# How do they measure visibility for weather reports? **URL:** \ **Category:** Factual Questions\ **Created:** [November 15, 2010, 1:38pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737 "2010-11-15T13:38:41Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 15, 2010, 1:38pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/1 "2010-11-15T13:38:41Z")
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? ---
**Author:** ![StrangerThanFiction](https://avatars.discourse-cdn.com/v4/letter/s/f05b48/32.png) [@StrangerThanFiction](https://boards.straightdope.com/u/StrangerThanFiction)\ **Post date:** [November 15, 2010, 2:40pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/2 "2010-11-15T14:40:29Z")
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. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [November 15, 2010, 2:50pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/3 "2010-11-15T14:50:20Z")
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. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [November 15, 2010, 3:58pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/4 "2010-11-15T15:58:04Z")
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. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [November 15, 2010, 4:42pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/5 "2010-11-15T16:42:48Z")
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. ---
**Author:** ![bizerta](https://avatars.discourse-cdn.com/v4/letter/b/3be4f8/32.png) [@bizerta](https://boards.straightdope.com/u/bizerta)\ **Post date:** [November 15, 2010, 4:54pm UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/6 "2010-11-15T16:54:12Z")
> [@Nametag](#): > > Yeah, but what does “visible” mean? 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. ---
**Author:** ![dzero](https://avatars.discourse-cdn.com/v4/letter/d/5f9b8f/32.png) [@dzero](https://boards.straightdope.com/u/dzero)\ **Post date:** [November 16, 2010, 6:31am UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/7 "2010-11-16T06:31:51Z")
Great explanation STF - thank you. ---
**Author:** ![StrangerThanFiction](https://avatars.discourse-cdn.com/v4/letter/s/f05b48/32.png) [@StrangerThanFiction](https://boards.straightdope.com/u/StrangerThanFiction)\ **Post date:** [November 17, 2010, 4:32am UTC](https://boards.straightdope.com/t/how-do-they-measure-visibility-for-weather-reports/560737/8 "2010-11-17T04:32:50Z")
My pleasure. To expand on **bizerta** ’s response, according to the US National Weather Service: > [@](#): > > Visibility is a measure of the horizontal opacity of the atmosphere at the point of observation and is expressed in terms of the horizontal distance at which a person should be able to see and identify: in the daytime, a prominent dark object against the sky at the horizon; at night, a known, preferably unfocused, moderately intense light source. > > Surface Visibility is the greatest visibility, taken at the surface, equaled or exceeded throughout at least half the horizon circle. from here: [Forecasts](http://www.weather.gov/forecasts/wfo/definitions/defineSfcVisibility.html)