December log (Arnøya / Kågen / Laukøya and Tromsø)
Notes
- air temperatures below are measured on sea level
- if clear sky, assume inversion
2025-12-01—2025-12-02 (T)
- Air: cold tail of the system, falling temp to -10 the second day
- Wind: first day S-SW light breeze to calm
- Precipitation: snow showers to clear sky
- Observations: stablizing slabs from the previous days on N-E slopes. Partially colder weather and time - TG, what are the lower layers doing? Growing surface hoar due to the clear sky
2025-12-03 (T)
- Air: raising temps towards zero
- Wind: mostly calm
- Precipitation: wintry mix and snow showers
- Observations: quite heavy and humid snow covering the surface hoar from previous days. How well will this new layer of about 30cm new snow bind?
2025-12-04—2025-12-05 (T)
- Air: dropping temps again
- Wind: N-NE light to moderate breeze,
- Precipitation: clear: sky and scattered clouds
- Observations: fresh wind slabs on S-W slopes where wind strong enough to transport the snow from last days. What is going on in the SP and where?
2025-12-06—2025-12-08 (A)
- Air: zero temps
- Wind: SE-S up to fresh and strong breeze
- Precipitation: clear sky and cloudy last day
- Observations: dozens of soft snow on top of crust of unknown nature. Spatial variability in snow strength. More blown places and less snow higher up, especially on edges and convex places
2025-12-09—2025-12-10 (A)
- Air: couple of degrees below zero
- Wind: E fresh to strong breeze ∕ gale winds higher up in the mountains
- Precipitation: snow showers night to the second day combined with cloudy n’ clear sky during the second day
- Observations: no so many places with fresh snow anymore, only lower down in the forest. Perhaps some isolated pockets or couloirs with soft and packed snow. A lot of blown places and small to middle-sized zastrugis. Snowpack feels stable in general.
2025-12-11—2025-12-12 (A)
- Air: around -5 to -10 degrees
- Wind: up to NE fresh light breeze and increasing to SE stong breeze the second evening
- Precipitation: mostly clear sky
- Observations: one profile in SE slope about 200 masl. reveals quite homogenous SP with some wet clustered crystals next to the ground. There is a MF crust couple of cm below surface and around this one there is a combo of low dense crystals above and similar crystals below (with some occasional squares in it?). There is a nice and growing surface hoar the second day but it's getting broken by the strong SE wind during the evening.
2025-12-13—2025-12-14 (A)
- Air: increasing temperature towards 0
- Wind: SE-S stong breeze and up to gale winds in gusts
- Precipitation: clear sky combined with scattered clouds
- Observations: approaching a thaw. Quite compact and strong SP. There might be problems around the crust if TG gets strong which won't happen during the coming thaw. Some fresh wind slabs on northern slopes where enough snow available for transport.
2025-12-15—2025-12-16 (T)
- Air: up to +5 degrees - thaw
- Wind: S-SV strong breeze and up to gale winds in gusts
- Precipitation: periods with heavy rain, especially the first day
- Observations: first cold shower on the dry SP, what does it mean? What will happen now and where to expect it? And why?
2025-12-17—2025-12-19 (T)
- Air: temps up to +4 degrees with overnight refreeze
- Wind: mostly S-SW up to light and fresh breeze, occasional rain showers
- Precipitation: cloudy and long periods with clear sky (frost on the rain crust lower down)
- Observations: There is a MF (radial recrystalization) layer of snow, let's say up to 400 masl., this slippery layer is covered by frost crystals, eventually broken by more rain. But this can be problem higher up in the mountains if covered by new snow. The topmost layer goes through the repetitive melt-freeze cycles but getting destroyed by rain showers. What is the situation higher up?
2025-12-20—2025-12-21 (T)
- Air: couple of degrees above zero
- Wind: S-SW up to light breeze
- Precipitation: cloudy and longer periods with rain
- Observations: drainage channels well established up to 400 masl. but how will the new snow bond to the crust higher up in the mountains?
