Haze is traitionally 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.