The Philosophy of the Weather Part 8

Lieutenant Maury, in the course of his investigations, and in order to ascertain the direction of the winds in the Mississippi valley during rain, addressed a number of gentlemen, and received their replies, which are published with his wind and current charts. Several answered, among other things, that, "whenever the lightning appears to linger at the north at eventide, rain almost invariably follows speedily; not so in the south." Thus it frequently is with us. If, during a hot, dry time, of a few days continuance, the lightning so lingers in the evening, and the wind continues to blow _fresh_ from the southward _after nightfall_, showers will generally follow within forty-eight hours, most commonly the next day, and a cool N. N. W. or N. W. wind with a favorable change ensue.

Such, at least, has been the result of my observation for many years.

Indeed this seems to be the general law in summer in the Mississippi valley, where the easterly winds are not so common as with us. To ill.u.s.trate this further, I copy from a recent work by T. Ba.s.snett, ent.i.tled the "Mechanical Theory of Storms," two short extracts, showing the manner in which belts of showers extend southerly, while progressing north-eastwardly, at Ottawa. The first occurred in August, 1853; the last, December, 1852. The first was a belt of showers; the latter would have been in August, but the lateness of the season changed its character somewhat, though not entirely, to a more regular rain, especially toward the close.

"AUGUST 6th.--Very fine and clear all day: wind from S. W.; a light breeze; 8 P.M. frequent flashes of lightning in the northern sky; 10 P.M., a _low bank of dense clouds in north_, fringed with cirri, visible during the flash of the lightning; 12 P.M., same continues.

"7th.--very fine and clear morning; wind S. W. moderate; noon, clouds acc.u.mulating in the northern half of the sky; _wind fresher_, _S.

W._; 3 P.M., a clap of thunder over head, and black c.u.muli in west, north, and east; 4 P.M., much thunder and scattered showers; six miles west rained very heavily; 6 P.M., the heavy clouds pa.s.sing over to the south; 10 P.M., clear again in north.

"8th.--Clear all day; wind the same (S. W.); a hazy bank visible all along on _southern horizon_.

"DECEMBER 21st, 1852.--Wind N. E., fine weather.

"22d.--Thick, hazy morning, wind east, much lighter in S. E. than in N. W.; 8 A.M., a clear arch in S. E. getting more to south; noon, _very black in W. N. W._; above, a broken layer of cirro-c.u.mulus, the sun visible sometimes through the waves; wind around to S. E., and fresher; getting thicker all day; 10 P.M., _wind south, strong_; thunder, lightning, and heavy rain all night, with strong squalls from south.

"23d.--Wind S. W., moderate, drizzly day; 10 P.M., wind west, and getting clearer."

It is obvious that the showers at the north pa.s.sed east on the evening of the 6th of August; that new showers, taking the same course, originated in the north, but more southerly next day, with S. W. wind, and that they pa.s.sed east, and others formed successively further south, which pa.s.sed over the place of observation late in the afternoon, and that others formed south and pa.s.sed east during the night and next day, visible in a bank on the southern horizon.

Later or earlier in the spring and autumn, these brisk afternoon southerly winds continuing after nightfall, indicate moderate rains from a rainy belt extending in a similar manner, without the c.u.muli and thunder which attend those of mid-summer. I shall recur to this cla.s.s of showers and storms when we come to their cla.s.sification.

Light surface winds from south-west to west are not often storm-winds, and are usually those which the trade near the earth draws after it. Sometimes the trade seems to draw the surface wind from the S. W. and W. S. W. with considerable rapidity, and some scud a little distance above the earth.

When this is so, it will be found that a storm has pa.s.sed to the north of us, or a belt of rains is pa.s.sing north, which may or may not have sufficient southern extension to reach us. When there have been heavy storms at the south in the spring, especially if of snow, the S. W. wind which the trade draws after it, and which comes from the snowy or chilled surface, is exceedingly "raw"--that is, damp and chilly, although not thermometrically very cold. Probably every one has noticed these "_raw_"

S. W. winds of spring.

Usually, when storms and showers, which have not a southern lateral extension, pa.s.s off, the trade is very near the earth, and a light S. W.

wind or calm follows for a longer or shorter period. Not unfrequently, however, our N. E. storms terminate with a S. W. wind, shifting suddenly, perhaps, just at the close of the storm, during what is sometimes called a "clearing-off-shower," or, more frequently, dying gradually away as a N.