2025-12-22—2025-12-23 (T)
- Air: couple of degrees above zero
- Wind: S-SW up to light breeze
- Precipitation: cloudy and periods with rain
- Observations: stabilizing rain on the old snow with well establish draining channels, fresh wind slabs on NE slopes higher up in the mountains
2025-12-24—2025-12-25 (A)
- Air: thaw, up to +7 degrees
- Wind: SW-W strong breeze and up to fresh gale in gusts
- Precipitation: cloudy and periods with heavy rain
- Observations: tons of rain, wet sluffs and slabs releasing on the crust in the SP (only higher up in the mountains). Snow and strong wind high up in the mountains might cause fresh slabs releasing on the E slopes. These might "transform" to wet-sluffs and slabs on their way down the slope
2025-12-26—2025-12-27 (A)
- Air: up to +5 degrees and falling the second day
- Wind: W-SW fresh and strong breeze in gusts, the second day “sudden” N strong breeze with whole gale gusts
- Precipitation: cloudy and rain and snow showers the second day
- Observations: the new snow does not bond well with the layer of wet snow and the very strong wind creates islands with rain crust and "shadow islands"where the snow bonds well. Much of the snow drifts off the mountains to the sea and to the leeward slopes, thus, very many wind eroded areas also high in the mountains.
2025-12-28—2025-12-29 (A)
- Air: couple of minus degrees
- Wind: “N” strong breeze and up to strong gale in gusts
- Precipitation: heavy snow showers and drifting snow
- Observations: the storm continues. Now there are also icy and wind eroded places covered by new snow. There was a crust in the SP, has it been eaten up by the rain? Now there is a new crust with poor bonding areas and there is also a wet snow underneath, this might cause a problem in the future. Fresh slabs on southern slopes but still a lot of wind eroded areas due to the very very strong winds.
2025-12-30—2025-12-31 (A)
- Air: couple of minus degrees
- Wind: "N” strong breeze and decreasing
- Precipitation: heavy snow showers, especially the second day
- Observations: still quite some wind eroded areas with the old rain crust, especially near ridges, in "Venturi" valleys and close to convex terrain formations. Wind slabs of various size, thickness, strength. Could be released where stress overcomes strength and where structure and energy levels are able to propagate eventual fracture.
Summary
Overall pattern:
December 2025 was highly variable, oscillating between cold clear spells and multiple warm–wet cycles. This produced a very complex early-season snowpack with repeated stages of wetting, refreezing, wind transport, and slab formation.
Temperatures:
Large fluctuations from around –10°C to +7°C. Cold clear nights produced TG and surface hoar, while repeated thaw pulses destroyed SH and reset the upper snowpack several times.
Wind:
Frequent strong wind events from varying directions (S, SE, SW, N, NE). Result: widespread wind erosion, heavy cross-loading, extensive zastrugi, and multiple generations of wind slabs (E, NE, N aspects especially affected).
Precipitation:
Everything from rain to heavy snow, graupel, wet snow, freezing rain, and convective showers. Several intense rain-on-snow events created multiple melt–freeze (MF) crusts at different elevations.
Snowpack evolution:
- Early December: stabilizing old slabs + SH growth.
- 3–8 Dec: humid snowfall buried SH; wind transported it unevenly.
- 9–12 Dec: strong winds → scouring + hard slabs.
- 13–16 Dec: significant rain → new MF crust.
- 17–23 Dec: repeated rain–freeze cycles → multiple MF layers.
- 24–25 Dec: heavy rain + strong wind → wet sluffs/slabs on crust.
- 26–29 Dec: very strong winds → scouring and poor slab bonding.
- 30–31 Dec: colder → new wind slabs on visible crusts and eroded surfaces.
Key layers entering January:
- Several MF crusts at low/mid elevations.
- SH layers formed during cold spells (destroyed in exposed zones).
- Graupel pockets.
- Hard/soft slab transitions.
- Possible crust-adjacent faceting where TG was strong.
- Highly mixed slab bonding due to repeated resets.
Overall:
December ended with a
complex, multi-layered snowpack full of crusts, wind slabs, wet interfaces, and structural discontinuities — primed for January’s colder, clearer weather to start forming persistent weak layers above/below crusts.
