The Project Gutenberg eBook ofBarometer and Weather GuideThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: Barometer and Weather GuideCompiler: Robert FitzroyRelease date: December 19, 2007 [eBook #23921]Language: EnglishCredits: Produced by Robin Monks, Stephen Blundell and the OnlineDistributed Proofreading Team at http://www.pgdp.net (Thisbook was produced from scanned images of public domainmaterial from the Google Print project.)*** START OF THE PROJECT GUTENBERG EBOOK BAROMETER AND WEATHER GUIDE ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: Barometer and Weather GuideCompiler: Robert FitzroyRelease date: December 19, 2007 [eBook #23921]Language: EnglishCredits: Produced by Robin Monks, Stephen Blundell and the OnlineDistributed Proofreading Team at http://www.pgdp.net (Thisbook was produced from scanned images of public domainmaterial from the Google Print project.)
Title: Barometer and Weather Guide
Compiler: Robert Fitzroy
Compiler: Robert Fitzroy
Release date: December 19, 2007 [eBook #23921]
Language: English
Credits: Produced by Robin Monks, Stephen Blundell and the OnlineDistributed Proofreading Team at http://www.pgdp.net (Thisbook was produced from scanned images of public domainmaterial from the Google Print project.)
*** START OF THE PROJECT GUTENBERG EBOOK BAROMETER AND WEATHER GUIDE ***
BOARD OF TRADE.
1859.
THIRD EDITION.
(WITH ADDITIONS.)
LONDON:
PRINTED BY GEORGE E. EYRE AND WILLIAM SPOTTISWOODE,
PRINTERS TO THE QUEEN'S MOST EXCELLENT MAJESTY.
FOR HER MAJESTY'S STATIONERY OFFICE.
AND SOLD BY
J. D. POTTER,Agent for the Admiralty Charts, 31, POULTRY,
AND 11, KING STREET, TOWER HILL.
1859.
Price One Shilling.
Transcriber's NoteMinor typographical errors have been corrected without note. Due to an omission in the original text, the anchor forfootnote #4has been placed in an assumed position.A brief table of contents, though not present in the original publication, has been provided below:PREFACEHOW TO FORETELL WEATHERMARINE BAROMETER ...
Transcriber's Note
Minor typographical errors have been corrected without note. Due to an omission in the original text, the anchor forfootnote #4has been placed in an assumed position.
A brief table of contents, though not present in the original publication, has been provided below:
A contraction of rules for foretelling weather—in accordance with the following pages—is submitted, for scales of common barometers.
A contraction of rules for foretelling weather—in accordance with the following pages—is submitted, for scales of common barometers.
RISEFALLFORFORN. Ely.NW.-N.-E.S. Wly.SE.-S.-W.DRYWETORORLESSWIND.MOREWIND.————————EXCEPTEXCEPTWET FROMN. Ed.WET FROMN. Ed.
Add one tenth for each hundred feet above the Sea.
LONG FORETOLD—LONG LAST,SHORT NOTICE—SOON PAST.
LONG FORETOLD—LONG LAST,SHORT NOTICE—SOON PAST.
LONG FORETOLD—LONG LAST,
SHORT NOTICE—SOON PAST.
FIRST RISE AFTER LOW,FORETELLS STRONGER BLOW.
FIRST RISE AFTER LOW,FORETELLS STRONGER BLOW.
FIRST RISE AFTER LOW,
FORETELLS STRONGER BLOW.
Manypersons have advocated placing barometers at exposed fishing villages; and the Board of Trade has sanctioned the principle of some assistance by Government to a limited extent, depending on the necessity of each case, and other contingencies, such as the care, publicity, and setting of the barometers.
It was thought advisable to substitute a few words on the scales of these instruments in place of those usually engraved (which are not the most suitable), and to compile brief and plain information respecting the use of weather-glasses.
The following pages were prepared; but only the first few were intended particularly for this purpose.
After writing these, it was suggested that some remarks might be added for the benefit of many persons, especially young officers at sea, and the suggestion was complied with; yet not so as to diminish the portability of this compilation, or increase its price.
These remarks, derived from the combined observation, study, and personal experience of various individuals, are in accordance, generally, with the results obtained by eminent philosophers.
The works of Humboldt, Herschel, Dové, Sabine, Reid, Redfield, Espy, and others, are appealed to in confirmation of this statement.
To obviate any charge of undue haste, or an insufficiently considered plan—which may be fairly brought against many novelties—the following testimony to the first published suggestion of such a measure is submitted.
In the First Report of the Committee on Shipwrecks (1843), at pages 1, 2, 3, the following evidence was printed by order of the House of Commons.
