Stratus and Fog are phenomena that are easy to differentiate for a human observer at ground level. The definition of each has been formulated as follows:

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.

Favoured synoptic environment for St/fog

Fog (or low Stratus) is formed, when moist air near ground level starts to condensate. This condensation can be produced in the atmosphere by three mechanisms:

• By ascent and resultant cooling of an air parcel.
• By radiative heat loss.
• By the mixing of two parcels of slightly unsaturated air initially having different temperatures.

The necessary condition for both the advection and radiation St/fog formation is a sufficient moisture content in the lowest layers of the atmosphere. Otherwise the conditions differ greatly for these Fog types.

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.

Diagram showing the synoptic conditions for night-time Radiation Fog (left) and Advection St/fog (right) with front and side view panels. The typical vertical distribution of temperature, dew point temperature and wind are presented in the front panels.

Radiation Fog

Radiation Fog forms, when the ground cools rapidly after sunset by longwave radiation to space. The cooling of the air near the ground makes it increasingly thermally stable, which weakens and eventually stops turbulence above ground. As radiative cooling continues, excess water vapour begins to condense into Fog droplets. As soon as the deepening Fog layer becomes optically thick (some tens of metres), the upper part of it becomes the effective interface to space.

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.

As the Fog gets thicker, it becomes more uniform in the vertical, with a well-defined top. Depending on the season and synoptic conditions the Radiation Fog may or may not develop into a mature Fog. For example in summer, Fog layers are most often thin due to short nights. In the winter, however, under stationary synoptic situations Radiation Fog or lifted Fog (Stratus) can last for days.

The dissipation of Radiation Fog can take place through the following mechanisms:

• The spreading of a cloud sheet above the Fog, which reduces the radiative cooling at the Fog top.
• Atmospheric layers with high amounts of water vapour can help to dissipate fog by long wave radiation.
• Heat from the ground can lift the Fog into low cloud.
• Increasing shear at Fog top caused by increasing wind above the Fog entrains drier air into the cloud and accelerates the dissipation.
• In mature Fogs the settling of droplets to the ground or onto vegetation contributes significantly to the dissipation.
• Solar radiation (excluding mid-winter) can penetrate to the ground in places where Fog is thinner (such as elevated places) and heat the ground and evaporate the Fog. Consequently, heat will be convected into the cloud. During summertime due to short nights the Fog has no time to thicken long enough, before the morning sun already starts heating the ground and breaks the Fogs into patches and later maybe into fair-weather cumulus.

In geostationary satellite image loops the dissipation of Radiation Fog can be seen as shrinking of the Fog from its outer edges. This is due to differential heating between the foggy and fog-free regions. If there is variability in the brightness of the Fog top, the brightest points will most likely be the most persistent.

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.

NOAA 124 images often show dissipating Radiation Fog turning from yellow towards brownish colours before dissipation. This can be explained as follows: as the Fog gets thinner, its reflectivity decreases, which decreases the contribution of Ch1 and, especially, of Ch2 components in RGB imagery. Local clearer patches within dispersing radiation fog have the same effect. These factors cause the brownish tones in thin St/fog layers. The satellite images show the dissipation of relatively shallow (thickness only 100-300 m) Radiation Fog sheet over Finland. The Radiation Fog is already thinning inland in the first image, and the charasteristic, brownish-yellow colour is evident there at 06.00 UTC. At the same time fog seems to be thicker near the Gulf of Finland, and in the last satellite image this is verified, as the Fog has dissipated totally over heated land, but still persisting over the sea. It has to be kept in mind, however, that the differentiation of the yellowish and brownish clouds may be affected by, e.g., solar angle.

Advection Fog

When moist and warm air is advected by the wind, turbulent mixing in the boundary layer moistens the air from below. This results in advection stratus clouds. When Stratus encounters cooler waters (with sea surface temperatures lower than air dewpoint temperature) or cold (possibly snow-covered) land, turbulence and thermal convection decrease significantly, but the water vapour-driven convection from the surface continues to mix the overlying air. This leads to cooling of the unsaturated air and lowering of the cloud base, eventually even to the formation of Advection Fog.

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.

The IR satellite image sequence shows a relatively extensive Stratus sheet being advected from the North Sea to Northern Germany and further southeast. The movement of the cloud sheet is easy to follow on this occasion; even in nighttime images the cloud sheet is distinguishable from its surroundings, especially in the cold, clear Alpine region. The air flow configuration shows typical features for wintertime advection St/fog: a very wide warm sector is covering southern Scandinavia, western and Central Europe. Upper level air is dry, whereas in the boundary layer (925 hPa) the numerical model finds extensive low-level cloud located over northern parts of Central Europe. The boundary layer wind is apparently quite strong. Comparison between the numerical model output and NOAA 124 image shows that Stratus cloud cover is, in reality, much more extensive than the model cloud at 925 hPa. For example over the southeastern part of the North Sea and Denmark there is stratus, even though the model does not show cloudiness there. This might be a model failure, but more likely the Stratus cloud tops are below 925 hPa. The example shows, however, that care has to be taken when diagnosing Stratus from mandatory main pressure level charts.