Part 6

Chapter 63,696 wordsPublic domain

CONSTANT WINDS.—_The Trade Winds._—The violent contrast between the temperature of the equator and the poles is well known, and from the vast area included within the tropics ascending currents of rarefied air are incessantly rising and being as incessantly replaced by a rush of cold air from the poles to the equator. Were the earth stationary, this interchange would be of the simplest kind; on arriving within the influence of the ascending equatorial current the air from the poles would be carried to the higher regions and turning over would proceed to the poles, and, becoming cold and dense in traversing the higher stratum, would descend and resume its course _ad infinitum_. The revolution of the earth on its axis changes all this: the first effect is that the air at the equator is borne along with the earth at the rate of seventeen miles a minute from west to east, a rate which diminishes at 60° of latitude to one-half that velocity, until at the poles it is nothing; consequently a _slow_ north wind flowing to the equator is continually passing over places possessing a higher velocity than itself, and not immediately acquiring that velocity, there is according to the law of the composition of forces a compromise effected resulting in a north-east wind. In a similar manner the same process in the southern hemisphere results in a south-east wind. These winds have acquired the name of _Trade Winds_ on account of the facilities afforded to navigation by their constancy. The _North Trades_ occur in the Atlantic between 9° and 30° and in the Pacific between 9° and 26°. The _South Trades_ occur in the Atlantic between lat. 4° N. and 22° S. and in the Pacific between latitude 4° N. and 23-1/2° S. These limits extend northward with the sun from January to June, and southward from July to December.

Parallel to the equator and extending between 2° and 3° on each side is a broad belt, where the north and south trades neutralize each other, producing what is called the “_Region or Belt of Calms_.” Though wind is absent, thunderstorms and heavy rains are of daily occurrence.

When Humboldt ascended Teneriffe the trade wind was blowing at its base in the usual direction, but on arriving at the summit he found a strong wind blowing in the opposite direction. Observation has shown that this upper current prevails north and south of the equator, and that, after passing the limit of the trade winds, it descends to form the south-_west_ winds of the north temperate zone and the north-_west_ winds of the south temperate zone; the _westing_ being due to the same cause as the _easting_ in the regular trades, viz., the rotation of the earth on its axis. These winds are called the Return Trades, but are not equal in constancy to the regular trade winds.

PERIODICAL WINDS.—_Land and Sea Breezes_ occur on the coasts, chiefly in tropical countries, but sometimes in Great Britain during the summer months when the land during the day becomes very hot, causing an ascending column of air, which is replaced by a comparatively colder stream flowing inwards from the sea. At sunset the conditions are reversed, the earth becomes rapidly cooled by radiation, the sea continuing comparatively warm, the air over it ascends, and its place is supplied by a cold breeze, which “blows off the shore,” as illustrated by the diagrams and the following experiment—In the centre of a large tub of water float a water plate containing hot water, imagine the former to be the ocean and the latter the heated land, rarefying the air over it. Light a candle and blow it out and hold it while still smoking over the cold water, when the smoke will be seen to move towards the plate. The reverse of this takes place if the tub be filled with hot water and the water plate with cold. When this phenomenon takes place on a large scale, as in the case of the north trade winds being drawn from their course by the heated shores of Southern Asia, the gigantic sea breeze thus produced is called the south-west monsoon. This occurs from April to October, when the sun is north of the equator. When the sun is south of the equator—that is, from October to April—the analogue of the land breeze is produced, and is called the north-east monsoon.

VARIABLE WINDS.—The character of this class of winds is determined by the physical configuration of the country in which they occur. Some tracts are marked by luxuriant vegetation, others are bare. Here mountains lift their awful fronts and “midway leave the storm,” there an arid plain extends itself to the seashore, or inland, towards a chain of lakes. Within the tropics these purely local conditions are insufficient to overcome the force of the prevalent atmospheric currents: such, however, is not the case beyond the tropical zone. There the variable winds prevail, for which space permits only the mention of their names:—The _Simoom_ (from the Arabic _samma_, hot), peculiar to the hot sandy deserts of Africa and Western Asia. The _Sirocco_ blows over the two Sicilies as a hot wind from the south. It extends sometimes to the shores of the Black and Caspian seas, spreading death among animals and plants. The _Solano_ prevails at certain seasons in the south of Spain: its direction is south-east. The _Harmattan_ is another wind of the same class, peculiar to Senegambia and Guinea. The _Puna Winds_ blow for four months over a barren tableland called the Puna, in Peru. They are a portion of the south-east trade winds, which, having crossed the Pampas, are thereby deprived of moisture, and become the most parching wind in the world. The _East Winds_, peculiar to the spring in Britain, blowing as they do through Russia, over Europe, are a portion of the great polar current, distinctive of that season of the year. They are dry and parching, every one being familiar with the unpleasant bodily sensations attendant on this much-abused and yet most beneficent wind.

