FOG AND STRATUS - CLOUD STRUCTURE IN SATELLITE IMAGES
- The satellite image shows cloud sheets, which vary in size from very small (only a few tens of kilometres or even
smaller, depending on satellite radiometer resolution) areas to extensive features (up to a few hundreds of thousands
of square kilometres). In geostationary satellite imagery the top of Stratus or Fog (abbreviated to St/fog) has a
homogeneous appearance both in VIS and IR imagery. The tops of extensive St/fog areas over the oceans often have a
particularly uniform appearance.
- In the VIS images the grey shades of St/fog are usually white or grey depending on the thickness of the cloud
sheet and the sun elevation. The overall flat appearance is characteristic for St/fog.
- In IR images (the wavelengths between 10 and 12 µm) the grey shades of cloud sheets are usually grey or dark
grey indicating relatively low (warm) cloud tops. The detection of cloud sheets using the IR image can be difficult,
because the temperature difference between cloud tops and land surface is often small. The temperature difference can
even be reversed, that is, St/fog top can show warmer temperature than surrounding cloud-free land surface areas
causing a black St/fog. This is especially the case in high latitudes during the winter when, in situations of
intensive low-level temperature inversion, Fog tops can be much warmer than adjacent land areas.
- Single geostationary satellite IR images may not show well the development of advection stratus, if the
temperature difference between stratus top and land surface is small. Animated loops greatly help in detecting these
clouds and their movement. VIS images show stratus clouds much better because of their strong reflectance.
- WV images cannot be used for the detection of St/fog, as low level cloudiness does not contribute to the WV image.
However, WV images can sometimes be used indirectly in combination with VIS images; the advection of moist air (even
without any cloudiness in IR or VIS images) above a St/fog-topped boundary layer can assist in the dispersion of
- The combination of images, such as the polar orbiting satellites daytime RGB images (combining wavelenghts 0.6
µm, 0.9 µm and 10.8 µm (AVHRR channels 1,2 and 4, respectively) - hereafter referred to as NOAA 124
images - or of individual wavelengths 3.7 µm, 10.8 µm and 12 µm (AVHRR channels 3,4, and 5,
respectively) - hereafter referred to as NOAA 345 images - greatly help in identifying low level St/fog (see
subchapter for St/fog in AVHRR Imagery)). The great advantage of these combination channels is that the low level
cloud layers can be seen even when middle or upper level clouds may be present. The yellowish or dark reddish colours
of the lowest cloud layers are easy to see in locations where the overlying cloud deck has breaks in it.
- The edges of radiation St/fog over land are sharp but irregular, frequently bounded by terrain features. Over the
oceans the edges are smoother. Valley fog is very easy to distinguish, since its dendritic edges follow the height
contours of the valleys. Advection St/fog has a more regular edge pattern than radiation fog which is restricted by
the terrain features. Another way to distinguish Stratus from Fog is by use of the animated VIS images. They can show
whether a low cloud deck is moving or merely changing shape. In the former case it is probably Stratus rather than
- The general advantages and disadvantages of polar orbiting satellite vs. geostationary satellite imagery
particularly apply to St/fog detection. The much better spatial resolution of the AVHRR instruments on board
NOAA-polar orbiting satellites is extremely useful for detecting and monitoring relatively small-scale St/fog areas.
However, the better temporal resolution (excluding the polar regions) of the geostationary satellites is a big
advantage when monitoring St/fog development (advection and dispersion), e.g., for nowcasting purposes. However, the
most effective use of geostationary satellites for this purpose is restricted to daytime hours for the time being.
05 November 1999/12.00 UTC - Meteosat IR image
05 November 1999/12.00 UTC - Meteosat VIS image
05 November 1999/12.00 UTC - Meteosat WV image
The images show a St/fog sheet over the Baltic States, White Russia and the western parts of Russia. In the IR image the St/fog is seen as a grey, flat, featureless cloud sheet. In the southern part of the sheet (marked with A in the IR image) the temperature difference between cloud top and the adjacent solar-heated land is relatively large allowing the cloud to be easily detected, while in the north (marked with B in the IR image) the temperature difference is small, making the detection of the cloud edge difficult.
By contrast, in the VIS image the St/fog layer is clearly distinguishable as a light grey, well-defined cloud sheet. Even in the VIS image the cloud top is rather flat and featureless. The Water vapour image shows only the water vapour content in the upper and middle troposphere and does not directly help the detection of St/fog. However, it can be used indirectly. For example, there is no Stratus beneath the moist air tongue (marked with C in the WV image). The advection of moist air towards the northeast may, in fact, be assisting the dissipation of the Stratus at point D (shown in VIS image).
