How To Foretell Weather With A Barometer Part 2

In this part of the world, towards the higher latitudes, the quicksilver

ranges, or rises and falls, nearly three inches--namely, between about

thirty inches and nine-tenths (30.9), and less than twenty-eight inches

(28.0) on extraordinary occasions; but the usual range is from about

thirty inches and a half (30.5), to about twenty-nine inches. Near the

Line, or in equatorial places, the range is but a few tenths, except in
< r /> storms, when it sometimes falls to twenty-seven inches.



The sliding-scale (vernier) divides the tenths into ten parts each, or

hundredths of an inch. The number of divisions on the vernier exceeds

that in an equal space of the fixed scale by one.



* * *



By a thermometer the weight of air is not shown. No air is within

the tube. None can get in. But the bulb of the tube is full of mercury,

which contracts by cold, and swells by heat--according to which effect

the thread of metal in the small tube is drawn down or pushed up so many

degrees: and thus shows the temperature.



If a thermometer have a piece of linen tied round the bulb, wetted

enough to keep it damp by a thread or wick dipping into a cup of water,

it will show less heat than a dry one, in proportion to the dryness of

the air, and quickness of drying. In very damp weather, with or

before rain, fog, or dew, two such thermometers will be nearly alike.



For ascertaining the dryness or moisture of air, the readiest, and

surest method is the comparison of two thermometers; one dry, the other

just moistened, and kept so. Cooled by evaporation as much as the

state of the air admits--the moist (or wet) bulb thermometer shows a

temperature nearly equal to that of the other one, when the atmosphere

is extremely damp, or moist; but lower at other times,--in proportion to

the dryness of air, and consequent evaporation,--as far as twelve or

fifteen degrees in this climate; twenty or even more elsewhere. From

four to eight degrees of difference is usual in England; and about seven

is considered healthy for living rooms.



The thermometer fixed to a barometer intended to be used only as a

weather-glass shows the temperature of air about it nearly--but does not

show the temperature of mercury within exactly. It does so however near

enough for ordinary practical purposes--provided that no sun, nor fire,

nor lamp heat is allowed to act on the instrument partially.



The mercury in the cistern and tube being affected by cold or heat,

makes it advisable to consider this when endeavouring to foretell coming

weather by the length of the column.



* * *



Briefly, the barometer shows weight or pressure of the air; the

thermometer--heat and cold, or temperature; and the wet thermometer,

compared with a dry one, the degree of moisture or dampness.



It should be remembered that the state of the air foretells, rather

than shows present weather (an invaluable fact too often overlooked);

that the longer the time between the signs and the change foretold by

them, the longer such altered weather will last; and, on the contrary,

the less the time between a warning and a change, the shorter will be

the continuance of such foretold weather.



* * *



If the barometer has been about its ordinary height, say near thirty

inches, at the sea level, 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 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 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.



A rapid rise of the barometer indicates unsettled weather. A slow rise,

or steadiness, with dryness, shows fair weather.



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.



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).



2. The amount, taken by itself, of vapour, moisture, wet, rain, hail, or

snow, in the wind or current of air (direction and strength remaining

the same) seems to cause a change amounting, in an extreme case, to

about half an inch.



3. The strength or force alone of wind from any quarter (moisture and

direction being unchanged) is preceded, or foretold, by a fall or rise,

according as the strength will be greater or less, ranging, in an

extreme case, to more than two inches.



Hence, supposing the three causes to act together--in extreme cases--the

mercury might range from about 31 (30.9) inches to near 27 inches, which

has happened occasionally.



Generally, however, as the three act much less strongly, and are less in

accord--ordinary varieties of weather (the wind varying as usual--with

more or less cloudiness, or rain) occur much more frequently than

extreme changes.



Another general rule requires attention; which is, that the wind usually

veers, shifts, or goes round, with the sun, (right-handed in northern

places, left-handed in the southern parts of the world,) and that, when

it does not do so, or backs, more wind or bad weather may be expected

instead of improvement.



In a barometer the mercury begins to rise occasionally before the

conclusion of gale, sometimes even at its commencement, as the

equilibrium of the atmosphere begins to be restored. Although the

mercury falls lowest before high winds, it frequently sinks considerably

before heavy rain only. The barometer falls, but not always, on the

approach of thunder and lightning, or when the atmosphere is highly

charged with electricity. Before and during the earlier part of

serene and settled weather, the mercury commonly stands high, and is

stationary.



Instances of fine weather, with a low glass, occur exceptionally, but

they are always preludes to a duration of wind or rain, if not both.



After very warm and calm weather, rain or a storm is likely to occur;

or at any time when the atmosphere has been heated much above the

usual temperature of the season.



Allowance should invariably be made for the previous state of the

instrument during some days as well as hours, because its indications

may be affected by remote causes, or by changes close at hand. Some of

these changes may occur at a greater or less distance, influencing

neighbouring regions, but not visible to each observer whose barometer,

nevertheless, feels their effect.



There may be heavy rains or violent winds beyond the horizon, out of

view of an observer, by which his instruments may be affected

considerably, though no particular change of weather occurs in his

immediate locality.



It may be repeated, that the longer a change of wind or weather is

foretold by the barometer before it takes place, the longer the presaged

weather will last; and, conversely, the shorter the warning, the less

time whatever causes the warning; whether wind or a fall of rain, hail,

or snow, will continue.



Sometimes severe weather from an equatorial 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.



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.



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).



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.



Generally speaking, natural, quiet, delicate tints or colours, with soft

undefined forms of clouds, foretell fine weather: but gaudy or unusual

hues, with hard, definite outlines, presage rain and wind.



Misty clouds forming, or hanging on heights, show wind and rain

coming--if they remain, or descend. If they rise, or disperse, the

weather will improve, or become fine.



When sea birds fly out early, and far to seaward, moderate wind and fair

weather may be expected. When they hang about the land, or over it,

sometimes flying inland, expect a strong wind, with stormy weather. As

many creatures, besides birds, are affected by the approach of rain or

wind, such indications should not be slighted by the observer of

weather.



There are other signs of a coming change in the weather known less

generally than may be desirable; and, therefore worth notice here.



When birds of long flight, such as swallows and others, hang about home

and fly low--rain or wind may be expected. Also when animals seek

sheltered places, instead of spreading over their usual range: when pigs

carry straw to their sties; and when smoke from chimneys does not ascend

readily, (straight upwards during a calm,) an unfavourable change may be

looked for.



Dew is an indication of fine weather. So is fog. Neither of these two

formations occurs under an overcast sky, or when there is much wind. One

sees the fog occasionally rolled away, as it were, by wind--but not

formed while it is blowing.



Remarkable clearness of atmosphere, near the horizon; distant objects,

such as hills, unusually visible; or raised (by refraction); and what is

called "a good hearing day" may be mentioned among signs of wet, if

not wind, to be expected.



More than usual twinkling of the stars; indistinctness or apparent

multiplication of the moon's horns; haloes; "wind-dogs;" and the

rainbow; are more or less significant of increasing wind, if not

approaching rain.



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.