Thunder-clouds are easily distinguishable. Their form is always similar. The duplicated outline in the masses of cloud above and below, the connecting darker neck between, and the leaf-like and invariable curve of strength where the neck runs out into the profile line of the upper mass, are sufficiently distinctive. A little further observation will enable their lines of system and the probable direction of their movement to be located. The lower mass of cloud may, in cases, be absent, but the truncated ‘neck’ and the leaf-like ‘curve’ will always enable us to identify them.
Wind.
Even more than the clouds, once we have discovered the habit of the year and the local signs, the winds are our firm basis for forecast. The north wind in the Alps is usually for good, though in a bad season of habit it may, if long continued, bring snow. The south wind is always fraught with suspicion, until it justifies it. It brings a succession of storms, but leaves fine intervals. The west wind, if continuous, means the continuance of unsettled weather, with an inclination to preserve whatever may be the habit of the year. It is perhaps the most forcible and trying of winds to encounter on the ridges exposed to it. The east wind is infrequent and rarely long continued. Its portent is favourable. The south-west wind means rain to follow. A change from north to north-west threatens rain. A change from south-west to north-west—generally a wind on its way to becoming a north wind—means a change for the better in bad seasons. South-east, and especially north-east, winds share the good qualities and projects of the east wind.
The current of the Föhn is recognizable by its accompanying oppression of warm-parched, or in cases of warm-moist, atmosphere, and by its depressing effect upon our spirits. It is a harbinger of evil of all kinds. Its own forerunners are often a massing of heavy-bordered clouds accompanied by a threatening vividness in all visible coloration. The Föhn is said to be able to infect, or more probably alternate with, any wind from any quarter; but we know of it in summer usually in the south or south-west winds. In the south it is ‘dry,’ in the south-west it is ‘wet,’ Föhn. Its presence ruins the best-directioned wind; it underthaws the snow dangerously and spoils our tempers inexplicably.
Sudden gusts and round-the-compass winds are of ill omen. As between cross-currents of wind, at different levels, the higher alone is worth attention. Valley currents, particularly those off glaciers, are only misleading.
The simple sky signs, familiar not only in mountains, give us further assistance.
Signs in General.
A red sunrise is bad; a red sunset good. Sunrise on a grey sky means a fair day; sunset on a grey or pale yellow sky means a rainy day, on a bright yellow sky a windy day.
A ‘high dawn,’ the sun showing first over a vapour belt, is an ill sign; a ‘low dawn,’ the sun leaping from the horizon, a good sign.
If the distant sky at dawn, especially to the west, is low and dark, there will be breaking weather by noon.