Haze is traditionally an

atmospheric phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the sun or moon with ...

in which dust, smoke, and other dry

particulates Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...

suspended in air obscure

visibility The visibility is the measure of the distance at which an object or light can be clearly discerned. In meteorology it depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time of ...

and the clarity of the sky. The

World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Internati ...

manual of codes includes a classification of particulates causing horizontal obscuration into categories of

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...

ice fog Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It occurs only in cold areas of the world, as water droplets suspended in the air can remain liquid down to . It should be distinguished from diamond dust, a pre ...

, steam fog,

mist Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a dispersion. It is most commonly seen where water vapor in warm, moist air meets sudden cooling, such a ...

, haze,

smoke Smoke is a suspension of airborne 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 ...

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

, and

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

. Sources for particles that cause haze include farming (

ploughing A plough or plow ( US; both ) is a farm tool for loosening or turning the soil before sowing seed or planting. Ploughs were traditionally drawn by oxen and horses, but in modern farms are drawn by tractors. A plough may have a wooden, iron or ...

in dry weather), traffic, industry,

volcanic activity Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

and

wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...

s. Seen from afar (e.g. an approaching airplane) and depending on the direction of view with respect to the Sun, haze may appear brownish or bluish, while mist tends to be bluish grey instead. Whereas haze often is thought of as a phenomenon occurring in dry air, mist formation is a phenomenon in saturated, humid air. However, haze particles may act as

condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This ca ...

that leads to the subsequent vapor condensation and formation of mist droplets; such forms of haze are known as "wet haze." In meteorological literature, the word ''haze'' is generally used to denote visibility-reducing

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...

s of the wet type suspended in the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

. Such aerosols commonly arise from complex

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

s that occur as

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

gases emitted during

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...

are converted into small droplets of

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

when exposed. The reactions are enhanced in the presence of sunlight, high relative humidity, and an absence of air flow (wind). A small component of wet-haze aerosols appear to be derived from compounds released by trees when burning, such as

terpenes Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ar ...

. For all these reasons, wet haze tends to be primarily a warm-season phenomenon. Large areas of haze covering many thousands of kilometers may be produced under extensive favorable conditions each summer.

Air pollution

Haze often occurs when suspended dust and smoke particles accumulate in relatively dry air. When weather conditions block the dispersal of smoke and other pollutants they concentrate and form a usually low-hanging shroud that impairs visibility and may become a

respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...

health threat if excessively inhaled. Industrial pollution can result in dense haze, which is known as

smog Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and '' fog'' to refer to smoky fog due to its opacity, and odor. The word was then inte ...

. Since 1991, haze has been a particularly acute problem in Southeast Asia. The main source of the haze has been smoke from fires occurring in Sumatra and Borneo which dispersed over a wide area. In response to the 1997 Southeast Asian haze, the ASEAN countries agreed on a Regional Haze Action Plan (1997) as an attempt to reduce haze. In 2002, all ASEAN countries signed the Agreement on Transboundary Haze Pollution, but the pollution is still a problem there today. Under the agreement, the ASEAN secretariat hosts a co-ordination and support unit. During the 2013 Southeast Asian haze, Singapore experienced a record high pollution level, with the 3-hour

Pollutant Standards Index The Pollutant Standards Index (PSI) is a type of air quality index, which is a number used to indicate the level of pollutants in air. Initially PSI was based on five air pollutants, but since 1 April 2014 it has also included fine particulat ...

reaching a record high of 401. In the United States, the Interagency Monitoring of Protected Visual Environments (IMPROVE) program was developed as a collaborative effort between the US EPA and the National Park Service in order to establish the chemical composition of haze in National Parks and establish air pollution control measures in order to restore the visibility of the air to pre-industrial levels. Additionally, the Clean Air Act requires that any current visibility problems be addressed and remedied, and future visibility problems be prevented, in 156 Class I Federal areas located throughout the United States. A full list of these areas is available on EPA's website.

International disputes

Transboundary haze

Haze is no longer just a confined as a domestic problem. It has become one of the causes of international disputes among neighboring countries. Haze can migrate to adjacent countries in the path of wind and thereby pollutes other countries as well, even if haze doesn't first manifest there. One of the most recent problems occur in Southeast Asia which largely affects the nations of Indonesia, Malaysia and Singapore. In

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, due to forest fires in Indonesia,

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and surrounding areas became shrouded in a pall of noxious fumes dispersed from Indonesia, that brings a smell of ash and coal for more than a week, in the country's worst environmental crisis since

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. The main sources of the haze are Indonesia's Sumatra Island, Indonesian areas of Borneo, and Riau, where farmers, plantation owners and miners have set hundreds of fires in the forests to clear land during dry weather. Winds blew most of the particulates and fumes across the narrow

Strait of Malacca The Strait of Malacca is a narrow stretch of water, 500 mi (800 km) long and from 40 to 155 mi (65–250 km) wide, between the Malay Peninsula (Peninsular Malaysia) to the northeast and the Indonesian island of Sumatra to the southwest, connec ...

to Malaysia, although parts of Indonesia in the path are also affected. The 2015 Southeast Asian haze was another major crisis of air quality, although there were occasions such as the

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and

2019 File:2019 collage v1.png, From top left, clockwise: Hong Kong protests turn to widespread riots and civil disobedience; House of Representatives votes to adopt articles of impeachment against Donald Trump; CRISPR gene editing first used to experim ...

haze which were less impactful than the three major Southeast Asian haze of 1997, 2013 and 2015.

Obscuration

Haze causes issues in the area of terrestrial photography and imaging, where the penetration of large amounts of dense atmosphere may be necessary to image distant subjects. This results in the visual effect of a loss of contrast in the subject, due to the effect of light

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

and

reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...

through the haze

particles In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

. For these reasons, sunrise and sunset colors and possibly the sun itself appear subdued on hazy days, and stars may be obscured by haze at night. In some cases, attenuation by haze is so great that, toward sunset, the sun disappears altogether before even reaching the horizon. Haze can be defined as an aerial form of the

Tyndall effect The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to ...

therefore unlike other atmospheric effects such as cloud,

and

, haze is spectrally selective in accordance to the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

: shorter (blue) wavelengths are scattered more, and longer (red/

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

) wavelengths are scattered less. For this reason, many super-telephoto lenses often incorporate yellow light filters or coatings to enhance image contrast. Infrared (IR) imaging may also be used to penetrate haze over long distances, with a combination of IR-pass optical filters and IR-sensitive detectors at the intended destination.

Notes

External links