Expert Details Foretelling Weather Part 3

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 dimini hes, 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.



From a slight motion at the commencement, affecting only that portion of

the fluid adjoining either of those places of diminution or repletion,

gradually all the water becomes influenced and acquires more or less

rapid movement. But suppose a long reservoir or canal of fluid which has

two such points of exhaustion or two of such repletion (as imagined

above), and that one of either is near each end of the vessel. If each

aperture be opened at the same moment, equal effects will be caused in

each half of the fluid towards either end of the vessel, but in the

middle there must be a neutral point at which the water falls, yet has

no horizontal motion. The converse takes place in raising the level. And

in the case of fluid drawn off or diminished in weight at one end while

increased by repletion at the other, the whole body of water will move

similarly to that in the former vessel, but unequally. Hence it is

evident, that before horizontal motion occurs, an augmentation or a

diminution of pressure must take place somewhere more or less remote;

and so it is with the lighter fluid atmosphere,--which has centres,

lines, or areas of depression towards which currents flow.



Such considerations show in some degree why the barometric changes

usually precede, but sometimes only accompany, changes of weather: and,

though very rarely, occur without any sensible alteration in the wind

current of the atmosphere. An observer may be near a central point

towards which the surrounding fluid tends,--or from which it diverges.

He may be at the very farthest limit of the portion of fluid that is so

influenced. He may be at an intermediate point--or he may be between

bodies of atmosphere tending towards opposite directions.



It has been said, that "a whirlwind which sets an extended portion of

the atmosphere into a state of rapid revolution diminishes the pressure

of the atmosphere over that portion of the earth's surface, and most of

all at the centre of the whirl. The depth of the compressing column of

air will, at the centre, be least, and its weight will be diminished in

proportion to the violence of the wind." Yet this has been controverted

with respect to the general effect of air in horizontal motion, and

the depth of the column in question.



Certainly there are two kinds of whirlwinds--one caused by rarefaction,

tending to lighten vertical pressure under the vortex, though not,

perhaps, under all the current drawn towards it; and the other, a

consequence of opposing winds, which occasion huge eddies or whirlwinds

of compression.



Some whirlwinds are accompanied by rushes from the upper atmosphere,

from the colder regions, which, mingling with warmer and moister air

near the sea, cause dense clouds. About their centre it sometimes

happens that the barometer falls as much as two or three inches, showing

a diminution of atmospheric pressure by nearly a tenth part; when it

should be expected, from physical considerations alone, that very dense

clouds would be formed.



* * *



The column of mercury falls about one tenth of an inch for each of the

first few hundred feet above the sea level, but varying when it becomes

much more elevated. Due allowance, therefore, should be made in

observing, when on high land.



The tides are affected by atmospheric pressure, so much that a rise of

one inch in the barometer will have a corresponding fall in the tides of

nine to sixteen inches, or about one foot for each inch.



* * *



Vessels sometimes enter docks, or even harbours, where they have

scarcely a foot of water more than their draught; and as docking, as

well as launching large ships, requires a close calculation of height of

water, the state of the barometer becomes of additional importance on

such occasions.



* * * * *



To render these pages rather more useful at sea, in any part of the

world, a few words about squalls and hurricanes are here offered to the

young seaman.



Generally, squalls are preceded, or accompanied, or followed by clouds;

but the very dangerous "white squall" (of the West Indies and other

regions), is indicated only by a rushing sound, and by white wave

crests.



"Descending squalls" come slanting downwards, off high land, or from

upper regions of atmosphere. They are dangerous, being sometimes

violently strong.



A squall cloud that can be seen through or under is not likely to bring,

or be accompanied by, so much wind as a dark continued cloud extending

beyond the horizon. How the comparative hardness or softness of clouds

foretells more or less wind or rain, was stated in pages 13 and 14.



The expressions "hardening up," "softening," or looking "greasy," are

familiar to seamen: and such very sure indications are the appearances

so designated, that they can hardly be mistaken.



The rapid or slow rise of a squall cloud--its more or less disturbed

look--that is, whether its body is much agitated, and changing form

continually, with broken clouds, or scud, flying about--or whether the

mass of cloud is shapeless and nearly quiet, though floating onwards

across the sky--foretells more or less wind accordingly.



An officer of a watch, with a good eye for clouds and signs of changing

weather, may save his men a great deal of unnecessary exposure, as well

as work, besides economising sails, spars, and rigging.



In some of the "saws" about wind and weather, there is so much truth

that, though trite and simple, their insertion here can do no harm.



Adverting to the barometer:--

When rise begins, after low,

Squalls expect and clear blow.

Or:--First rise, after very low,

Indicates a stronger blow.

Also:--Long foretold, long last:

Short notice, soon past.

To which may be added:--In squalls--

When rain comes before wind,

Halyards, sheets, and braces mind.

And:--When wind comes before rain,

Soon you may make sail again.



* * *



Also, generally speaking:--

When the glass falls low,

Prepare for a blow;

When it rises high,

Let all your kites fly.



* * *



To these short expressions--well known, in practice, to the experienced;

a very concise but sure rule may be added, for avoiding the central or

strongest part of a hurricane, cyclone, typhoon, tornado, or circling

storm.



With your face towards the wind, in North latitude, the centre of the

circling, or rotatory storm, will be square to your right. In South

latitude, square to your left.



The apparent veering of the wind, and the approach or retreat of the

dangerous central circle, depend on your position in the curvilinear

whirl or sweep.



Draw a circle;--mark the direction of the rotation or circulation, by an

arrow with the head towards the left hand (against the movement of a

watch's hands) in North latitude; but towards the right (or with the

hands of a watch) if in South latitude. The direction of the wind, and

the bearing of the centre, show your position in the meteor, for such it

is, though perhaps hundreds of miles in diameter; and the veering of the

wind, or the contrary, and its change in strength, will show how the

meteor is moving bodily--over an extensive region, revolving

horizontally--or inclined at a certain angle to the horizontal plane.



If the observer be stationary, in North latitude, and the centre pass on

his polar side, he will experience a change of wind from Southward by

the West towards North; but if it pass between him and the Equator, the

change will be from Southward by the East towards North; but otherwise

in South latitude, as his place in circles sketched will show more

clearly than words. The roughest sketch or diagram, indicating the

various directions of wind, and the course of the meteor's centre, will

show more plainly than descriptions--which must necessarily vary with

each case, and are tedious.



Cyclonology, or really meteorology, is simple enough in these great

characteristic effects; but their causes must be the philosopher's

study, rather than that of the young practical seaman.



Were it not for this reflection, one might endeavour to show how all the

great Easterly trade winds--the no less important anti-trades, or

nearly constant Westerly winds,--and their complicated eddying offsets,

are all (on greater or smaller scales) breadths, or zones of atmosphere,

alternating, or circulating, or crossing (superposed or

laterally)--between which, at distant intervals, occur those strong

eddies, or storms, called hurricanes--typhoons--tornadoes--or cyclones.



The great easterly and westerly movements--so clearly shown by

philosophers to be the consequences of cold polar currents of air--warm

equatorial currents--and diurnal rotation of the earth; are grand

ruling phenomena of meteorology--to which storms, and all local changes,

occurring but occasionally, are subordinate and exceptional. Further

investigations into electrical and chemical peculiarities will probably

throw additional light, perhaps the strongest, on meteorological

science.