by FMI
The reason why these two phenomena are discussed together is the fact that for satellite sensors viewing them from above the differentiation is very difficult, at least with current techniques. The only real difference between Fog and Stratus is the different altitude of the cloud base, which for Stratus lies a few hundred meters above ground, whereas in Fog the cloud base descends to ground level.
This chapter mainly focuses on advection Stratus/fog and Radiation Fog.
Radiation Fog formation typically calls for clear skies, ample moisture in the surface layer and light winds. Preferred locations to fullfill these conditions can be found adjacent to high pressures over land (mostly during the winter season) with associated weak pressure gradient. Wetness of soil significantly increases the chances of Radiation Fog, for which reason a very favoured situation for Fog formation is the sky clearing and wind decreasing in the evening after a rainy day. Too strong wind will most likely create Stratocumulus than Stratus.
For the Advection Fog the important factors are the advection of moisture and temperature by the wind. Depending on the wind speed and fetch over water the interaction between the cooler surface layer and the overlying air results in low-level Stratus cloudiness and even Fog. As an average moderate (4-7 m/s) surface winds are typical for Advection Fog occurrences. Moist and warm southwesterly or southerly air streams over cool waters such as over cold ocean currents can create very extensive Fog sheets. Advection Fog is also frequently observed in wintertime low pressure warm sector areas over the continents.
Radiation fog
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Advection fog
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The formation phase of Radiation Fog may be very hard to follow in satellite images, as the development often takes place at night, when visible satellite images are not usable. In cases where there is enough sunlight, very thin Fog may be distinguished with NOAA 124 images, thin Fog layers being very deep yellow, even brownish in colour (see Cloud structure in satellite image). The poor temporal resolution of polar satellite imagery greatly restricts the usefulness of this method. Better temporal, but poorer spatial resolution of geostationary satellites reduces their capability to show the initial stages of Radiation Fog.
Enhanced IR data can in some cases be used for the detection of moist boundary layers and, hence, for forecasting the probable locations for Radiation Fog a few hours in advance. There are two reasons. Firstly, in a moist air mass the ground cools slower than in a dry regime. Secondly, even if the temperature of dry and moist regimes is equal, the moist boundary layer would still appear warmer in IR imagery due to the radiation from water vapour which is detected at these wavelengths.
The dissipation of Radiation Fog can take place through the following mechanisms:
Radiation Fog duration after sunrise can be approximated by comparing pixel brightness differences between Fog and adjacent land. The brightness difference can further be normalised with daily incoming solar radiation to account partially for variations in solar radiation with latitude, time of year and time of day. The following image shows a graph presented by Gurka (1978). The graph is applicable in for USA during late summer and autumn, and certain geographical tuning is needed when using it in other environments. For example, Anthis and Cracknell (1999) have used rather similar techniques for forecasts of Fog dissipation in Greece.
05 September 1999/04.34 UTC - NOAA RGB image (channel 1, 2 and 4)
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05 September 1999/05.52 UTC - NOAA RGB image (channel 1, 2 and 4)
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05 September 1999/07.32 UTC - NOAA RGB image (channel 1, 2 and 4)
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Important factors for the formation of Advection Fog are the fetch over the sea (or over land with wet snow) and wind speed. A long fetch together with not too high a wind speed allows the cloud to sink lower and eventually form Fog. Over land areas the snow temperature is an important factor: snow having below-zero temperatures can easily absorb moisture from overlying air. This is due to the smaller water vapour pressure for ice surfaces compared to liquid water surfaces. Fog formation over dry snow is, therefore, much more common than over wet snow, as wet snow is a source of moisture.
Advection Fog can be expected to be persistant, since the synoptical situation does not change rapidly. In wintertime warm sectors the middle and upper troposphere are normally relatively dry, which helps the Advection Fog to develop further through longwave cooling. Either a frontal passage and change of air-mass, increasing wind/turbulence or the advection of mid-level or low-level cloud over the St/fog top are needed for dissipation to take place. The advection of sea Fog over warmer water can also lead to lifted stratus and possibly dispersal of cloud.
Advection Fog has the same tendency to sink from the outer edges as Radiation Fog, but the advection of the clouds has to be taken into account. Brighter spots indicating thicker and more persistent cloud may be carried downwind, while the upwind edges of cloud will clear more rapidly.
Below: Advection St/fog in Central Europe and on the North Sea. Rough outlines of Stratus sheet are shown with a dashed line.
25 January 2000/12.00 UTC - Meteosat IR image
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25 January 2000/18.00 UTC - Meteosat IR image
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26 January 2000/00.00 UTC - Meteosat IR image
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26 January 2000/06.00 UTC - Meteosat IR image
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26 January 2000/12.00 UTC - Meteosat IR image
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26 January 2000/18.00 UTC - Meteosat IR image
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27 January 2000/00.00 UTC - Meteosat IR image
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27 January 2000/06.00 UTC - Meteosat IR image
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26 January 2000/12.00 UTC - HIRLAM 500 hPa height (black), temperature (C, red/blue) and cloud water (g/kg)
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26 January 2000/12.00 UTC - HIRLAM 925 hPa height (black), layer-averaged temperature 850-1000 hPa (C, red/blue)
and cloud water (g/kg)
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