Helsinki ice update Feb. 17th: Optical view
Disclaimer: This is not safety advice or an advocacy to venture onto the ice. The writer is not professionally qualified to analyse ice conditions or study SAR imagery. Special safety precautions are required to travel on ice.
Net assessment (17th Feb)
This update follows yesterday’s SAR ice surface analysis and clearly shows the wet, fractured area around Koirasaari, as well as wetter areas between Koirasaari-Stenskär-Kytö.
Context in last 7 days
Conditions favour refreezing and surface smoothing rather than further large-scale break-up:
Temperatures have remained below at least -3.6 degrees (Harmaja / FMI)
Snowfall of roughly 15-20cm of light snow, which strongly reduces ice thickening
Calm south-westerly/southerly winds
9th Feb: Media reported large area fracturing into floes near Koirasaari
FMI Chart: Fast ice or thin level ice (5-15cm) in the study area
Compact 20–40 cm ice reported south toward Estonia
About optical interpretation
True colour
True colour is an unaltered view: Basically what you see with your eyes. Best for understanding extent of ice surface and larger open water areas. Visible ice cover is not an indication of load-bearing strength.
True colour view February 17th
NDWI - Water Body Index
This is a powerful lens that shows water or high moisture content in blue. NDWI highlights moisture, open water, thin new ice, slush or water above the ice surface.
Light green areas likely represent relatively dry snow-covered ice surfaces. Thick, dry snow would mask any moisture below from optical detection. It must not be interpreted as “safe” area.
Any of the above can correlate with weaker or more dynamic ice, but the index does not tell us about thickness or load-bearing strength.
Clouds and their shadows may give strong “false positives” seen as blue, especially over bright snow and ice.
NDWI - Water Body Index. Blue shows water and moisture (but also cloud shadows)
Observations in key study areas
Area 1. Lauttasaari-Rysäkari corridor
Wetness
Most of the corridor is under cloud. Southern end is clear with a wet “wedge” visible, created by possible influence from shipping lane.
Fracturing
Not assessable near Melkki due to cloud. None observed in the otherwise clear corridor.
External influences
None observed.
Open water
None observed.
Area 2. Rysäkari-Koirasaari
Wetness
The areas Southwest, South and East appear wet. This is in line with reported large scale fracturing event on February 9th.
Update: A separate SWIR analysis on cloud cover confirms that the blue features between Koirasaari–Kytö–Stenskär reflect wetter/thinner ice, and are not cloud cover interference as previously considered.
Old information: The blue features between Koirasaari–Kytö–Stenskär are inconclusive: they may reflect wetter/thinner ice, but cloud edge effects or cloud shadow interference cannot be ruled out.
Fracturing
The areas Southwest, South and East show the fractured ice field after February 9th event.
The NDWI signal is weaker than typical open water (compare with the deeper blues directly east near the shipping lanes), consistent with wet/slushy or thin ice conditions rather than large continuous open leads.
External influences
While not a direct observation, the proximity to the shipping lane is a constant source of waves and current that may contribute to surface weakening.
Open water
None observed.
Area 3. Gåsgrund-Kytö-Stenskär
Wetness
No structural observations that relate to moisture.
Update: A separate SWIR analysis on cloud cover confirms that the blue features between Koirasaari–Kytö–Stenskär reflect wetter/thinner ice, and are not cloud cover interference as previously considered.
Old information: The blue features between Koirasaari–Kytö–Stenskär are inconclusive: they may reflect wetter/thinner ice, but cloud edge effects or cloud shadow interference cannot be ruled out.
Fracturing
None observed.
External influences
None observed. Porkkala-Helsinki shippin lane that passes horisontally through the area between Kytö and Gåsgrund does not appear to be in use.
Open water
Permanent open water spot south of Gåsgrund is the Blominmäki waste water outflow location and has the potential to weaken ice around a larger area.
Other areas
Not studied in this analysis.
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See you,
Orbital Vantage