"I think that the neglect of the use of the barometer has led to the loss of many ships. From a want of attention to the barometer, they have either closed the land (if at sea), or have put to sea (being inharbour in safety) at improper times; and in consequence of such want of precaution the ships have been lost, owing to bad weather coming on suddenly, which might have been avoided had proper attention been paid to that very simple instrument. While alluding to the use of barometers, I may remark, that if such weather-glasses were put in charge of the Coast-guard, at the principal stations round the coast, so placed as to allow any one passing by to look at them, they might be the means, not only of preventing ships from going to sea just before bad weather was coming on, but of preventing the great losses of life which take place every year on our coasts (particularly in the Orkney Islands and on the coasts of Scotland and Ireland), owing to fishing vessels and boats going to sea when bad weather is impending. No bad weather ever comes on our coasts without timely warning being given by the barometer. The oldest seaman are often deceived by the look of the weather, but there is no instance on record of very bad weather, such as would have involved loss of life to the extent we have heard of in several years, having come on without the barometer having given timely warning. By the very small expense of an establishment of barometers, so placed as to be accessible to any fishermen, boatmen, or others on the coasts, much loss of life, as well as loss of boats, and even shipping, might be prevented."What state of the barometer indicates danger?—It varies in different climates according to the range. The range is small between the tropics, but very large in the higher latitudes. In our climate the range is usually about two inches. The barometer falling considerably below its average height is at once an indication that some considerable change is going to take place, and when it falls low, as for instance (in our climate) to near 29 inches, or below 29 inches, a gale is certain to follow."Are the Committee to understand that you are of opinion that every ship ought to have a barometer on board?—I think that every ship ought to have either a barometer or sympiesometer, which is an efficient substitute for a barometer."Does the barometer show a sudden change of wind as well as the coming on of bad weather? Supposing a gale of wind is blowing, and you are sailing with a fair wind, does the barometer show any change of wind?—Decidedly."Supposing the wind was at West-north-west and it shifted suddenly to West-south-west, would the barometer indicate that?—It requires some practice to be able to sayexactly what is likely to take placeafter a change in the barometer; but the principal point for a seaman is, that no violent wind will blow without the barometer giving warning. He may not know exactly from what quarter the wind will come, but no strong wind will come on without warning being given."You recommend that at the Coast-guard stations there should be a barometer, by means of which people would know when a violent wind was coming on; but as it would not indicate the quarter fromwhich it was coming, would you have the merchant ship always remain in port till the barometer showed fine weather?—Being accustomed to the barometer on our coast, one could tell from what quarter the wind would probably come by the height of the barometer, taken in connexion with its previous height, and the state of the weather, and the strength of winds that had prevailed before. Taking the state of the barometer in connexion with the appearance of the weather one could make a satisfactory conclusion as to the quarter from which any violent wind would come. And the barometer, after very little practice, can be used by any man. There is no difficulty in using it sufficiently to know that danger is coming on; and if danger is coming on, a man refrains, of course, from exposing himself to it; the quarter from which the wind comes being of minor consequence."With a North-easterly wind, in this part of the world, the barometer stands, on an average, about half an inch higher than with the same strength of wind from the South-westward. All over the world there is a similar difference proportionate to the range of the mercury for which allowance should always be made in considering the height of the barometer."[1]
"I think that the neglect of the use of the barometer has led to the loss of many ships. From a want of attention to the barometer, they have either closed the land (if at sea), or have put to sea (being inharbour in safety) at improper times; and in consequence of such want of precaution the ships have been lost, owing to bad weather coming on suddenly, which might have been avoided had proper attention been paid to that very simple instrument. While alluding to the use of barometers, I may remark, that if such weather-glasses were put in charge of the Coast-guard, at the principal stations round the coast, so placed as to allow any one passing by to look at them, they might be the means, not only of preventing ships from going to sea just before bad weather was coming on, but of preventing the great losses of life which take place every year on our coasts (particularly in the Orkney Islands and on the coasts of Scotland and Ireland), owing to fishing vessels and boats going to sea when bad weather is impending. No bad weather ever comes on our coasts without timely warning being given by the barometer. The oldest seaman are often deceived by the look of the weather, but there is no instance on record of very bad weather, such as would have involved loss of life to the extent we have heard of in several years, having come on without the barometer having given timely warning. By the very small expense of an establishment of barometers, so placed as to be accessible to any fishermen, boatmen, or others on the coasts, much loss of life, as well as loss of boats, and even shipping, might be prevented.
