Heavy sky over pack ice near Cape Adare, Ross Sea, Antarctica

Cape Adare, lying at about 71º South, marks the northwestern extremity of Antarctica’s Ross Sea. Even in midsummer the cape is frequently surrounded by varying amounts of pack ice, constantly in motion with changing winds and tidal currents. Further offshore, larger tabular icebergs occasionally run aground where they may remain for an entire season or even longer before winds and tides set them free again.

The effect of light reflecting off pack ice onto an overcast sky, known as ‘ice blink’, is familiar to polar navigators who use this to their advantage when following areas of open water, or ‘leads’, through heavy ice. Low cloud over extensive sea ice receives much reflected light from the ice itself, while cloud over open water appears much darker. It is this darker cloud that signals to an experienced navigator that open water may lie not too far ahead, and it is through experience that a navigator also learns to distinguish this effect from the darkening skies caused by changes in weather conditions.

The darkening sky I photographed here at Cape Adare was a good example of this effect when a snow-laden southerly front arrived early one summer afternoon. Although the cloud was moving swiftly towards me when I took this photograph, the darkest cloud in the distance seemed not to move closer at all even after the sun had been blotted out – confirming that it was dark because it lay over open water not too far away, just over the horizon.