E. wind, and coming out gently from the S. W., following the retreating cloud of the storm. In such cases it is said to "clear off warm."

With us the wind rarely blows from the west, except while slowly hauling from some southerly point to the N. W. It is probably otherwise east of the lakes and in some other localities to the north-west.

Occasionally, and most frequently in March, a W. to W. N. W. wind follows storms, and blows with considerable severity, with large irregular, squally ma.s.ses of scud, and sometimes a gale. Such was the character of the dry gale which crossed the country, particularly Northern New York, in March, 1854, doing great damage. These westerly winds are always accompanied by a continued depression of the barometer, and peculiar, foggy, scuddy, condensation, and should be distinguished with care from the regular and peculiar N. W. wind, as they may be, by the continued depression of the barometer, and the character of the scud. They are doubtless magnetic storms.

The remaining surface wind, the N. W., the genuine Boreas of our climate, the invariable fair-weather wind, is one of great interest. It is unique and peculiar. It is not the left-hand wind of a rotary gale, and has no immediate connection with the storm. I have known it blow moderately, fifteen successive days in winter; rising about ten A.M., and dying away at nightfall. Occasionally, but very rarely indeed, a light wind exists from the N. W. during a storm, owing probably to a focus of intensity in relation to some surface the storm covers, like the focus which exhibits itself as a clearing-off shower near the close of a storm; but the real fair-weather Boreas is a different affair altogether. Let us observe with care its peculiarities; they are instructive.

1st. It rarely blows with any considerable force beneath the trade while there are storm clouds, or any considerable condensation in it. It does not interfere with that reciprocal action which takes place between the trade and the earth, during approaching or existing storms. I have frequently seen it with its peculiar scud clouds in the N. W., waiting for the storm condensation of the trade to pa.s.s by, that full of positive electricity it might commence its sports; rushing and eddying along the surface, licking up the warm, south polar, electric rain, which stood in pools upon the ground, or rose in steamy vapor from the surface, and with its cool breath dry up the muddy roads as no degree of heat can dry them.

The annexed figure (14) shows the appearance of the northern edge of a stratus storm cloud, pa.s.sing off E. N. E. at the close of the storm, which was "_clearing off from the north-west_." It is from a daguerreotype view, looking W. N. W., taken at eight o'clock in the morning, in the fall of the year. Near the horizon maybe seen the N. W. scud, forming in the N. W.

wind, which is about to follow the retreating edge of the storm cloud.

Figure 15 is from a daguerreotype view, taken at eleven o'clock the same day, when the storm cloud had pa.s.sed off and its edge remained visible only south of the zenith, and the north-east scud had risen up and covered the northern half of the sky, and the wind was blowing a gale from that quarter.

[Ill.u.s.tration: Fig. 14.]

Another view was taken about two P.M. of the same day, when the scud had a very dark, gloomy appearance--as _dark_ and _gloomy_ as those of a Mexican norther--too dark to represent by a cut.

Not unfrequently in a moist summer season, after a day of showers or rain, which have had an extending formation or lateral extension from north to south, it will commence blowing in the morning, and encourage the hay-maker with the hope of fine weather. But often before noon, the milky stratus condensation above with c.u.muli below, will appear in the trade; the N. W. wind die away and variable airs from the east or south appear, to be followed toward night by an enlargement of the c.u.muli and showers.

It rarely, if ever, blows fresh till the storm condensation of the trade has pa.s.sed; or continues to blow after that condensation reappears. When it commences blowing after a storm, and the northern edge of the storm is not over us, we may frequently see the latter low down in the S. E.

pa.s.sing eastward.

[Ill.u.s.tration: Fig. 15. NORTH VIEW.]

2d. Its scud are peculiar. Every one, probably, has noticed them. They are distinct, more or less disconnected, irregular, with every form between those of the easterly scud, c.u.mulus, and stratus, according to the season.

