If the barometer has been about its ordinary height, say near thirty inches, at the sea level,[11] and is steady, or rising—while the thermometer falls, and dampness becomes less—North-westerly, Northerly, or North-easterly wind—or less wind—may be expected.
On the contrary—if a fall takes place, with a rising thermometer and increased dampness, wind and rain may be expected from the South-eastward, Southward, or South-westward.
A fall, with a low thermometer, foretells snow.
Exceptions to these rules occur when a North-easterly wind, with wet (rain or snow) is impending, before which the barometer often rises (on account of the direction of the coming wind alone), and deceives persons who, from that sign only, expect fair weather.
When the barometer is rather below its ordinary height, say, below twenty-nine inches and nine-tenths (at the sea level only), a rise foretells less wind, or a change in its direction towards the Northward,—or less wet; but when the mercury[12] has been low, say near 29 inches—the first rising usually precedes, and foretells, strong wind—(at times heavy squalls)—from the North-westward—Northward—or North-eastward—after which violence a rising glass foretells improving weather—if the thermometer falls. But, if the warmth continue, probably the wind will back (shift against the sun's course), and more Southerly, or South-westerly wind will follow. "Backing" is a bad sign, with any wind.
The most dangerous shifts of wind, and the heaviest Northerly[13] gales happen after the mercury first rises from a very low point.
Indications of approaching changes of weather, and the direction and force of winds are shown less by the height of mercury in the tube, than by its falling or rising. Nevertheless, a height of about 30 inches (at the level of the sea) with a continuance of it, is indicative of fine weather and moderate winds.
The barometer is said to be falling when the mercury in the tube is sinking, at which time its upper surface is sometimes concave or hollow. The barometer is rising when the mercurial column is lengthening; its upper surface being then, as in general, convex or rounded.[14]
A rapid rise of the barometer indicates unsettled weather. A slow rise, or steadiness, with dryness, shows fair weather.