Near land, in sheltered harbours, in valleys, or over low ground, there is usually a marked diminution of wind during part of the night—and a dispersion of clouds. At such times an eye on an overlooking height may see an extended body of vapour below; which the cooling of night has rendered visible.
Although the preceding remarks are probably sufficient for their principal purpose—these pages may fall into the hands of persons familiar with the subject, to whom the following observations may be addressed, as some of the reasons for what has been so briefly, if not too positively outlined.
As the mercurial column rises with increase of pressure by the atmosphere, and descends when the pressure diminishes, it indicates a greater or less accumulation of air, which, like other fluid, such as water (when heaped above its average level or reduced below it, from whatever cause),—will have a tendency to fall or rise till the general equilibrium is restored. An observer may be under the centre of such accumulation or depression, he may be more or less distant from it, though within the influence of whatever horizontal movement of air may be caused by such temporary increase or diminution of pressure. Hence the barometer shows, and generally foretells, changes of wind; but as complications always occur, and as changes are of greater or less extent, affecting or extending through a wider or more limited area, accompanied by hygrometric and electrical alterations, it is extremely difficult at times to say beforehand what particular change of weather is to be expected, and at what interval of time; although after the event the correspondence of barometric changes with those of the weather can be readily traced. However, notwithstanding occasional perplexity, the general character of weather during the next few days may be predicted by an observer who understands the nature and use of this instrument and the thermometer, and has watched them in the few immediately preceding days.
In endeavouring to foretell weather, the general peculiarity should always be remembered, that the barometric column usually stands higher with easterly than it does with westerly winds; and with winds from the polar regions higher than with those from the direction of the equator. Hence the highest columns are observed with north-east winds in northern latitudes, and with south-east in the southern hemisphere.
In middle latitudes there is an average difference (unreduced or observed height as read off) of about half an inch, other things being similar, between the heights of the mercury with North-easterly, and with South-westerly winds.
The steadier the column, or the more gradually it moves, the more settled in character will the weather be, and conversely: because it shows a quiet settled state of the atmosphere; or, if otherwise, the reverse. In the tropics, when the barometric column moves contrary to its usual daily motion, inferior weather may be expected (temporarily), because the usual air currents are disturbed.
This regular movement, whether tidal, or otherwise connected with the sun's influence—sensible in tropical latitudes, but more or less masked elsewhere—amounts to nearly two-tenths of an inch near the equator, the highest being at about nine, and the lowest near three o'clock.
Some movements of the atmosphere may be illustrated by reference to the motion of water drawn off from a reservoir by a small opening below; or by similar upward draught through a syphon; or by a gradual pouring in at the upper surface.