© George Anderson
Bracknell, England, United Kingdom of Great Britain and Northern Ireland
Latitude: 51° 25' 26'' N
Longitude: 0° 48' 25'' W
01 November 2016 1038 (Local Time)
Camera direction: towards ESE
Image P/S code: P.11.1.1.1
Image I.D.: 5186
CL = 6, CM = /, CH = /
At the time of this photograph in the late morning, overnight fog was slowly lifting into mist and low Stratus. The horizontal visibility was estimated to be 1 100 m.
Mist is a suspension of very small, usually microscopic water droplets in the air that reduce the horizontal visibility at the Earth's surface to not less than 1 km. When the visibility is less than 1 km, the obscuration is due to fog. In practice, mist is considered synonymous with “light fog”.
The bank of trees on the right are about 450 m away from the observation point. In the distance, trees at 800 m are visible, and more distant ground features are just discernible on the far left at around 1 100 m. The overcast layer of low cloud is Stratus nebulosus and, as the Sun is not visible, the variety is opacus.
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A ridge of high pressure covered the British Isles, in association with an anticyclone (1 039 hPa) centred over the eastern Atlantic Ocean. Fog formed widely over southern England during the night and only slowly lifted during daytime into low cloud. A weak cold front moved slowly southwards into the area later in the day.
This picture shows radiation fog that has formed overnight due to radiational cooling from the ground. The fog is deep enough (greater than 2 m) to restrict the horizontal visibility such that it obscures the lower parts of the trees in the distance. Above the fog the visibility is greater, but is restricted by mist.
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Ground fog, also known as shallow fog, is a radiation fog that is shallow in depth. Over land, the depth is defined as 2 m or less; above this height, the horizontal visibility is not restricted by the fog.
This picture was taken at Jersey Airport, Jersey (WMO 03895), about 15 minutes before sunrise. In the background, the fog at 2 and 3 has thickened to a depth greater than 2 m. The complex microphysics of fog formation is shown here, as shallow fog has formed over grass (grass minimum: 2.0 °C), but has not formed over the taxiway or runway (concrete minimum: 4.9 °C). The air minimum was 8.4 °C.
With the general relative humidity of the air measured as 97%, the reduction in horizontal visibility above the ground fog to 7 km is due to mist.
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A change in wind direction brought humid maritime air into Hong Kong Airport (China), resulting in a rapid deterioration in the visibility, together with the formation of upslope Stratus on the hills.
From the perspective of the airport meteorological observer, mist was reported. At 0700 hours, the prevailing visibility was 6 km but had deteriorated to 1 600 m to the west. At 0730 and 0800 hours, the general visibility was 2 900 m and 2 400 m respectively.
As moist air ascended the hills, Stratus cloud formed due to the air being adiabatically cooled to its saturation temperature. The result was upslope fog. Upslope fog is a type of hill fog. When viewed from beneath, it is seen as Stratus. However, to anyone on the hill ascending into the cloud, the visibility from that person's perspective is reduced to fog.
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Widespread mist was reported across southern China (see the accompanying synoptic chart). At Hong Kong airport, at the time of the photograph, the visibility was 1 200 m.
It is likely that a significant amount of particulate matter from industry has been favourable to condensation at well below 90 %RH. The significantly reduced visibility here is probably a combination of both haze and mist, with particulates acting as condensation nuclei for water droplets.
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