Geophysics of Some Inuit Observations

Temperature effects

Young ice formed at higher temperatures is weaker than that formed at lower temperatures.
This appears to be in contrast to one geophysical argument for sea ice strength.  Brine volume is generally thought to be the greatest factor in sea ice strength.  This is because brine pockets have no tensile strength.  Since ice grown under warm temperatures has a lower brine content, the warmer sea ice should be stronger.  Perhaps the reasoning behind the weakness in the warm sea ice comes from the fact that (according to Assur in the 1960 CRREL Research Report 44) there is an abrupt transition in the ice strength (not counting brine pockets) at -8.2 degrees C.  This corresponds to a precipitation of Na2SO4.10 H2O salt.  The ice above -8.2 degrees is described as dark and wet and "significantly weaker" than the colder ice.  Not knowing how warm Nelson was implying means that I can't take this much further.

This may have more to do with a lack of knowledge of ice thickness.  The Inuits had no way of observing ice thickness except at openings.  Perhaps the differences in strength actually come from the difference in thickness of similarly aged ice under different temperatures.

Unsafe black ice versus safe gray ice

Black ice is dangerous to walk on, but after it changes to gray ice it is safe.
This distinction is probably one between dark and light nilas, although there are other differences in ice which could give the same conclusion of dark ice being unsafe.  Dark nilas is a thin young ice layer which contains large amounts of brine.  After the ice reaches a certain thickness (approximately 5 cm) the ice rides high enough in the water that the albedo goes up drastically.  The phenomenon mentioned above of the -8.2 degrees transition from dark, wet ice to greenish-gray normal strength ice would also lead to this test of ice safety.  Finally there is the fact that columnar ice crystals appear dark if there is no layer of random granular ice above.  The columnar ice would have many brine channels and would be weak.  All three of these physical transitions imply that lighter colored ice would be safer than dark ice.

Wet ice surface layer

There is a wet salty layer on the surface of the ice even at -35 degrees Celcius.  This becomes apparent when sitting on the ice waiting for a seal at an opening in the ice.  (Moisture which soaks up from the ice is called masahok.)
Assur refers to this as mushy ice which can remain so in the presence of extremely cold temperatures.  Once an ice surface has been flooded, it stays mushy due to the exess salt at the surface.  The fact that Nelson comments on this phenomenon near open water where hunting goes on suggests that maybe sea water gets splashed, blown or flooded onto the surface of the ice at some point in time; brine could also reach the upper surface of the ice by upward expansion.  From then on, the surface will feel wet.

Locating open water

When the ice edge is far off, one can note where the open water is by the transition from light to dark cloud bottoms above the water or by observing water fog above the leads.
The albedo of open sea-water is around 0.06, whereas bare first year ice has an albedo of 0.52.  Accordingly at least eight times more light will be reflected back up from the sea ice surface than from open water.  The albedo of clouds varies, but is around 0.5 or more.  The varying brightness of the upwelling visible light would be noticible even after reflecting off the bottom of a cloud layer. 

The sharp contrast in fog as seen in the image here is due to the difference in the vapor pressure of the ice and the open water and the difference in their surface temperatures.  The open water surface is at the freezing point of sea-water, whereas the ice surface may be much colder.  The air will also likely be much colder.  Thus the evaporation of water from the surface will very easily saturate the air above and fog, or ice "smoke," will form.

Ekman spiral

The ice moves with the current or with the wind, but does not seem to demonstrate a deviation from them as the Ekman spiral would imply.
The Ekman spiral describes how wind (or currents) in a boundary layer weaken and change direction close to the surface.  The velocity approaches zero as the height above the surface goes to zero.  The depth of the Ekman layer is on the order of a kilometer in the air and 100m in the ocean.  Thus one would not likely notice an Ekman spiral unless you measured the wind velocity fairly far from the surface.  (When Nansen--who first reported this effect--was drifting toward the North Pole in 1893, he probably took wind measurements from the top of his mast and thus observed that there was a deviation in the sea ice motion from the direciton of the wind.)

Crack propagation

When cracks form, they open from the same direction as the current and wind come from.  The cracks generally open parallel to the wind.
Once a flaw in the ice yields, the upwind portion of the ice will be in tension while the downwind portion of the ice will be in compression.  The tensile strength of the ice is much lower than the compressive strength.  Thus the crack will propagate downwind like a zipper being pulled apart at the top.  Once the ice is free to drift, it will be subject mainly to the forces of the wind, currents, and internal stresses.

Monitoring ice motion

When on an ice floe, hunters are very concerned whether or not the ice floe is drifting.  Sometimes they hunt many kilometers from the shore and a crack could open up without their knowing it and set them adrift.
Sometimes they place a compass on a stool with wind breaks around it.  Somebody checks every so often that the compass still points toward the same feature on the ice.  If the compass begins to drift, they know the ice block is rotating.  At that point they would attempt to find the direction of the drift.  One method to do this is to drop a sinker on a line into the water and feel which way it drags on the bottom.  The line would bow in the current, but they could feel the drag on the ocean floor.  They would then move in the same direction as the floe drift for the same reason mentioned above that the cracks form upwind first and propagate downwind.  The hope is to arrive at the leading edge of the crack before it completely strands them.

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