If, again, the sky above is purely a snow sky, but is traversed by a band of slightly modified quality or tone, leading away from the eye, there will exist on the far side a correspondingly inclined great snow ridge, from one side of which the sun, in a given position, will be reflecting high light, but from the other, lower. By repeating the observation at different times of day, so that the light will have fallen and been reflected from different directions, we can confirm the existence of such a ridge, and may be able to locate its position and determine its magnitude with some accuracy.
A second and characteristic appearance is visible on the sky above the unseen side, if this far side consists of two large snow fields divided by a long rocky ridge extending away from us. We then have a snow sky divided by a band of sky which is not catching any reflected higher light, and which we may assume to be a ‘rock’ sky.
The Rock Sky.—If the whole expanse of the sky above is seen to have a uniform and normal tone with no local alterations, such as would be produced by partial snow reflection, then the unseen side will consist of a large wall of rock—probably, in such case, steep rock.
Under favourable conditions, a very practised observer may detect in a uniform rock sky a band of slightly modified tone going away from the eye on the far side. This will be produced by another great rock ridge extending in the corresponding direction. By watching the sky above this ridge, and observing the differences produced by the reflections of light at different hours from its different aspects, we may even be able to discover to our satisfaction whether the invisible ridge is all rock or has one side covered with snow.
The Ice Sky.—Dry glacier or large fields of ice betray themselves upon the sky in a slightly greyer tinge, distinguishable more by contrast than by an absolute tone from a snow sky under the same conditions.
There is also a ‘water’ sky—the unmistakable look in a sky which is reflecting great unseen sheets of water; but its identification is more familiar and of more service to arctic or desert travellers than to mountaineers.
Apart from these larger sky signs there are some more local indications that are of particular value to the climber. The appearance of the sky as seen across the ridge will in most cases give us evidence of a mixed character—that the unseen side is partly rock, partly snow. It is thus essential to know, if we propose to use the ridge we are prospecting for our ascent, whether the rock just over or on the ridge is bare or ice glazed, and whether the snow discovered on the far side rises up to the edge of the ridge or leaves a crest of clear rock.
If there are bare rocks close up to the edge on the unseen side, these will be at certain hours heated by the sun, and a hot current of air will be ascending. The skyline above will have a wavering appearance, showing a band of darker tint between the ridge and the normal sky.
If there is snow close up on the far side, the air will not be disturbed, and the skyline will be steady and clean.
If the rocks beyond are free from snow but glazed with ice, the skyline will remain undisturbed, but it will have a brilliant glistening appearance in strong sunlight, like a strip of polished blue steel. This last indication, if the eye can learn its significance, may often be of service on climbs where the unseen mountain structure is already known, but when there is uncertainty as to the actual condition of the rocks on the day. Many fruitless ascents might have been saved if the ice glazing on such unseen sections of a ridge could have been detected in time.