Appearance in AVHRR imagery
- Fog/stratus banks are very bright in the 0,6 µm (Ch1) and 0,9 µm (Ch2) images. The detailed structure
(sharp edges) with possible breaks and thinner places can easily be seen. However, their use is restricted to daytime
only. If St/fog cloud lies beneath middle or upper level clouds, its detection can be difficult due to the small grey
scale variability between different cloud layers. On the other hand the upper level cloudiness can cast a shadow on
St/fog cloud layer and thus make its detection easier.
- In 10,8 µm (Ch4) images the appearance of St/fog is smoother than in visible images, and the detection
sometimes is difficult, if the temperature contrast between cloud top and surrounding sea or land surface is small.
However, in some cases slight differences can be enhanced and the cloud area thus becomes easier to see.
- "Black St/fog" phenomenon (the cloud is darker than the surrounding cloud-free area) in Ch4 images
during night and cold season is very often seen in northern latitudes, especially during the winter season.
- In nighttime 3,7 µm (Ch3B) images the St/fog clouds are lighter than in Ch4 due to the spectral variation of
the emissivity of small liquid water droplets in Stratus clouds. This difference is inversely proportional to the
droplet size of the clouds. Over the sea the droplet size tends to be bigger, which causes the difference between
nighttime Ch3B and Ch4 imagery be smaller than over land.
- The difference in cloud-top temperature of nighttime images between Ch3B and Ch4 can be enhanced and thus used fo
nighttime St/fog detection. The difference can be shown either as a greyscale image, in which the St/fog clouds having
a negative temperature difference are shown as different tones of grey-to-white and the cloud-free surfaces as
mid-grey. Some false-colour techniques can also be used: for example, the negative temperature differences can be
shown in variable non-greyscale colours (red, blue, green), while the areas with zero or positive temperature
differences are shown in greyshades.
- In daytime inverted Ch3B- images the fog is often seen as almost or totally black cloud due to strong reflectance
of Fog droplets. Pronounced sharp edges are charasteristic.
- During daytime, combination images 124 show Fog and Stratus sheets in deep yellow colours. If the (radiation) Fog
sheet gets very thin, the resulting colour in 124 imagery becomes brownish yellow. This is due to the reduced
intensity of channel 1 (red component) and, especially, channel 2 (green component). Also, a less reflective earth
surface can be seen in the visible channel through (even sub-pixel size) breaks in Fog cover.
- Combination images 345 can be used both day and night. The appearance of St/fog is dark red or almost red-black
during nighttime, dark blue during daytime.
- The texture of the St/fog cloud top is remarkably less detailed than the top of Stratocumulus Sheets, at least in
reduced-resolution images. Full resolution images show sometimes waves or lines within the Fog top.
30 August 1999/03.18 UTC - NOAA Ch3B image
30 August 1999/03.18 UTC - NOAA Ch4 image
30 August 1999/03.18 UTC - NOAA difference image (Ch3B - Ch4). Negative values enhanced: orange-yellow-green
colours correspond to smallest-middle-largest negative differences, respectively.
30 August 1999/05.07 UTC - NOAA RGB image (channel 1, 2 and 4)
30 August 1999/05.07 UTC - NOAA RGB image (channel 3, 4 and 5)
30 August 1999/06.26 UTC - NOAA RGB image (channel 3, 4 and 5)
A series of satellite images illustrate the main points listed above. During the night of 30 August radiation Fog sheets were formed in Germany and Poland. These sheets are seen as light cloud banks in the Ch3B image over Germany, while the Fog in Northern Poland is almost invisible. In the Ch4 image the Fog sheet over Germany is darker compared to the respective sheet seen in Ch3B image. This is a good example of the visual differences in Stratus cloud appearance in these channels. The Fog over Northern Poland appears to be Black Fog in the Ch4 image, as the surrounding area is colder than the Fog top.
The left image in the middle row shows an enhanced image difference product, in which the negative temperature differences between Ch3B and Ch4 images are shown in orange (the smallest difference; less than -1 degree) , yellow and green (largest difference; around -3 degrees). The yellow and green areas clearly show the Fog sheets over Germany and Poland.
A combination 124 image taken less than two hours later (just after dawn) shows the real locations of St/fog. The Fog layer over Northern Poland has very brownish tones suggesting that the cloud sheet is very thin.
The images in the bottom row show combination 345 images. The varying tones of dark red in St/fog turn towards varying tones of dark blue as the sun starts to illuminate the cloud sheet.