FOG AND STRATUS - CLOUD STRUCTURE IN SATELLITE IMAGES

by FMI


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

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.


SUB-MENU OF FOG AND STRATUS
METEOROLOGICAL PHYSICAL BACKGROUND