Haze is traditionally an atmospheric 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, volcanic ash, dust, sand and snow. Sources for haze particles include farming (ploughing in dry weather), traffic, industry, forest fires and peat field fire.
Seen from afar (e.g. approaching airplane), haze appears brownish, while mist is bluish-grey. Whereas haze formation is a phenomenon of dry air, mist formation is a phenomenon of humid air. However, haze particles may act as condensation nuclei for later mist droplet formation.
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. In response the ASEAN countries agreed on a Regional Haze Action Plan (1997) and later signed the Agreement on Transboundary Haze Pollution (2002). Under the agreement the ASEAN secretariat hosts a co-ordination and support unit.
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.