Haze often occurs when 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 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, Indonesian forest fires burnt to clear land being the reason. 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. [2] 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 to pre-industrial levels.

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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. [4] Obscuration[edit] Haze causes issues in the area of terrestrial photography, 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 through the haze particles. For these reasons, sunrise and sunset colors appear subdued on hazy days, and stars may be obscured at night. In some cases, attenuation by haze is so great that, toward sunset, the sun disappears altogether before reaching the horizon. 5] Haze can be defined as an aerial form of the Tyndall effect therefore unlike other 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 many super-telephoto lenses often incorporate yellow 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 (such as the Wratten 89B) and IR-sensitive detector.