Haze
Primary reference(s)
WMO, 2017. Haze. International Cloud Atlas, World Meteorological Organization (WMO). Accessed 13 October 2021.
Additional scientific description
As light is scattered by haze particles, distant bright objects or lights seen through the haze appear yellowish or reddish, while dark objects appear bluish. Haze particles may have a colour of their own that also contributes to this effect (WMO, 2017). In addition, haze has been described as particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog (AMS, 2012). Some specific types of haze include:
- Dust haze: A suspension in the air of dust or small sand particles, raised from the ground prior to the time of observation by a dust storm or sandstorm. The dust storm or sandstorm may have occurred either at or near the observation site or far from it (WMO, 2017).
- Sand haze: Haze caused by the suspension in the atmosphere of small sand or dust particles, raised from the ground prior to the time of observation by a sandstorm or dust storm (WMO, 1992).
Many aerosols increase in size with increasing relative humidity due to deliquescence, drastically decreasing visibility. On Köhler curve plots of saturation relative humidity versus aerosol particle radius, equilibrium haze particles are to the left of the peak, while growing cloud droplets are to the right (AMS, 2012).
Many haze formations are caused by the presence of an abundance of condensation nuclei which may grow in size, due to a variety of causes, and become mist, fog, or cloud. Distinction is sometimes drawn between dry haze and damp haze, largely on the basis of differences in optical effects produced by the smaller particles (dry haze) and larger particles (damp haze), which develop from slow condensation upon the hygroscopichaze particles (AMS, 2012).
Dry haze particles, with diameters of the order of 0.1 micron, are small enough to scatter shorter wavelengths of light preferentially (although not according to the inverse fourth-power law of Rayleigh scattering). Such haze particles produce a bluish colour when the haze is viewed against a dark background, for dispersion allows only the slightly bluish scattered light to reach the eye. The same type of haze, when viewed against a light background, appears as a yellowish veil, for here the principal effect is the removal of the bluer components from the light originating in the distant light-coloured background (AMS, 2012).
Haze may be distinguished by this same effect from mist, which yields only a grey obscuration, since the particle sizes are too large to yield appreciable differential scattering of various wavelengths (AMS, 2012).
Metrics and numeric limits
Not identified.
Key relevant UN convention / multilateral treaty
Not applicable.
Examples of drivers, outcomes and risk management
In meteorology, haze is caused by suspended dry solid particles in the air such as dust or salt. Haze is an atmospheric phenomenon in meteorological observation and the particles of which it is formed are termed lithometeors. It can obscure vision and directly influence horizontal visibility (AMS, 2012).
In terms of national alerting parameters, one example is the alerting parameters for sandstorm warning in China (China Meteorological Administration, 2012).
References
AMS, 2012. Haze. Glossary of Meteorology. American Meteorological Society (AMS). Accessed 22 November 2019.
China Meteorological Administration, 2012. Weather Warnings: Gale. Accessed 22 November 2019.
WMO, 1992. International Meteorological Vocabulary. WMO-No. 182. World Meteorological Organization (WMO). Accessed 27 November 2019.
WMO, 2017. Haze. International Cloud Atlas, World Meteorological Organization (WMO). Accessed 13 October 2020.