The _Etesian Winds_ are drawn from the north across the Mediterranean by the great heat of the African desert. The _Mistral_ is a strong north-west wind peculiar to the south-east of France. The _Pampero_ is a north-west wind, blowing in summer from the Pampas of Buenos Ayres.

As long ago as the year 1600 Lord Bacon remarked that the preponderating tendency of the wind was decidedly to veer _with_ the sun’s motion, thus passing from N. through N.E., E., S.E., to south, thence through S.W., W.N.W., to N.; also, that it often makes a complete circuit in that direction, or more than one in succession (occupying sometimes many days in so doing), but that it rarely backs, and very rarely or never makes a complete circuit in the contrary direction. The merit of having first demonstrated that this tendency is a direct consequence of the earth’s rotation is due to Professor Dove, of Berlin, who has also shown that the three systems of atmospheric currents just treated of, viz., the constant, periodical, and variable winds, are all amenable to the same influence.

As to the _mode_ of observing the wind, Admiral Fitzroy recommends that a true east and west line should be marked _about the time of the equinox_, and the north, south, and other points of the compass being added, to take the bearings of the wind in relation to a dial so prepared, the indications of the _lower_ stratum of clouds in conjunction with vanes and smoke being preferred to any other.

The direction of the wind should always be given according to _true_, and not to _compass bearings_. Two points to the westward nearly represents the amount of “Variation of the Compass” for the British Isles, which yields the following table for the conversion of directions observed by the compass in Great Britain and Ireland to approximate true bearings.

+---------------------+-----+-----+-----+-----+-----+-----+-----+ |Compass bearings. N | NNE | NE | ENE | E | ESE | SE | SSE | |True bearings. NNW| N | NNE | NE | ENE | E | ESE | SE | +---------------------+-----+-----+-----+-----+-----+-----+-----+ |Compass bearings. S | SSW | SW | WSW | W | WNW | NW | NNW | |True bearings. SSE| S | SSW | SW | WSW | W | WNW | NW | +---------------------+-----+-----+-----+-----+-----+-----+-----+

“One may call a very simple diagram, a circle divided by a diameter from north-east to south-west, the _thermometer compass_. While the wind is shifting from south-west, by west, north-west, and _north to north-east_, the thermometer is falling, but while shifting from north-east, by east, south-east and south, towards _south and south-west_, the thermometer is rising. Now the barometric column does just the reverse. From north-east the barometer falls as the wind shifts through the east to south-east, south, and south-west, and from the south-west, as the wind shifts round northward to north-east, the barometer rises—it rises to west, north-west, north, and north-east.

“The effect of the wind thus shifting round when traced upon paper by a curve, seems certainly wave-like to the eye; but I believe it to be simply consequent on the wind shifting round the compass, and indicating alteration in the barometric column.

“If the wind remained north-east, say three weeks, there would be no wave at all—there would be almost a straight line along a diagram (varying only a little for _strength_). The atmospheric line, in such a case, remains at the same height, and the barometer remains at 30 inches and (say) some three or four-tenths, for weeks together. So likewise when the wind is south-westerly a long time, or near that point, the atmospheric line remains _low_, towards 29 inches. Thus, such ‘atmospheric waves’ may be an optical delusion.

“The diagram alluded to above shows how the barometer and thermometer may be used in connection with each other in foretelling wind, and consequently weather, that is coming on, because _as the one rises, the other_ generally _falls_, and if you take the two together and confront with their indications the amount of moisture in the air at any time, you will scarcely be mistaken in knowing what kind of weather you are likely to have for the _next two or three days_, which for the gardener, the farmer, soldier, sailor, and traveller must be frequently of considerable importance.”[14]

[Footnote 14: The late Admiral Fitzroy.]

We are indebted to M. Buys Ballot, a Dutch meteorologist, for an invaluable generalization, the importance of which it is almost impossible to over-estimate. This distinguished _savant_ says:—“It is a fact above all doubt that the wind that comes is nearly at right angles to the line between the places of highest and lowest barometer readings. The wind has the place of lowest barometer at its left hand, and is stronger in proportion as the difference of barometer readings is greater.” These facts have been variously stated by other writers; for example: “Stand with your back to the wind, and the barometer will be lower on your left hand than on your right;” “Facing the wind the centre of depression bears in the right-hand direction,” statements which can be verified at any time by a brief study of the “Weather Charts” now published in the daily journals. The value of the law consists in its connecting the surface winds of our planet with the actual pressure of the air itself, and it admits of the following tabulation:—

which can be verified by the reader from the daily Weather Charts in the newspapers.

The above are deductions from Buys Ballot’s Law, still further impressed on the memory by taking four outline maps of the British Isles, inserting the names of Thurso, Penzance, Yarmouth, and Valentia, with barometer readings of the kind above named at each place, and then drawing a large arrow in red ink across the centre of each map in the direction appropriate to the readings.