"What state of the barometer indicates danger?—It varies in different climates according to the range. The range is small between the tropics, but very large in the higher latitudes. In our climate the range is usually about two inches. The barometer falling considerably below its average height is at once an indication that some considerable change is going to take place, and when it falls low, as for instance (in our climate) to near 29 inches, or below 29 inches, a gale is certain to follow.
"Are the Committee to understand that you are of opinion that every ship ought to have a barometer on board?—I think that every ship ought to have either a barometer or sympiesometer, which is an efficient substitute for a barometer.
"Does the barometer show a sudden change of wind as well as the coming on of bad weather? Supposing a gale of wind is blowing, and you are sailing with a fair wind, does the barometer show any change of wind?—Decidedly.
"Supposing the wind was at West-north-west and it shifted suddenly to West-south-west, would the barometer indicate that?—It requires some practice to be able to sayexactly what is likely to take placeafter a change in the barometer; but the principal point for a seaman is, that no violent wind will blow without the barometer giving warning. He may not know exactly from what quarter the wind will come, but no strong wind will come on without warning being given.
"You recommend that at the Coast-guard stations there should be a barometer, by means of which people would know when a violent wind was coming on; but as it would not indicate the quarter fromwhich it was coming, would you have the merchant ship always remain in port till the barometer showed fine weather?—Being accustomed to the barometer on our coast, one could tell from what quarter the wind would probably come by the height of the barometer, taken in connexion with its previous height, and the state of the weather, and the strength of winds that had prevailed before. Taking the state of the barometer in connexion with the appearance of the weather one could make a satisfactory conclusion as to the quarter from which any violent wind would come. And the barometer, after very little practice, can be used by any man. There is no difficulty in using it sufficiently to know that danger is coming on; and if danger is coming on, a man refrains, of course, from exposing himself to it; the quarter from which the wind comes being of minor consequence.
"With a North-easterly wind, in this part of the world, the barometer stands, on an average, about half an inch higher than with the same strength of wind from the South-westward. All over the world there is a similar difference proportionate to the range of the mercury for which allowance should always be made in considering the height of the barometer."[1]
In the first Number of Meteorological Papers, published by the Board of Trade, 1857, is the following passage respecting the use of weather-glasses:—
"The variety of interesting and useful, if not always important, subjects included within the range of meteorology, is not perhaps sufficiently realized in the minds of active participators in the world's stirring work. Irrespective of any scientific object, how much utility is there to all classes in what is commonly called 'weather wisdom'? In our variable climate, with a maritime population, numbers of small vessels, and especially fishing boats, how much life and property is risked unnecessarily by every unforeseen storm? Even animals, birds, and insects have a presaging instinct, perhaps a bodily feeling, that warns them; but man often neglects his perceptive and reasoning powers; neither himself observes, nor attends to the observations of others, unless special inclination or circumstances stimulate attention to the subject. Agriculturists, it is true, use weather-glasses: the sportsman knows their value for indicating a good or bad scenting day; but the coasting vessel puts to sea, the Shetland fisherman casts his nets, without the benefit of such a monitor, and perhaps without the weather wisdom which only a few possess, and cannot transfer to others."Difficult as it is to foretell weather accurately, much useful foresight may be acquired by combining the indications of instruments (such as the barometer, thermometer, and hygrometer) with atmosphericappearances. What is more varying than the aspect of the sky? Colour, tint of clouds, their soft or hard look, their outline, size, height, direction, all vary rapidly, yet each is significant. There is a peculiar aspect of the clouds before and during westerly winds which differs from that which they have previous to and during easterly winds, which is one only of the many curious facts connected with the differing natures of easterly and westerly currents of air throughout the world, which remain unchanged, whether they blow from sea to land, or the reverse.[2]"Perhaps some of those who make much use of instruments rather undervalue popular knowledge, and are reluctant to admit that a 'wise saw' may be valuable as well as a 'modern instance;' while less informed persons who use weather-glasses unskilfully too often draw from them erroneous conclusions, and then blame the barometer."Not only are reliable weather-glasses required at the smaller outlying ports and fishing places, but plain, easily intelligible directions for using them should be accessible to the seafaring population, so that the masters of small vessels, and fishermen, might be forewarned of coming changes in time to prepare for them, and thus become instrumental in saving much property and many lives."
