How to read barometers

explanation on how to read a barometer properly in order to predict weather.>

Familiar as the practical use of weather-glasses is, at sea as well as on land, only those who have long watched their indications and compared them carefully, are really able to conclude more than that the rising glass usually fortells less wind or rain, a falling barometer more rain or wind or both, a high one fine weather, and a low one the contrary. But useful as these general conclusions are in most cases, they are sometimes erroneous, and then remarks may be rather hastily made, tending to discourage the inexperienced.

By attention to the following observation (the results of many years' practice, and many persons' experience), any one not accustomed to use a barometer may do so without difficulty. The barometer shows whether the air is getting lighter or heavier, or is remaining in the same state. The quicksilver falls as the air becomes lighter, rises as it becomes heavier, and remains at rest in the glass tube while the air is unchanged in weight. Air presses upon everything within about 40 miles of the world's surface, like a much lighter ocean, at the bottom of which we live, not feeling its weight because our bodies are full of air, but feeling its currents, the winds. Towards any place from which the air has been drawn by suction, air presses with a force or weight of nearly 15 lbs. on a square inch of surface. Such a pressure holds the limpit to the rock when, by contracting itself the fish has made a place without air under its shell. Another familiar instance is, that of the fly, which walks on the ceiling with feet that stick. The barometer tube, emptied of air and filled with pure mercury, is turned down into a cup or cistern containing the same fluid, which feeling the weight of air, is so pressed by it as to balance a column of about 30 inches (more or less) in the tube, where no air presses on the top of the column.

If a long pipe, closed at one end only, were emptied of air, filled with water, the open end kept in water, and the pipe held upright, the water would rise in it more than 30 feet. In this way water barometers have been made. A proof of this effect is shown by any well with a sucking-pump, up which, as is commonly known, the water will rise nearly 30 feet by what is called suction, which is, in fact, the pressure of air towards an empty place.

The words on scales of barometers should not be so much regarded for weather indications as the rising or falling of the mercury, for if it stand at "changeable," and then rise towards "fair," it presages a change of wind or weather, though not so great as if the mercury had risen higher; and, on the contrary, if the mercury stand above "fair," and then fall, it presages a change, though not to so great a degree as if it had stood lower; besides which, the direction and force of the wind are not in any way noticed. It is not from the point at which the mercury may stand that we are alone to form a judgment of the state of the state of the weather, but from its rising or falling, and from the movements of immediately preceding days, as well as hours, keeping in mind effects of change of direction and dryness or moisture as well as alteration of force or strength of wind

In this part of the world, towards the higher latitudes, the quicksilver ranges, or rises and fails nearly three inches--namely, between about thirty inches and ninetenths (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 10 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 round the bulb, wetted enough to keep it dump 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, a wet and dry bulb thermometer 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 12 or 15 in this climate, 20 or even more elsewhere. From 4 to 8 of difference is usual in England, and about 7 is considered healthy for inhabited rooms. The wet and dry bulb thermometer on the same frame, the water being supplied by a bird fountain, constitutes August's or Mason's hygrometer.

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 endeavoring 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 always be remembered that the state of the air foretells coming weather rather than shows the weather that is present--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.

To know the state of the air not only barometers and thermometers should be watched, but the appearance of the sky should be vigilantly noticed.

If the barometer has been about its ordinary height, say near 30 inches (at the sea level), and is steady, or rising while the thermometer falls, and dampness becomes less, northwesterly or northerly wind, or less wind, less rain or snow 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 southwestward. A fall with a low thermometer foretells snow. A rise during frost indicates snow.

Exceptions to these rules occur when a north-easterly wind with wet (rain, hail 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 (the rising) expect fair weather.

When the barometer is rather below its ordinary height, say down to near 29 1/2 inches (at the sea level), a rise foretells less wind, or a change in its direction toward the northward, or less wet, but when it has been very low, about 29 inches, the first rising usually precedes or indicates strong wind; at times heavy squalls from the northwestward, northward or northeastward, after which violence a gradually 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 southwesterly wind will follow, especially if the barometer's rise is sudden.

The most dangerous shifts of wind or the heaviest northerly gales happen soon after the barometer first rises from a very low point, or if the wind veers gradually at some time afterwards.

Indications of approaching changes of weather and the direction and force of winds are shown less by the height of the barometer than by its falling or rising. Nevertheless, a height of more than thirty (30.0) inches (at the level of the sea) is indicative of fine weather and moderate winds, except from east to north occasionally or during frost, when northeast winds and snow are indicated.

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; or when the hand of the wheel barometer or Aneroid moves to the left. The barometer is rising when the mercurial column is lengthening, its upper surface being convex or rounded, or when the hand moves to the right.

A rapid rise of the barometer indicates unsettled weather, a slow movement the contrary; as likewise a steady barometer, which, when continued and with dryness, foretells very fine weather.

The greatest depressions of the barometer are with gales from S.E., S., or S.W.; the greatest elevations, with wind from N.W., N., or N.E., or with calm.

Though the barometer generally falls with a southerly and rises with a northerly wind, the contrary sometimes occurs; in which cases, the southerly wind is usually dry with fine weather, or the northerly wind is violent and accompanied by rain, snow or hail; perhaps with lightning.