If large, with _dark under surfaces_; forming _rapidly_ and as _rapidly dissolving_; rarely dropping any rain, sometimes dropping a flurry of snow, in November or March, oftener than at any other period; sailing away to the S. E., and casting a traveling shadow as they pa.s.s on over the surface of the earth. Their electricity, particularly when white, is probably always positive, as that of all whitish clouds is supposed to be.

3d. _It is emphatically a surface wind._ The incident storm winds, the N.

E. and S. E., frequently _commence blowing_ under the storm, toward its point of greatest intensity, _up near the line of cirro-stratus condensation_, evidenced by the running scud; or blow there with most rapidity, and so continue for hours before the whole surface atmosphere from thence to the earth becomes involved in the movement; and sometimes without being felt below at all. Not so with the N. W. wind; it _begins at the surface_ and blows there with more rapidity than above; it seems to be attracted by the earth; it interposes between the earth and the trade, wedging the trade up and occupying its place. It blows under at all seasons of the year, but most readily and strongly from a surface of snow whose electricity is always positive. Hence it blows most strongly and _continuously_ when snow has fallen at the north, and prevails during winter very much in proportion to the extent and continuance of the covering of snow which invests the earth in that direction. It follows after storms, and particularly warm rains, during the autumn, winter, and spring months, which have a lateral southern extension. Whether it is increased by the snow from the surface from which it blows, or is caused by the same magnetic action which causes the great fall of snow, is a question we shall consider hereafter.

4th. It does not connect or mingle with the trade current in any way, or change or divert the course of that current; but interposes between it and the earth, elevating the trade in proportion to its own volume, above the influences of the earth (when the trade becomes free from condensation, and singularly, clear); and raising _proportionately_ the barometer. An experienced observer can frequently estimate, with considerable accuracy, the rise of the barometer, by measuring with his eye, (when the clouds will enable him to do so,) the depth of this interposed N. W. current. The barometer rarely rises after a storm, for twenty-four or forty-eight hours if the wind continues at any point from S. W. to W. N. W., but always rapidly as soon as the genuine N. W. current with any considerable depth interposes and elevates the trade.

It will be obvious to every one, I think, certainly, if they will hereafter study the subject and observe for themselves, that the N. W.

wind does not blow away the storm; and that it follows after it, blowing over the surface which is uncovered by the storm; rarely, if ever, with any force when the body of the storm pa.s.sed south of us; and that it is a purely surface wind, seemingly attracted by the peculiar magneto-electric state in which the surface of the earth is left, compared with a snow-clad surface to the north, by a recent storm, or that peculiar state of the trade which is left by the action of the storm. It seems to follow that magnetic wave which, pa.s.sing from north to south, acts in its course upon the counter-trade, producing the storm, or belt of showers, and giving them their southern lateral extension, and will well repay future telegraphic investigation. Its electricity is intensely positive--that of the earth by the action of the storm as intensely negative.

5th. This N. W. wind occurs in all parts of the northern hemisphere, so far as we have data to determine, and its corresponding wind from the S.

W. occurs in the southern hemisphere. It is identical with a cla.s.s of the northers of the Gulf of Mexico, as a brief a.n.a.lysis of the character of the latter will show.

1st. The fall and winter _norther_ is a dry wind without rain or falling weather--so is our N. W. wind.

2d. It is preceded by a falling barometer; S. E. scud and rain at the point where it blows, or to the eastward of it. So is ours when it blows a gale in the fall and spring months, which bear the nearest resemblance in climatic character to the periods when the northers blow. With this distinction, however, that our precedent rains either pa.s.s over us or to the southward, the direction of storms being E. N. E.; their precedent storms pa.s.sing over or to the eastward of them as they move more to the northward.

3d. It is often preceded by a copious dew; so is ours--such dews often following light fall rains in our climate, and preceding N. W. wind.

4th. The most peculiar characteristic, however, is that the barometer rises rapidly and invariably while the norther prevails, and very much in proportion to its violence. The same is true of our genuine N. W. wind, and is not true _of any other wind_ on this continent which I have observed or read of.

5th. While they are thus alike in these respects, they are unlike in no respect.

Mr. Redfield has traced them in _supposed_ connection with storms which continue from that vicinity across the United States to the E. N. E., and endeavored to connect them with those storms, as the left-hand winds of a rotary gale. Obviously, I think, they are identical with our N. W. winds which also _follow_, indeed, but _are distinct from the storms_.

