Visibility

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Visibility

In meteorology

Meteorology

Meteorology is the interdisciplinary scientific study of the Earth's atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century....
, visibility is a measure of the distance

Distance

Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria ....
at which an object or light can be clearly discerned. It is reported within surface weather observation

Surface weather observation

Surface weather observations are the fundamental data used for safety as well as climatology reasons to weather forecasting and issue warnings worldwide....
s and METAR

METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by Aviators in fulfillment of a part of a pre-flight weather briefing, and by Meteorologys, who use aggregated METAR information to assist in weather forecasting....
code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing and aviation. Meteorological visibility refers to transparency of air: in dark, meteorological visibility is still the same as in daylight for the same air.

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In meteorology

Meteorology

Distance

Surface weather observation

Surface weather observations are the fundamental data used for safety as well as climatology reasons to weather forecasting and issue warnings worldwide....
s and METAR

METAR

Definition

ICAO Annex

Annex

An annex is a building that is an addition to another building.Annex or Annexe may also refer to:* Annexation, the incorporation of territory into another geo-political entity...
3 Meteorological Service for International Air Navigation contains the following definitions and note:

a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background;

b) the greatest distance at which lights of 1,000 candela

Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....
s can be seen and identified against an unlit background.

Note.— The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR).

Annex 3 also defines Runway Visual Range (RVR)

Runway visual range

Runway Visual Range is a term used in aviation meteorology to define the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line....
as:

The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.

In extremely clean air in Arctic or mountainous areas, the visibility can be up to to . However, visibility is often reduced somewhat by air pollution

Air pollution

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment, into the Earth's atmosphere....
and high humidity

Humidity

Humidity is the amount of water vapor in the air. In daily language the term "humidity" is normally taken to mean relative humidity. Relative humidity is defined as the ratio of the partial pressure of water vapor in a Air parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature....
. Various weather station

Weather station

A weather station is a facility with instruments and equipment to make observations of Earth's atmosphere conditions in order to provide information to make weather forecasting and to study the weather and climate....
s report this as haze

Haze

Haze is traditionally an Earth's atmosphere phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The WMO manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, vog, dust, sand and snow....
(dry) or mist

Mist

Mist is a phenomenon of small droplets suspended in air. It can occur as part of natural weather or volcanic activity, and is common in cold air above warmer water, in Exhalationd air in the cold, and in a steam room of a sauna....
(moist). Fog

Fog

Fog is a cloud bank that is in contact with the ground. A cloud may be considered partly fog; for example, the part of a cloud that is suspended in the air above the ground is not considered fog, whereas the part of the cloud that comes in contact with higher ground is considered fog....
and smoke

Smoke

File:Bling-Bling Skywriting David Shankbone.jpgSmoke is the collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrainment or otherwise mixed into the mass....
can reduce visibility to near zero, making driving

Driving

Driving is the controlled operation of a land vehicle, such as a automobile, truck or bus. Although direct operation of a bicycle, a mounted animal or a motorcycle is commonly called riding, such operators are usually legally considered to be drivers and are required to obey the rules of the road which apply to all drivers....
extremely dangerous. The same can happen in a sandstorm

Sandstorm

Sandstorm can refer to:* Dust storm, a storm caused by strong winds and blowing sand or dust* Sandstorm , an electronica song by Darude*...
in and near desert

Désert

?D?sert? is ?milie Simon's debut single, released in October 2002. The song was a huge success both critically and commercially in her homeland....
areas, or with forest fires. Heavy rain

Rain

Rain is liquid precipitation . On Earth, it is the condensation of atmospheric water vapor into droplet heavy enough to fall, often making it to the surface....
(such as from a thunderstorm

Thunderstorm

File:FoggDam-NT.jpgA thunderstorm, also known as an electrical storm or a lightning storm, is a form of weather characterized by the presence of lightning and its effect: thunder....
) not only causes low visibility, but the inability to brake

Brake

A brake is a device for applying a force against the friction of the road, slowing or stopping the motion of a machine or vehicle, or alternatively a device to restrain it from starting to move again....
quickly due to hydroplaning

Hydroplaning

Hydroplaning and hydroplane may refer to:* Hydroplaning , a loss of steering or braking control when a layer of water prevents direct contact between road vehicle or aircraft tires and the road or runway surface...
. Blizzard

Blizzard

A blizzard is a severe winter storm condition characterized by low temperatures, strong winds, and heavy blowing snow. Blizzards are formed when a high pressure area, also known as a ridge, interacts with a low pressure area; this results in the advection of air from the high pressure zone into the low pressure area....
s and ground blizzards (blowing snow) are also defined in part by low visibility.

Fog, mist, and haze

The international definition of fog is a visibility of less than ; mist is a visibility of between and and haze from to . Fog and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size; this has implications for sensors such as Thermal Imagers (TI/FLIR) operating in the far-IR at wavelengths of about 10 µm which are better able to penetrate haze and some smokes because their particle size is smaller than the wavelength; the IR radiation is therefore not significantly deflected or absorbed by the particles

Very low visibility

Visibility of less than are usually reported as zero. In these conditions, road

Road

A road is an identifiable Road number, way or Trail between Location . Roads are typically smoothed, Pavement , or otherwise prepared to allow easy travel; though they need not be, and historically many roads were simply recognizable routes without any formal construction or Maintenance, repair and operations....
s may be closed, or automatic warning lights and signs may be activated to warn drivers. These have been put in place in certain areas that are prone to repeatedly low visibility, particularly after massive pile-up accidents involving collision

Collision

A collision is an isolated event in which two or more bodies exert relatively strong forces on each other for a relatively short time....
s of several (or even dozen

Dozen

Dozen is another word for the number 12 . The dozen may be one of the earliest primitive groupings, perhaps because there are approximately a dozen cycles of the moon or months in a cycle of the sun or year....
s) of automobile

Automobile

An automobile or motor car is a wheeled motor vehicle for transportation passengers, which also carries its own car engine or motor. Most definitions of the term specify that automobiles are designed to run primarily on roads, to have seating for one to eight people, to typically have four wheels, and to be constructed principally f...
s have occurred there.

Low visibility warnings

In addition, an advisory is often issued by a government weather agency for low visibility, such as a dense fog advisory from the U.S. National Weather Service

National Weather Service

The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States Federal government of the United States....
. These generally advise motorists to avoid travel

Travel

Travel is the change in Location of people on a trip through the means of transport from one location to another. Travel is most commonly for recreation , for business trip or for commuting; but may be for numerous other reasons, such as migration, fleeing war, etc....
until the fog burns off or other conditions improve. Airport

Airport

An airport is a location where aircraft such as Fixed-wing aircraft, helicopters, and Non-rigid airship take off and land. Aircraft may also be stored or maintained at an airport....
travel is also often delayed by low visibility, sometimes causing long waits due to instrument flight rules

Instrument flight rules

Instrument flight rules are a set of regulations and procedures for flying aircraft whereby navigation and obstacle clearance is maintained with reference to aircraft instruments only, while separation from other aircraft is provided by Air Traffic Control....
and wider spacing of aircraft

Aircraft

An aircraft is a vehicle which is able to flight by being supported by the air, or in general, the atmosphere, of a planet. Examples include balloons, airplanes and helicopters....
.