Buoys in the NERACOOS Region: The following buoys provide real-time visibility:
- Gulf of Maine N - Northeast Channel
- Gulf of Maine I - Eastern Maine Shelf
- Gulf of Maine M - Jordan Basin
- Gulf of Maine E - Central Maine Shelf
- Gulf of Maine A - Massachusetts Bay
The purpose of the visibility observation is to identify reduced visibility events. For readings that come in at 3000 meters, it is implied that the visibility is 3000 meters or greater. An observational value for visibility is based upon a "homogeneous" air mass and not specific to any given direction. For example, if a cloud bank pops in front of the sensor on an otherwise clear-as-can-be day, the visibility might be reported as 3000m because the local air mass in general is still clear, but the visibility in the direction of said cloud bank might not be 3000 meters.
NWS Graphical Forecast: The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye 1.) in the daytime, a prominent dark object against the sky at the horizon, and 2.) at night a known, preferably unfocused, moderately intense light source. After visibilities have been determined around the entire horizon circle, they are resolved into a single value of prevailing visibility for reporting purposes.