Visibility
Encyclopedia
In meteorology
, visibility is a measure of the distance
at which an object or light can be clearly discerned. It is reported within surface weather observation
s and METAR
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.
ICAO Annex 3 Meteorological Service for International Air Navigation contains the following definitions and note:
Annex 3 also defines Runway Visual Range (RVR)
as:
In extremely clean air in Arctic or mountainous areas, the visibility can be up to 70 kilometres (43.5 mi) to 100 kilometres (62.1 mi). However, visibility is often reduced somewhat by air pollution
and high humidity
. Various weather station
s report this as haze
(dry) or mist
(moist). Fog
and smoke
can reduce visibility to near zero, making driving
extremely dangerous. The same can happen in a sandstorm
in and near desert
areas, or with forest fires. Heavy rain
(such as from a thunderstorm
) not only causes low visibility, but the inability to brake
quickly due to hydroplaning. Blizzard
s and ground blizzards (blowing snow) are also defined in part by low visibility.
object being viewed against a perfectly white background. The visual contrast
, CV(x), at a distance x from the black
object is defined as the relative difference between the light intensity of the background and the object
where FB(x) and F(x) are the intensities of the background and the object, respectively. Because the object is assumed to be perfectly black
, it must absorb all of the light incident on it. Thus when x=0 (at the object), F(0) = 0 and CV(0) = 1. Between the object and the observer, F(x) is affected by additional light that is scattered
into the observer's line of sight and the absorption
of light by gases and particles. Light scattered by particles outside of a particular beam may ultimately contribute to the irradiance
at the target, a phenomenon known as multiple scattering. Unlike absorbed light, scattered light is not lost from a system. Rather, it can change directions and contribute to other directions. It is only lost from the original beam traveling in one particular direction. The multiple scattering's contribution to the irradiance
at x is modified by the individual particle scattering coefficient, the number concentration of particles, and the depth of the beam. The intensity change dF is the result of these effects over a distance dx. Because dx is a measure of the amount of suspended gases and particles, the fraction of F that is diminished is assumed to be proportional to the distance, dx. The fractional reduction in F is
where bext is the extinction coefficient
. The scattering
of background light into the observer's line of sight can increase F over the distance dx. This increase is defined as b' FB(x) dx, where b is a constant. The overall change in intensity is expressed as
Since FB represents the background intensity, it is independent of x by definition. Therefore,
It is clear from this expression that b must be equal to bext. Thus, the visual contrast
, CV(x), obeys the Beer–Lambert law
which means that the visual contrast
decreases exponentially with the distance from the object:
Lab experiments have determined that contrast ratios between 0.018 and 0.03 are perceptible under typical daylight viewing conditions. A contrast ratio of 2% (CV = 0.02) is usually used to calculate visual range. Plugging this value into the above equation and solving for x produces the following visual range expression (the Koschmeider equation):
with xv in units of meters
. At sea level, the Rayleigh atmosphere
has an extinction coefficient of approximately 13.2 X 10−6 m−1 at a wavelength
of 520 nm. This means that in the cleanest possible atmosphere, visibility is limited to about 296 km.
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 arepii e to repeatedly low visibility, particularly after traffic collisions or pile-ups involving multiple vehicles.
. These generally advise motorists to avoid travel
until the fog burns off or other conditions improve. Airport
travel is also often delayed by low visibility, sometimes causing long waits due to instrument flight rules
and wider spacing of aircraft
.
and scattering
of light by particles and gases in the atmosphere. Absorption
of electromagnetic radiation
by gases and particles is sometimes the cause of discolorations in the atmosphere but usually does not contribute very significantly to visibility degradation. Scattering
by particulate, on the other hand, impairs visibility much more readily. Visibility is reduced by significant scattering
from particles between an observer and a distant object. The particles scatter
light from the sun
and the rest of the sky through the line of sight of the observer, thereby decreasing the contrast between the object and the background sky. Particles that are the most effective at reducing visibility (per unit aerosol
mass) have diameters in the range of 0.1-1.0 µm. The effect of air molecules on visibility is minor for short visual ranges but must be taken into account for ranges above 30 km.
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
, 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, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical distance...
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 climatological reasons to forecast weather and issue warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using...
s and METAR
METAR
METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, 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.
Definition
- 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 candelaCandelaThe 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 . A common candle emits light with a luminous intensity of roughly one candela...
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.
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 cause damage to the natural environment or built environment, into the atmosphere....
and high humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
. Various weather station
Weather station
A weather station is a facility, either on land or sea, with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind...
s report this as haze
Haze
Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...
(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 exhaled air in the cold, and in a steam room of a sauna. It can also be created artificially with aerosol canisters if the...
