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
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
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
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
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
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
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
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
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
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
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
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
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
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