"The variety of interesting and useful, if not always important, subjects included within the range of meteorology, is not perhaps sufficiently realized in the minds of active participators in the world's stirring work. Irrespective of any scientific object, how much utility is there to all classes in what is commonly called 'weather wisdom'? In our variable climate, with a maritime population, numbers of small vessels, and especially fishing boats, how much life and property is risked unnecessarily by every unforeseen storm? Even animals, birds, and insects have a presaging instinct, perhaps a bodily feeling, that warns them; but man often neglects his perceptive and reasoning powers; neither himself observes, nor attends to the observations of others, unless special inclination or circumstances stimulate attention to the subject. Agriculturists, it is true, use weather-glasses: the sportsman knows their value for indicating a good or bad scenting day; but the coasting vessel puts to sea, the Shetland fisherman casts his nets, without the benefit of such a monitor, and perhaps without the weather wisdom which only a few possess, and cannot transfer to others.
"Difficult as it is to foretell weather accurately, much useful foresight may be acquired by combining the indications of instruments (such as the barometer, thermometer, and hygrometer) with atmosphericappearances. What is more varying than the aspect of the sky? Colour, tint of clouds, their soft or hard look, their outline, size, height, direction, all vary rapidly, yet each is significant. There is a peculiar aspect of the clouds before and during westerly winds which differs from that which they have previous to and during easterly winds, which is one only of the many curious facts connected with the differing natures of easterly and westerly currents of air throughout the world, which remain unchanged, whether they blow from sea to land, or the reverse.[2]
"Perhaps some of those who make much use of instruments rather undervalue popular knowledge, and are reluctant to admit that a 'wise saw' may be valuable as well as a 'modern instance;' while less informed persons who use weather-glasses unskilfully too often draw from them erroneous conclusions, and then blame the barometer.
"Not only are reliable weather-glasses required at the smaller outlying ports and fishing places, but plain, easily intelligible directions for using them should be accessible to the seafaring population, so that the masters of small vessels, and fishermen, might be forewarned of coming changes in time to prepare for them, and thus become instrumental in saving much property and many lives."
June 1858.
Familiaras 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[3]USUALLYforetells less wind or rain, a falling barometer more rain or wind, or both; a high one fine weather, and a low, the contrary. But useful as these general conclusions arein most cases, they aresometimeserroneous, and then remarks may be rather hastily made, tending to discourage the inexperienced.
By attention to the following observations (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[4]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 on everything within about forty miles of the world's surface, like amuchlighter 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,[5]air presses with a force or weight of nearly fifteen pounds on a square inch of surface. Such a pressure holds the limpet to the rock when, by contracting itself, the fish has made a place without air[6]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 turneddown 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 thirty 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 thirty 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 thirty 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 atChangeable, and then rise towardsFair, 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 abovefairand 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 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 weather, but from itsrisingorfalling; and from the movements of immediately preceding days as well as hours, keeping in mind effects of change ofdirection, 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 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.[7]
By a thermometer theweightof air isnotshown. No air is within the tube. None can get in. But the bulb of the tube isfull 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.[8]
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.[9]In very damp weather, with orbeforerain, 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 otherjustmoistened, andkept 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.[10]
It should be remembered that the state of the airforetells, rather than shows present weather (an invaluable fact too often overlooked); that the longer the time between the signs and thechange 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,[11]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 thedirectionof 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 levelonly), a rise foretells less wind, or a change in its direction towards the Northward,—or less wet; but when the mercury[12]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—afterwhich 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[13]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 befallingwhen the mercury in the tube is sinking, at which time its upper surface issometimesconcave or hollow. The barometer isrisingwhen the mercurial column is lengthening; its upper surface being then, as ingeneral, convex or rounded.[14]
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 contrarysometimesoccurs; in which cases the Southerly wind is dry and the weather fine; or the Northerly wind is wet and violent.[15]
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 thanpoles?)
The range, or difference of height, of the mercury, due to change of directiononly, 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 happenedoccasionally.
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, butnot always, on the approach of thunder and lightning, or when the atmosphere is highly charged with electricity.[16]Before and during the earlier part of serene and settled weather, the mercury commonly stands high, and is stationary.[17]
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 likelyto occur; or at any time when the atmosphere has beenheatedmuch 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[18]direction, not lasting long, may cause no great fall of the barometer, because followed by adurationof wind from polar regions:—and at times it may fall considerably with polar winds and fine weather, apparently against these rules, because acontinuanceof 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.[19]
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, thesofterclouds 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.[20]
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).[21]
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: butgaudy 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 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 goodhearingday" may be mentioned among signs of wet, if not wind, to be expected.[22]
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.[23]
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 fortheir 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 thereasonsfor 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 movescontrary 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 similarupwarddraught 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, thewholebody 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 revolutiondiminishes 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 thegeneraleffect 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.[24]
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.[25]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.[26]
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, inanypartof 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,[27]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 pages13and14.
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.