(moist). Fog
Fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...
and smoke
Smoke
Smoke is a 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 entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
can reduce visibility to near zero, making driving
Driving
Driving is the controlled operation and movement of a land vehicle, such as a car, truck or bus.Although direct operation of a bicycle and a mounted animal are commonly referred to as riding, such operators are legally considered drivers and are required to obey the rules of the road...
extremely dangerous. The same can happen in a sandstorm
Dust storm
A dust / sand storm is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Particles are transported by saltation and suspension, causing soil to move from one place and deposition...
in and near desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
areas, or with forest fires. Heavy rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
(such as from a thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
) not only causes low visibility, but the inability to brake
Brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
quickly due to hydroplaning. Blizzard
Blizzard
A blizzard is a severe snowstorm characterized by strong winds. By definition, the difference between blizzard and a snowstorm is the strength of the wind. To be a blizzard, a snow storm must have winds in excess of with blowing or drifting snow which reduces visibility to 400 meters or ¼ mile or...
s and ground blizzards (blowing snow) are also defined in part by low visibility.
Derivation
To define visibility, we examine the case of a perfectly blackBlack body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
object being viewed against a perfectly white background. The visual contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
, CV(x), at a distance x from the black
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
object is defined as the relative difference between the light intensity of the background and the object
where FB(x) and F(x) are the intensities of the background and the object, respectively. Because the object is assumed to be perfectly black
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
, it must absorb all of the light incident on it. Thus when x=0 (at the object), F(0) = 0 and CV(0) = 1. Between the object and the observer, F(x) is affected by additional light that is scattered
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
into the observer's line of sight and the absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of light by gases and particles. Light scattered by particles outside of a particular beam may ultimately contribute to the irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
at the target, a phenomenon known as multiple scattering. Unlike absorbed light, scattered light is not lost from a system. Rather, it can change directions and contribute to other directions. It is only lost from the original beam traveling in one particular direction. The multiple scattering's contribution to the irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...
at x is modified by the individual particle scattering coefficient, the number concentration of particles, and the depth of the beam. The intensity change dF is the result of these effects over a distance dx. Because dx is a measure of the amount of suspended gases and particles, the fraction of F that is diminished is assumed to be proportional to the distance, dx. The fractional reduction in F is
where bext is the extinction coefficient
Extinction coefficient
Extinction coefficient refers to several different measures of the absorption of light in a medium:*In chemistry, biochemistry, molecular biology, or microbiology, the mass extinction coefficient and the molar extinction coefficient are parameters defining how strongly a substance absorbs light...
. The scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
of background light into the observer's line of sight can increase F over the distance dx. This increase is defined as b' FB(x) dx, where b is a constant. The overall change in intensity is expressed as
Since FB represents the background intensity, it is independent of x by definition. Therefore,
It is clear from this expression that b must be equal to bext. Thus, the visual contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
, CV(x), obeys the Beer–Lambert law
which means that the visual contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
decreases exponentially with the distance from the object:
Lab experiments have determined that contrast ratios between 0.018 and 0.03 are perceptible under typical daylight viewing conditions. A contrast ratio of 2% (CV = 0.02) is usually used to calculate visual range. Plugging this value into the above equation and solving for x produces the following visual range expression (the Koschmeider equation):
with xv in units of meters
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
. At sea level, the Rayleigh atmosphere
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
has an extinction coefficient of approximately 13.2 X 10−6 m−1 at a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 520 nm. This means that in the cleanest possible atmosphere, visibility is limited to about 296 km.
Fog, mist, and haze
The international definition of fog is a visibility of less than 1 kilometres (3,280.8 ft); mist is a visibility of between 1 kilometre (0.621372736649807 mi) and 2 kilometres (1.2 mi) and haze from 2 kilometres (1.2 mi) to 5 kilometres (3.1 mi). 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 100 metres (328.1 ft) are usually reported as zero. In these conditions, roadRoad
A road is a thoroughfare, route, or way on land between two places, which typically has been paved or otherwise improved to allow travel by some conveyance, including a horse, cart, or motor vehicle. Roads consist of one, or sometimes two, roadways each with one or more lanes and also any...
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 arepii e to repeatedly low visibility, particularly after traffic collisions or pile-ups involving multiple vehicles.
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 ServiceNational 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 government...
. These generally advise motorists to avoid travel
Travel
Travel is the movement of people or objects between relatively distant geographical locations. 'Travel' can also include relatively short stays between successive movements.-Etymology:...
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 blimps take off and land. Aircraft may 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 one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
and wider spacing of aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
.
Visibility and Air Pollution
A visibility reduction is probably the most apparent symptom of air pollution. Visibility degradation is caused by the absorptionAbsorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
and scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
of light by particles and gases in the atmosphere. Absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
by gases and particles is sometimes the cause of discolorations in the atmosphere but usually does not contribute very significantly to visibility degradation. Scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
by particulate, on the other hand, impairs visibility much more readily. Visibility is reduced by significant scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
from particles between an observer and a distant object. The particles scatter
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
light from the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the rest of the sky through the line of sight of the observer, thereby decreasing the contrast between the object and the background sky. Particles that are the most effective at reducing visibility (per unit aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
mass) have diameters in the range of 0.1-1.0 µm. The effect of air molecules on visibility is minor for short visual ranges but must be taken into account for ranges above 30 km.