Haze is traditionally an atmospheric phenomenon where dust, smoke and additional 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 ash, dust, sand and snow. Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires.
Seen from afar (e.g. approaching airplane) and depending upon the direction of view with respect to the sun, haze might appear brownish or bluish, while mist tends to be bluish-grey. Whereas haze often is thought of as a phenomenon of dry air, mist formation is a phenomenon of humid air. Notwithstanding haze particles might act as condensation nuclei for the subsequent formation of mist droplets; such forms of haze are known as "wet haze."
The term "haze", in meteorological literature, generally is used to denote visibility-reducing aerosols of the wet type. Such aerosols commonly arise from complex chemical reactions that occur as sulfur dioxide gases emitted throughout combustion are converted into small droplets of sulphuric acid. The reactions are enhanced in the presence of sunlight, high relative humidity, and stagnant air flow. A small component of wet haze aerosols appear to be derived from compounds released by trees, such as terpenes. For all these reasons, wet haze tends to be primarily a warm-season phenomenon. Large areas of haze covering a large number of thousands of km might be produced under favourable conditions each summer.
Haze often occurs when dust and smoke particles accumulate in relatively dry air. When weather conditions block the dispersal of smoke and additional pollutants they concentrate and form a usually low-hanging shroud that impairs visibility and might become a respiratory health threat. Industrial pollution can result in dense haze, which is known as smog.
Since 1991, haze has been a particularly acute problem in Southeast Asia. The main source of the haze has been fires occurring in Sumatra and Borneo. In response to the 1997 Southeast Asian haze, the ASEAN countries agreed on a Regional Haze Action Plan (1997). In 2002, all ASEAN countries except Indonesia signed the Agreement on Transboundary Haze Pollution, but the pollution is still a problem 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 Pollution Standards Index reaching a record high of 401.
In the United States, the Interagency Monitoring of Protected Visual Environments (IMPROVE) programme 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 to pre-industrial levels. Additionally, the Clean Air Act requires that any current visibility problems be 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.
Haze is no longer a domestic problem. It has become one of the causes of international disputes among neighbouring countries. Haze migrates to adjacent countries and thereby pollutes additional countries as well. One of the most recent problems concerned the two neighbouring countries Malaysia and Indonesia. In 2013, due to forest fires in Indonesia, the capital city of Malaysia Kuala Lumpur and surrounding areas became shrouded in a pall of noxious fumes, smelling of ash and coal for more than a week, in the country’s worst environmental crisis after 1997. The main sources of the haze are Indonesia’s Sumatra Island, Kalimantan, and Riau, where farmers, plantation owners and miners have set hundreds of fires in the forests to clear land throughout dry weather. Winds blow most of the fumes across the narrow Strait of Malacca to Malaysia, although parts of Indonesia are additionally affected. The 2015 Southeast Asian haze constitutes an ongoing crisis.
Haze causes issues in the area of terrestrial photography, where the penetration of large amounts of dense atmosphere might 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 through the haze particles. For these reasons, sunrise and sunset colours appear subdued on hazy days, and stars might be obscured at night. In a few cases, attenuation by haze is so great that, toward sunset, the sun disappears altogether before reaching the horizon. Haze can be defined as an aerial form of the Tyndall effect therefore unlike additional atmospheric effects such as cloud and fog, haze is spectrally selective: shorter (blue) wavelengths are scattered more, and longer (red/infrared) wavelengths are scattered less. For this reason a large number of super-telephoto lenses often incorporate yellow filters or coatings to enhance image contrast. Infrared (IR) imaging might additionally be used to penetrate haze over long distances, with a combination of IR-pass optical philtres (such as the Wratten 89B) and IR-sensitive detector.