Russian cold air reaches Central Europe.
21 October 2020
22 January 2006
by Jochen Kerkmann (EUMETSAT)
In January 2006 several TV stations showed dramatic winter pictures from Eastern Europe: people in thick clothes huddled up against biting wind and snow. For days, Moscow experienced temperatures around -30 °C. The high consumption of gas even led to a shortage of gas supply from Russia to Western Europe.
On 22–23 January the cold air finally reached Central Europe. Several stations in Germany reported minimum temperatures below -20 °C. The Meteosat-8 image below shows the so-called Fog RGB composite, which combines the IR3.9, IR10.8 and IR12.0 channels. It is mainly used for the detection of low-level fog or stratus, which appears in a light green colour (see Interpretation ). The most interesting feature in this image, however, is the intense red colour of the cold, snow-covered surfaces in Eastern Europe, compared to the pink colour of the warmer, snow-free surfaces in Western Europe.
This colour difference is a combined effect of lower surface temperatures (i.e. less blue) and lower Brightness Temperature Difference (BTD) between the IR10.8 and IR3.9 channels (less green) of the cold, snow-covered ground. Indeed, the IR10.8–IR3.9 BTD depends on the emissivity of the ground at these two wavelength, which is nearly identical for snow-free, vegetated ground, but which is different for snow-covered ground (depending on snow grain size and snow temperature). Humidity effects can be excluded in this case as the moisture content was very low over Europe.
In summary, the combined temperature/emissivity effect described above allows, to a certain degree, the detection of snow-covered surfaces during night-time, which is probably a new application of MSG imagery. Conditions, however, are a snow cover of at least 10 cm (vegetation has to be covered) and low surface temperatures. Further case studies on this topic will be presented in the coming months.
Meteosat-8 RGB Composite Image
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