In general, clouds in the east alone are a fair sign; clouds in the west, especially dark clouds, mean rain soon.
High-travelling cirrus clouds portend rain. In the Alps, where the winds are our real prophets, a distinction is introduced: cirrus, high and fast on a south or south-west wind, has its usual meaning; but cirrus on a north, east or south-east wind may accompany or, more rarely, precede good weather. Castellated cirrus is always a bad sign, and, where the horizon can be seen, the appearance of cirrus in bands lengthening up towards the zenith (cirro-stratus) is an unwelcome event. Dappled cirrus, the so-called ‘roses’ of the south and the ‘sheep flocks’ of the north, implies a change of wind and weather. As the presence of high cirrus, upon which the change of wind can produce the dappling, usually means, in the mountains, that uncertain or bad weather has been preceding the change, the dappling is most often, and with justice, taken as a good sign. If, however, the weather has been dull, with merely clouds of heat, and the wind has been good before, the change portended will be probably for the worse.
Cumulus clouds, when they tower rapidly, and more especially when they ‘topple over,’ or show a tendency to do so, are signals of a change to rain. A sure method of observing cumulus is to watch some small portion of the cloud for a space of time. If it grows larger, the sign is a change for worse weather; if it ravels out and disappears on the warm air round it, the sign is for good, and the cloud can be disregarded.
Stratus, the long stratified bands of cloud at any height, means bad weather, and the tendency of cirrus or cumulus to stratify is ominous.
The edges of clouds tell us much of their import: harmless if they are filmy and ravel out on the warmer air; suspicious if they are hard, heavily outlined or charged.
An approaching rain-cloud in action is recognizable by the vertical or slanting lines in the air below it, or by the ‘torn lace’ on the lower edge. A hail-cloud shows heavier lines. A snow-cloud carries fuller, harder marginal protuberances, and may be lighter in tone. Heavy cumulus of this cumbrous sort may often surprise strong sunlight by a sudden snow-break. Usually we go by temperature and height in predicting if a ‘wet’ cloud will break in rain or snow.
High ‘mare’s-tails’ betray wind, and predict rain according to their direction.
‘Fish’ clouds pointing east and west usually mean foul weather; pointing north and south, fair weather.
Black wisps of cloud before sunrise, especially in a clear sky, mean early rain. Lighter-toned wisps at this hour have no signification. But if the lower edges of cloud-films are charged and dark, looking like ink which has run down into the borders of blotting-paper, we look out for rain.
Long fingers of cloud radiating from a peak portend storm, particularly if any one of them betrays the typical thunder-curve. Thunder on a peak means snow, so we must look out for a cold and electric day.