Fog
Primary reference(s)
WMO, 2017. Fog. International Cloud Atlas: World Meteorological Organization (WMO). Accessed 20 November 2019.
Additional scientific description
Under light wind, stable and humid conditions, if the air near the ground cools sufficiently, water vapour in the air may condense into tiny water droplets. These droplets reduce the visibility near ground level. This phenomenon is called fog (Hong Kong Observatory, 2019).
Fog differs from cloud only in that its base is at the Earth’s surface while the cloud base is above the Earth’s surface. When composed of ice crystals, it is termed ice fog (WMO, 2017). Visibility reduction in fog depends on the concentration of cloud condensation nuclei and the resulting distribution of droplet sizes. According to American Meteorological Society (AMS) weather observing practice, fog that hides less than 0.6 of the sky is called ground fog. If fog is so shallow that it is not an obstruction to vision at a height of 6 feet above the surface, it is called simply shallow fog (AMS, 2012).
Fog is easily distinguished from haze by its higher relative humidity (near 100%, having physiologically appreciable dampness) and grey colour. Haze does not contain activated droplets larger than the critical size according to Köhler theory (AMS, 2012).
Mist may be considered an intermediate between fog and haze; its particles are smaller (a few microns maximum), it has lower relative humidity than fog, and does not obstruct visibility to the same extent (AMS, 2012).
There is no distinct line, however, between any of these categories. Near industrial areas, fog is often mixed with smoke, and this combination has been known as smog. However, fog droplets are usually absent in photochemical smog, which only contains inactivated haze droplets (AMS, 2012).
In aviation weather observations fog is encoded F, and ground fog GF (AMS, 2012). Metrics and numeric limits Not identified.
Metrics and numeric limits
Not identified.
Key relevant UN convention / multilateral treaty
Not applicable.
Examples of drivers, outcomes and risk management
Fogs of all type originate when the temperature and dew point of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dew point (producing advection fog, radiation fog or upslope fog), or by adding moisture and thereby elevating the dew point (producing steam fog or frontal fog). Fog seldom forms when the dew point spread is greater than 4°F (AMS, 2012).
Fog is defined as ‘obscurity in the surface layers of the atmosphere, which is caused by a suspension of water droplets’. By international agreement (particularly for aviation purposes) fog is the name given to resulting visibility of less than 1 km (0.6 mile), however in forecasts for the public this generally refers to visibility of less than 180 m (0.1 mile) (UK Met Office, 2019).
In terms of national alerting parameters, examples include alerting parameters for issuing a fog advisory in Canada (Government of Canada, 2019) and alerting parameters for a heavy fog warning in China (China Meteorological Administration, 2012).
References
AMS, 2012. Fog. Glossary of Meteorology. American Meteorological Society (AMS). Accessed 20 November 2019.
China Meteorological Administration, 2012. Weather Warnings: Heavy Fog. Accessed 20 November 2019.
Government of Canada, 2019. Alerting parameters Environment Canada uses for issuing a Fog Advisory. Accessed 20 November 2019.
Hong Kong Observatory, 2019. Fog Explained. Accessed 20 November 2019.
UK Met Office, 2019. What is the difference between mist, fog and haze? Accessed 20 November 2019.
WMO, 2017. Ice Fog. Accessed 23 March 2021.