Widespread cold air advection from the north east occurred across central Europe in the second week of February, bringing cold and snowy conditions from Russia to Portugal.
21 February 2023
17 February 2023
By Ivan Smiljanic and Djordje Gencic (Exostaff)
Seen through the Meteosat-11 Airmass RGB, cold, dry air masses appear in blue shades, while humid and warm air masses show in more green tones (Figure 1). The loop shows how the usual westerly airflow is reversed over central and southern parts of Europe, with the air stream being mainly easterly.
Large areas of the eastern European continent were covered in snow, plus the higher terrains in central and west Europe (Alps and Pyrenees), depicted in strong green shades of Cloud Type RGB. Green is the dominant colour with this RGB product, since red (NIR1.3 channel) and blue (NIR1.6 channel) components show little, to no, reflection from low-laying snow. The NIR1.6 channel reflects very few photons from ice crystals, and the NIR1.3 channel, due to strong absorption by water vapour molecules in that spectral region, shows very little, or no, reflection from the surfaces in the lower troposphere. Hence, in the cloud-free areas, a good contrast is achieved between green snow-covered and darker blue snow-free ground with the Cloud Type RGB, since the green component consist of the highly reflective VIS0.6 channel.
However, zooming in to snow surfaces at higher terrains, the colour of snow changes from green to yellow. This is also true for snow that lies under the thin layer of high cirrus clouds.
In every RGB product, the yellow colour is the result of a stronger contribution in the red and green RGB component (less blue contribution). The red component of yellow snow, in the case of very high-lying snow, comes from the fact that the NIR1.3 channel starts to detect photon reflection from the snow surface since the optical path through the water vapour aloft is shorter, for every photon coming from the Sun and bouncing back to the satellite. If the snow is covered by a thin layer of semi-transparent ice clouds, the reflection in the red channel from those clouds is combined with the reflection from the snow in the green component, resulting in the aforementioned yellow shades.
In theory, if there isn't any water vapour in the photon path between the satellite and snow, every snow surface would result in yellow shades in the Cloud Type RGB. Closest to that would be the existence of a very dry air mass aloft. That is exactly what was observed with cold (and dry!) air advection from the north-east — looking at the same scene before and after the stronger dry air advection on 8 and 9 February (Figures 2-4), shows that the snow turns yellow, even in the lower laying terrains of the Carpathians, Pyrenees and Alps.