Mr. Strachan, in his able pamphlet on “Weather Forecasts,” puts the matter thus: “It follows from Ballot’s Law that in the northern temperate zone the winds will circulate around an area of low atmospherical pressure in the _reverse direction_ to the movement of the hands of a watch, and that the air will flow away from a region of high pressure, and cause an apparent circulation of the winds around it, _in the direction_ of watch hands.” And as the result of a careful digest of data contained in the eleventh number of meteorological papers, published by the Board of Trade, he has established the following valuable propositions. As introductory to the propositions, it should be stated that the positions of observations were the following:—

Places. Latitude. Longitude. Nairn 57° 29´ N. 4° 13´ W. Brest 48 „ 28 4 „ 29 W. Valentia 51 „ 56 10 „ 19 W. Yarmouth 52 „ 37 1 „ 44 E. Portrush (or Greencastle) 55 „ 12 6 „ 40 W. Shields 55 „ 0 1 „ 27 W.

Nairn and Brest are situated nearly on the same meridian, about 540 geographical miles apart. Valentia and Yarmouth are nearly on the same parallel of latitude, about 450 miles apart. Portrush and Shields, distant 180 miles, are on a parallel which is nearly as remote from the parallel of Nairn as that of Valentia and Yarmouth is from the one passing through Brest; and Shields is about as much to the westward of Yarmouth as Portrush is to the eastward of Valentia. When observations have not been obtainable for Brest, those made at Penzance have been used instead.

_Proposition 1._—Whenever the atmospherical pressure is greater at Brest than at Nairn, while it is of the same or nearly the same value at Valentia and Yarmouth, being gradually less from south to north, the winds over the British Isles are _westerly_.

_Proposition 2._—Whenever the pressure at Nairn is greater than at Brest, while its values at Valentia and Yarmouth are equal, or nearly so, the winds over the British Isles are _easterly_.

_Proposition 3._—Whenever the pressure at Valentia is greater than at Yarmouth, while its values at Brest and Nairn are nearly equal, the winds over the British Isles are _northerly_.

_Proposition 4._—Whenever the pressure at Yarmouth exceeds that at Valentia, while there is equality of pressure at Nairn and Brest, the winds of the British Isles are _southerly_.

_Proposition 5._—Whenever the pressure of the atmosphere is equal, or nearly so, at Brest, Valentia, Nairn, and Yarmouth, and generally uniform, the winds over the British Isles are variable in direction and light in force.

The data from which the foregoing propositions were deduced, and indeed all other cases calculated by Mr. Strachan, show in every well marked instance that when the atmospherical pressure was

(1) greater in the south than in the north, the wind had westing;

(2) greater in the north than in the south, the wind had easting;

(3) greater in the east than in the west, the wind had southing;

(4) greater in the west than in the east, the wind had northing;

(5) uniformly high, or uniformly low, variable light winds (with fine weather in the former case, and vapoury or wet weather in the latter).

Conditions (1) and (3) give winds from the S.W. quarter.

Conditions (1) and (4) give winds from the N.W. quarter.

Conditions (2) and (4) give winds from the N.E. quarter.

Conditions (2) and (3) give winds from the S.E. quarter.

These principles may be employed to set forth the mode of foretelling the impending change of wind as regards its direction and force; for the atmospherical pressure may change—

(_a_) uniformly over the whole area of observation;

(_b_) by increasing in the south, or (which causes a similar statical force) by decreasing in the north;

(_c_) by increasing in the north, or (which has the same effect) by decreasing in the south;

(_d_) by increasing in the west, or (which has the same effect) by decreasing in the east;

(_e_) by increasing in the east, or (which has the same effect) by decreasing in the west;

Scale, 0 to 6. | |Pressure in pounds | per square foot. | | | |Miles per hour. | | | | | |Seaman’s Nomenclature. | | | | | | | |Scale, 0 to 12. | | | | | | | | | | Beaufort Scale. ---+-----+---+---------+----+------------------------------------------- 0·0| 0·00| 2 |Calm | 0 | | | | | | 0·5| 0·25| 5 |Light Air| 1 | Just sufficient to make steerage way. | | | | | | | | Breeze | | 1·0| 1·00|10 | Light | 2 | ⎧With which a ship with ⎫ 1 to 2 knots. 1·5| 2·25|15 | Gentle | 3 | ⎨ all sail set would go ⎬ 3 to 4 „ 2·0| 4·00|20 | Moderate| 4 | ⎩ in smooth water. ⎭ 5 to 6 „ 2·5| 6·25|27 | Fresh | 5 | ⎧ ⎫ Royals, &c. 3·0| 9·00|35 | Strong | 6 | ⎪In which ⎮ Single Reefs and T.G. Sails. -- | -- |42 | -- -- | | ⎪ ⎮ Double Reefs and Jib, &c. | | | | | ⎪ ⎮ | | | Gale | | ⎨ she could ⎬ 3·5| -- |50 |Moderate | 7 | ⎪ ⎮ 4·0|16·00|60 |Fresh | 8 | ⎪ ⎮ Triple Reefs, &c. 4·5|20·25|-- |Strong | 9 | ⎩ just carry⎭ Close Reefs and Courses. 5·0|25·00|70 |Whole | 10 | ⎧In which she could just bear close-reefed | | | | | ⎩ Maintopsail and reefed Foresail. 5·5|30·25|80 |Storm | 11 | Under Storm Staysails or Trysails. 6·0|36·00|90 |Hurricane| 12 | Bare Poles. ---+-----+---+---------+----+-------------------------------------------

