A considerable and rapid fall is a sign of stormy weather and rain. Alternate rising and sinking show very unsettled weather.
The greatest depressions of the barometer are with gales from the S.E., Southward, or S.W.; the greatest elevations, with winds from the N.W., Northward, or N.E., or when calm.
Although the barometer generally falls with a Southerly, and rises with a Northerly wind, the contrary sometimes occurs; in which cases the Southerly wind is dry and the weather fine; or the Northerly wind is wet and violent.[15]
When the barometer sinks considerably, high wind, rain, or snow will follow: the wind will be from the Northward if the thermometer is low (for the season)—from the Southward if the thermometer is high.
Sudden falls of the barometer, with a Westerly wind, are sometimes followed by violent storms from N.W. or North.
If a gale sets in from the Eastward or S.E., and the wind veers by the South, the barometer will continue falling until the wind becomes S.W., when a comparative lull may occur; after which the gale will be renewed; and the shifting of the wind towards the N.W. will be indicated by a fall of the thermometer as well as a rise of the barometer.
Three things appear to affect the mercury in a barometer:—
1. The direction of the wind—the North-east wind tending to raise it most—the South-west to lower it the most, and wind from points of the compass between them proportionally as they are nearer one or the other extreme point.
N.E. and S.W. may therefore be called the wind's extreme bearings (rather than poles?)
The range, or difference of height, of the mercury, due to change of direction only, from one of these bearings to the other (supposing strength or force, and moisture, to remain the same) amounts in these latitudes to about half an inch (shown by the barometer as read off).