Ancient sailors used the stars to navigate around the globe. However, the sky itself
can give polar travelers the means to plot a safe course through icy waters. When the
weather is mildly overcast, bright and dark patches on the clouds’ lower surface are
actually reflections of the terrain below. These shadows are caused by the different
light absorption capacities of snow, ice, and water. For instance, when light hits
bright white snow, most of it is cast back upward to brighten the clouds. Dark surfaces
like open water reflect less light, so the corresponding clouds overhead remain darker.
The resulting cloud map is very useful. The bright areas represent snow or ice below,
while dark sky reveals open water. Under ideal conditions, this cloud form can mirror
points as much as thirty miles beyond the normal range of human vision.
Arctic travelers and sailors have long ago adapted this cool phenomenon for their own
purposes. Sailors watch for “ice blinks” - white patches on darker clouds that show
icebergs and require a change in course. Conversely, if the sky shows a dark area ahead,
something known as a “water sky” because it reveals open water ahead, a traveler on foot
will be readily warned beforehand.
This way of navigation has proven to work. Explorer and anthropologist Vilhjalmur
Stefansson, a traveler who journeyed often around the North Pole, reported in 1914 that
the water sky showed cracks and treacherous places all around his party. However,
“by keeping our eyes on the cloud map above we were able to travel sometimes a day at
a time without even seeing water.”