There are a cla.s.s of northers in the Gulf of Mexico--the "Nortes del Muero Colorado"--sometimes occurring in the summer months, beginning at N. E., veering about and settling at N. N. W., and as they decline hauling round by the west to the southward. These winds correspond precisely with the hurricane winds of the West Indies, and are doubtless the incident winds of a storm traveling thence to the N. N. W. precisely as our N. E. or E.

N. E gales are incident storm winds to the N. E. storms of our lat.i.tude.

In this connection we will look at the peculiarities of a West India hurricane.

"It is not a little remarkable," says Mr. Espy, speaking of the storms and hurricanes of the West Indies, "that all these storms, and _all others which have been traced to the West Indies_, traveled N. W. almost at right angles to the direction of the trade-wind in those lat.i.tudes, but very nearly, if not exactly, in the direction of an upper current of the air known to exist there toward the N. W." Substantially the same facts have been repeated by Mr. Redfield, and demonstrated by his able investigations, both there and in the Eastern Pacific, and are confirmed by the observations of Edwards, Lawson, and others, while residents there.

It is a matter of surprise that gentlemen like Messrs. Redfield and Espy, who have certainly displayed great ability in the investigations of meteorological phenomena, should fail to recognize a more intimate relation between this upper current and the storms they were investigating, and to detect the general laws which govern both. The storms and hurricanes of the West Indies are comparatively of small diameter, and have little advance condensation. When they pa.s.s on to the south-western portion of North America and curve to the N. E., as they frequently do, they enlarge in front and at the sides, and their advance condensation, which is not dense enough to drop rain, extends in some cases from one to three hundred miles; and the storm itself, by the time it reaches the Alleghanies, may extend one thousand to fifteen hundred miles, and perhaps in certain magnetic states of the surface, and occasionally, may cover the entire portion of the continent, from north to south. Such, probably, was very nearly the extension of the storm investigated by Professor Loomis. In the West Indies, however, at the commencement, they vary from twenty to one hundred miles, or possibly more, in width.

First, they are preceded by a hot, sultry and oppressive atmosphere--_as are electric storms every where_--a peculiar electric state of the earth and adjacent air.

Second, the black clouds and lightning which indicate the approaching hurricane are seen to the S., S. E., and E. S. E., according to the season of the year, as we see them at the westward. During the rainy season, and when the storm, as is usual at that period, is small, and the S. E. trade blows more eastwardly, the wind at the Windward Islands, possibly, may set in at the north, and back round by the east as it progresses. So Colonel Reid thinks it sometimes does, at Barbadoes. But when the belt of rains is south, and the hurricane comes from the south-east, and is larger and more violent in its action, and the north-east winds prevail, the first effect is an increase of these trades. Soon, however, the wind hauls to the north and north-west, in opposition to its course, bearing the same relation to it that our east and north-east winds bear to storms in the United States; and the wind hauls around during the pa.s.sage of the storm to the west, south-west, and south-east, and at the latter point it clears off. Mr.

Edwards in his History of Jamaica says--and as a resident, his authority should be decisive as to this Island--"_that all hurricanes begin from the north_, veer back to the W. N. W., W., and S. S. W., and when they get around to the S. E. the foul weather breaks up." Doubtless the same is true of the cla.s.s of northers of which we are speaking on the Gulf of Mexico. _But with this cla.s.s the barometer does not rise during the gale, and in proportion to its length and violence._ With the other cla.s.s of N.

W. winds--the northers of winter--it does.

The following description of two winter northers, copied from Colonel Reid's valuable work, will ill.u.s.trate what has been said. _Precisely such changes from S. E. rains to N. W. winds, with blue sky and detached dark clouds--fair-weather N. W. scud--occur every autumn in October and November_, and the falling of the thermometer and rising of the barometer, after rain, and a change of the wind, are perfectly characteristic.

------------------------------------------------------------------------ 1843. Wind. Force. Weather. Bar. Ther. ------------------------------------------------------------------------ Jan. 30. A.M. 4. S. S. W. 2 b. c. 29.90 77 Off Tampico.