When the barometer sinks considerably, much wind, rain (perhaps with hail) or snow will follow; with or without lightning. The wind will be from the northward, if the thermometer is low, (for the season), from the southward if the thermometer is high. Occasionally a low glass is followed or attended by lightning only, while a storm is beyond the horizon.

A sudden fall of the barometer with a westerly wind, is sometimes followed by a violent storm from N.W., or N., or N.E.

If a gale sets in from the E. or S.E., and the wind veers by the S., the barometer will continue falling until the wind is near a marked change, when a lull may occur, after which the gale will soon be renowed, perhaps suddenly and violently, and the veering of the wind towards the N.W., N., or N.E., will be indicated by a rising of the barometer with a fall of the thermometer.

Three causes (at least) appear to affect a barometer:

1. The direction of the wind; the northeast wind tending to raise it the most, the southwest 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.

The range or difference of height shown, 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 1/2 an inch (as read off).

2. The amount taken by itself of vapor (moisture, wet, rain, or snow in the wind remaining the same), seems to cause a change amounting in an extreme case to about 1/2 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 loss, ranging in extreme cases to more than 2 inches.

Hence, supposing three causes to act together, in extreme cases, the height would vary from near 31 in. (30.9) to about 27 in. (27.0), which has happened, though rarely (and even in tropical latitudes).

In general the three causes act much less strongly, and are less in accord, so that ordinary varieties of weather occur much more frequently than extreme changes.

Another general rule requires attention, which is, that the wind usually appears to veer, shift, or go round with the sun (right-handed, or from left to right), and that when it does not do so, or backs, more wind or bad weather may be expected, instead of improvement.

It is not by any means 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, and will find that the more accurately the two sources of foreknowledge are compared and combined, the more satisfactory their results will prove.

A barometer begins to rise considerably before the conclusion of a gale, sometimes even at its commencement. Although it falls lowest before high winds, it frequently sinks very much before heavy rain. The barometer falls, but not always on the approach of thunder and lightning. Before and during the earlier part of settled weather it usually stands high and is stationary, the air being dry.

Instances of fine weather with a low glass occur, however, rarely, but they are always preludes to a duration of wind or rain, if not both.

After very warm and calm weather, a storm or squall, with rain, may follow; likewise at any time when the atmosphere is heated much above the usual temperature of the season.

Allowance should invariably be made for the previous state of the glasses during some days, as well as some hours, because their indications may be affected by distant causes, or by changes close at hand. Some of these changes may occur at a greater or less distance, influencing neighboring regions, but not visible to each observer whose barometer feels their effect.

There may be heavy rains or violent winds beyond the horizon, and the 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 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 or snow, will continue.

Sometimes severe weather from the southward, not lasting long, may cause no great fall, because followed by a duration of wind from the northward, and at times the barometer may tall with northerly winds and fine weather, apparently against these rules, because a continuance of southerly wind is about to follow. By such changes as these one may be misled, and calamity may be the consequence if not duly forewarned.

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 (perhaps rain); a gray 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 gray tints the coming weather may be foretold very nearly; indeed, if aided by instruments, almost exactly.

Small, inky-looking clouds foretell rain; light scudclouds driving across heavy masses show wind and rain, but, if alone, may indicate wind only.

High, upper clouds crossing the sun, moon, or stars, in a direction different from that of the lower clouds, or the wind then felt below, foretell a change of wind.

After clear, fine weather, the first signs in the sky of a coming change are usually light streaks, curls, wisps, or mottled patches of white distant clouds, which increase and are followed by an overcasting of murky vapor that grows into cloudiness. This appearance, more or less oily, or watery, as wind or rain will prevail, is an infallible sign.

Usually the higher and more distant such clouds seem to be, the more gradual but general the coming change of weather will prove.

Light, delicate, quiet tints or colors, with soft, undefined forms of clouds, indicate and accompany fine weather, but gaudy or unusual hues, with hard, definitely outlined clouds, foretell rain, and probably strong wind.

Misty clouds forming or hanging on heights, show wind, if they remain, increase, 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 an observer who wishes to foresee weather.

There are other signs of a coming change in the weather, known less generally than may be desirable, and therefore worth notice, such as when birds of long flight, rooks, swallows, or others, hang about home, and fly up and down, or 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 styes; when smoke from chimneys does not ascend readily (or straight upwards during calm), an unfavourable change is probable.

Dew is an indication of fine weather; so is fog. Neither of these two formations occur under an overcast sky, or when there is much wind. One sees fog occasionally rolled away, as it were, by wind, but seldom or never formed while it is blowing.

Remarkable clearness of atmosphere near the horizon; distant objects, such as hills, usually visible, or raised (by refraction), and what is called "a good hearing day," may be mentioned among the 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, it not approaching rain, with or without wind.

Near land, in sheltered harbors, 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 vapor below (rendered visible by the cooling of night), which seems to check the wind.

Lastly, the dryness or dampness of the air and its temperature (for the season) should always be considered, with other indications of change, or continuance of wind and weather.





Return to The Household Cyclopedia of General Information