately measuring clearer conditions.
At airports with control towers, the controllers are certified weather observers. They get a report of the weather conditions from the automated weather reporting station on the field each hour (or whenever there is a significant change in the weather). The controller can then augment the report to make it more accurate. Most control towers have a set of landmarks that are at known distances, just as you suspect. The tower controller can edit the automated visibility report based on which landmarks he/she can see clearly.
in addition to airports government agencies for transportation/roads, water, sewerage will collect weather data. civilian trained observers also collect weather data in the USA.
some areas are prone to fog and observations taken specific to those locations. a siting point and known distance to identifiable objects and a person’s judgment would be used.
Yeah, but what does “visible” mean? I’ve seen weather reports citing “heavy fog” and “reduced visibility” only to find that, despite the fog, I can see everything that I could have seen in clear weather. When you’re driving behind SUVs, visibility is limited – rain or shine.
fog can be localized or of much greater intensity near bodies of water or in lower local elevations. fog can also disappear in a 15 minute time frame.
At an airport, it’s the distance that an object is discernible. If a building is two miles away and the person can barely make out the outline of the building, then the visibility is two miles. If he/she can’t even see the outline, then the visibility is less than two miles.
Great explanation STF - thank you.
My pleasure. To expand on bizerta’s response, according to the US National Weather Service: