Sometimes severe weather from an equatorial[18] direction, not lasting long, may cause no great fall of the barometer, because followed by a _duration_ of wind from polar regions:--and at times it may fall considerably with polar winds and fine weather, apparently against these rules, because a _continuance_ of equatorial wind is about to follow. By such changes as these one may be misled, and calamity may be the consequence if not thus forewarned. The veering of the winds is a direct consequence of the earth's rotation, while currents of air from the polar regions are alternating or contending with others from the equator. The polar currents are cold, dry, and heavy. Those from the equatorial parts of the world are warm, moist, and comparatively light. Their alternate or combined action, with the agencies of solar heat and electricity, cause the varieties of weather that we experience. It is not intended to discourage attention to what is usually called "weather wisdom." On the contrary, every prudent person will combine observation of the elements with such indications as he may obtain from instruments. The more carefully and accurately these two sources of foreknowledge are compared and combined, the more satisfactory will the results prove. A few of the more marked signs of weather--useful alike to seaman, farmer, and gardener, are the following: Whether clear or cloudy, a rosy sky at sunset presages fine weather; a red sky in the morning, bad weather, or much wind (if not rain):--a grey sky in the morning fine weather; a high dawn, wind; a low dawn; fair weather.[19] Soft-looking or delicate clouds foretell fine weather, with moderate or light breezes;--hard edged oily-looking clouds, wind. A dark, gloomy, blue sky is windy;--but a light, bright blue sky indicates fine weather. Generally, the _softer_ clouds look, the less wind (but perhaps more rain) may be expected;--and the harder, more "greasy," rolled, tufted, or ragged, the stronger the coming wind will prove. Also, a bright yellow sky at sunset presages wind; a pale yellow, wet:--and thus by the prevalence of red, yellow, or grey tints, the coming weather may be foretold very nearly: indeed, if aided by instruments, almost exactly.[20] Small inky-looking clouds foretell rain; a light scud, driving across heavy clouds, wind and rain; but if alone, wind only. High upper clouds crossing the sun, moon, or stars, in a direction different from that of the lower clouds, or wind then blowing, foretell a change of wind (beyond tropical latitudes).[21] After fine clear weather the first signs (in the sky) of change are usually small, curled, streaked, or spotty clouds, followed by an overcasting of vapour, that grows into cloudiness. This murky appearance, more or less oily or watery, as wind or rain will prevail, is a sure sign. The higher and more distant the clouds seem to be, the more gradual, but extensive, the coming change of weather will prove.