Clouds over sea and ice. Credit: Andrei Pozharskiy

Ice melting in the Baltic Sea

27 March 2018–6 April 12:12 UTC

Clouds over sea and ice. Credit: Andrei Pozharskiy
Clouds over sea and ice. Credit: Andrei Pozharskiy

After a dark Nordic winter, the Meteosat Natural Colour RGB provides a view of day time ice through the combination of 0.8µm, 1.6µm and 3.9µm — optimal to separate land, ice or cloud.

Last Updated

05 December 2022

Published on

27 March 2018

By Vesa Nietosvaara and Jose Prieto (EUMETSAT)

At the end of March, the Gulf of Bothnia was frozen at its surface, with a thickness of around 40 to 50cm (Figure 1, source: FMI).

 FMI Ice charts of 27 March (left) and 6 April (right). The blue colours stand for open water, the grey colours ice. The dates in maps are boundaries for evolution.
Figure 1: FMI Ice charts of 27 March (left) and 6 April (right). The blue colours stand for open water, the grey colours ice. The dates in maps are boundaries for evolution.

Over the lake Meteosat's sensors in the solar domain measured ice surface reflectivities higher than for liquid water.

The highest ice-liquid difference occurred for 0.8µm, which explains the red hues at the most northern part of the Baltic Sea on Figure 2. The sea between continental Finland and the Aland island was also frozen and appeared red.

Between 27–31 March the ice started to melt (eventually turning to liquid) in large areas of the east side of the Gulf of Finland — the same date as the colour composite showing no red colouration apart from the easternmost part of the Gulf of Finland, near Saint Petersburg.

The analysis does not match the satellite image. It could be that wet ice is similar to liquid water in its reflectivity, so it is also dark in the satellite composite.

 Meteosat-11, Snow RGB, 31 March, 12:00 UTC, with zoomed in image inset, bottom right
Figure 2: Meteosat-11 Snow RGB, 31 March 12:00 UTC, with zoomed in image inset, bottom right

The main advantage of the composite based on 0.8µm, 1.6µm and 3.9µm (Snow RGB) is the clear separation between snow on the ground and ice cloud, both looking similar in hue in the solar channels composite.

In the Snow RGB the second albedo at 1.6µm is higher for the small crystals in the cloud as for the rough snow on the ground, creating the hue difference: red for snow (reflectivity at 1.6µm ca. 15%), peach for ice crystals (ca. 40%). See the areas A (snow) and B (ice cloud) in the slider, Figure 3.

Image comparison

Meteosat Solar Channel 1.6 µm compare1
compare2
 

Figure 3: Comparison of Snow RGB and solar channels imagery, 6 April 12:12 UTC