With (_a_) similar wind and weather will continue. „ (_b_) winds will veer towards west. „ (_c_) „ „ east. „ (_d_) „ „ north. „ (_e_) „ „ south.

“The probable strength of wind will be in proportion to the rate of increase of statical force, or differences of barometrical readings. The position of least pressure must be carefully considered; as, in accordance with the law, the wind will blow around that locality. The same remark applies to areas of high pressure, which, however, very rarely occur in a well-defined manner over the British Isles.”

Referring to the table on page 76, the scale 0 to 6 was formerly used by meteorological observers at land stations, and it was intended to express, when the square of the grade was obtained, the pressure of the wind as given in the second column.

“The velocity is an approximation as near as can be obtained, from the values assigned by Neumayer, Stow, Laughton, Scott, Harris, James, &c.”[15]

[Footnote 15: Strachan’s “Portable Meteorological Register,” 4th edition.]

Few meteorological axioms are better established than that which embodies the fact that “every wind brings its weather,” and the primary cause of wind being the motion of the air induced by rarefaction, it is obvious that there is a constant tendency for the equatorial and polar currents in any locality to establish an equilibrium, and this consideration is found to facilitate weather predictions for extended periods. Thus, in consequence of the unusual prevalence of _east_ winds in the spring of 1862, a wet summer was predicted. The prediction was fully borne out by an incessant continuance of _south-west winds_, with clouded skies and the usual accompaniment of deluges of rain. These winds continuing, with slight intermissions only, till the spring of the following year, less than the usual number of south-west winds was looked for during the summer; the result fully justified the anticipation, the summer of 1863 being fine and warm, especially during the earlier portion. Similarly, without committing the inaccuracies of Murphy in 1838, the summer of 1877 may be reasonably expected to be a dry and cool one from the long continuance of warm and wet months in the winter of 1876-7.

The scientific research and mechanical ingenuity directed of late years to producing trustworthy estimates of the direction, pressure, and velocity of the wind, have resulted in the production of a series of instruments, possessing great precision and accuracy.

The _direction_ of the wind is indicated by vanes, a very efficient form of which is shown at Fig. 54, the _velocity_ by revolving cups, and the _pressure_ by the pressure plate and by calculation from the known velocity.

The Pendulum Anemometer (Fig. 56) shows in a simple manner the direction and pressure of the wind. The peculiarly shaped vane ensures the surface of the swinging pressure plate B being always kept towards the wind. The pendulum plate hangs, during a calm, quite vertically, indicating zero, and as the pressure increases it will be raised through all degrees of elevation from 1 to 12. The vane is perforated with holes large enough to be visible at some distance from the ground, the 5 and 10 being specially larger, so that the angle to which the pressure plate is raised can be quickly noted.

There is a simple contrivance (for the convenience of travellers) called a Portable Wind Vane, or Anemometer, It is furnished with a compass and bar needle, &c., and will tell the true direction of the wind to within a half point.

Lind’s Anemometer or Wind Gauge ranks among the earliest forms of instruments designed to estimate the force of the wind. It consists of a glass syphon, the limbs of which are parallel to each other, mounted on a vertical rod, on which it freely oscillates by the action of the vane which surmounts it. The upper end of one limb of the syphon is bent outward at right angles to the main direction, and the action of the vane keeps this open end of the tube always towards the quarter from whence the wind blows. Between the limbs of the syphon is placed a scale graduated from 0 to 3 in inches and 10ths, the zero being in the centre of the scale. When the instrument is used, it is only necessary to fill the tube with water to the zero of the scale, and then expose it to the wind. The natural consequence of wind acting on the surface of the water is to depress it in one limb and raise it in the other, and the sum of the depression and elevation is the height of a column of water which the wind is capable of sustaining at the time of observation. Sudden gusts of wind are apt to produce a jumping effect on the water in the tube, and to diminish this the bend of the syphon is contracted. A brass plate is attached to the foot of the instrument, bearing the letters indicating the cardinal points of the compass, to show the direction of the wind.