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Part 9

Weather Warnings for Watchers · Anonymous — chapter 9 of 20 · ~2,112 words · public domain

Since the more scientific “Pediment” has attained so high a degree of popularity, a certain amount of unmerited obloquy has attached itself to the Dial or Wheel Barometer invented by Dr. Hooke. It must be conceded that the standard form of pediment barometers in which the height of the mercury is seen at a glance is more strictly an “instrument of precision,” but it should not be forgotten, although a delicate mechanism intervenes between the mercury and the observer, it is so arranged that a tenth of an inch rise or fall causes a movement of the index over an inch of space.

The Aneroid Barometer indicates variations in atmospheric pressure by the elevation and depression of the sides of an elastic metallic box from which the air is exhausted and which is kept from complete collapse by a powerful spring. In cases where extreme accuracy is not indispensable, the portability and sensibility of this instrument recommend it for use by tourists and fishermen. It is “quick in showing the variations of atmospheric pressure.” “The Aneroid readings may be safely depended upon.” “Its movements are always consistent.” “Atmospheric changes are indicated first by the Aneroid.” It is especially adapted for determining mountain altitudes, some being furnished with a scale of feet, enabling the observer to read off the height by direct observation, and if adjusted once a year by comparison with a mercurial standard is quite trustworthy. It is fully described in a small pamphlet entitled “The Aneroid Barometer: How to Buy, and How to Use it,” by a Fellow of the Meteorological Society.

By a suitable arrangement of clockwork, revolving a cylinder bearing prepared paper, the aneroid barometer forms an admirable self-recording instrument, showing at a glance the height of the barometer: whether it is falling or rising, for how long it has been doing so, and at what rate the change is taking place, whether at the rate of 1/10th per hour, or 1/10th in twenty-four hours—facts which can only be obtained by very frequent and regular observations from an ordinary barometer, but which are nevertheless essential to a reliable “weather forecast.”

The height of mountains may also be determined by the temperature at which water boils, as this depends on the pressure of the atmosphere, and according to Deschanel, “just as we can determine the boiling-point of water when the external pressure is given, so if the boiling-point be known we can determine the external pressure,” and as this varies with the elevation above sea-level, the boiling-point of water also varies.

These facts induced Wollaston to attempt the determination of heights of mountains by an apparatus which he called the Barometric Thermometer, subsequently modified by Regnault and called a Hypsometer, but now more generally known as a Boiling-point Thermometer.

A portable form of boiling-point thermometer is shown at Fig. 33, which is much used by Alpine travellers, and forms a trustworthy check on the aneroid and barometer.

CONCISE TABLES FOR CALCULATING HEIGHTS BY MEANS OF BAROMETER OR ANEROID, AND ALSO BY THE BOILING-POINT THERMOMETER.

+-----------+--------+---------------------------------------+ |Boiling- |Bartr. | | |point of |at lower| BAROMETER AT UPPER STATION.--INCHES. | |Water for |Station.| | |pressure in| +----+----+----+----+----+----+----+----+ |next col. | In. | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | +-----------+--------+----+----+----+----+----+----+----+----+ | 213·78 | 31 | 998|1020|1043|1068|1095|1124|1155|1188| | 212·13 | 30 |1015|1038|1062|1087|1115|1144|1176|1210| | 210·43 | 29 |1033|1056|1081|1107|1135|1165|1198|1233| | 208·67 | 28 |1051|1075|1100|1127|1156|1187|1220|1257| | 206·87 | 27 |1073|1097|1122|1150|1180|1211|1246|1283| | 205·01 | 26 |1093|1118|1145|1173|1203|1236|1271|1309| | 203·09 | 25 |1116|1141|1169|1198|1229|1262|1299|1338| | 201·11 | 24 |1140|1166|1194|1224|1256|1290|1327|1367| | 199·05 | 23 |1164|1191|1220|1251|1284|1319|1358|1399| | 196·92 | 22 |.... 1218|1248|1280|1314|1350|1390|1433| | 194·71 | 21 |.... .... 1278|1310|1346|1383|1424|1469| | 192·41 | 20 |.... .... .... 1343|1380|1419|1461|1507| | 190·00 | 19 |.... .... .... .... 1416|1457|1500|1548| | 187·50 | 18 |.... .... Factor A. .... 1497|1542|1592| | 184·87 | 17 |.... .... .... .... .... .... 1588|1639| | 182·10 | 16 |.... .... .... .... .... .... .... 1690| | 179·20 | 15 |.... .... .... .... .... .... .... ....| +-----------+--------+---------------------------------------+

+--------------------------------------+ | Height | D. | | | | in 1,000 |additive.|Latitude.| C. +---------------------+ | feet. | | | | Mean | Factor | |----------+---------+---------+-------|Temperature.| B. | | 2 | 5 | 0° | †2·7 |------------+--------+ | 4 | 11 | 10 | †2·5 | 10° | 0·951 | | 6 | 17 | 20 | †2·0 | 20 | 0·973 | | 8 | 23 | 30 | †1·4 | 30 | 0·996 | | 10 | 30 | 40 | †0·6 | 40 | 1·018 | | 12 | 37 | 45 | 0·0 | 50 | 1·040 | | 14 | 44 | 50 | -0·5 | 60 | 1·062 | | 16 | 52 | 60 | -1·3 | 70 | 1·084 | | 18 | 60 | 70 | -2·0 | 80 | 1·127 | +----------+---------+---------+-------+------------+--------+

RULE I.—If the temperature of boiling water be observed at either or both Stations, find the equivalent pressure in the 2nd column, and calculate the height as for barometer.

RULE II.—The readings of the Barometer being corrected and reduced to 32° F., multiply the difference of pressure between the Stations by factor A, found in line with pressure at lower Station, and under that at upper Station; multiply again by factor B, corresponding to the mean temperature of the air at the Station; apply as many times C as there are thousand feet in the height, corresponding to the latitude; and add D, the correction for gravity.

EXAMPLE.—At the top of Snowdon, lat. 53° N., an aneroid read 26·48, correction -0·18, the pressure at sea-level was 29·91; the temperature of the intermediate air was 57°; find the height.

Lower Station 29·91 inches. Upper „ 26·30 ----- 3·61 Factor A 933 ----- 1083 1083 3249 ----- 3368 (neglecting decimals.) Factor B 1.055 ----- 16840 N.B.—In taking out the quantities, if accuracy 16840 is aimed at, it will be necessary to 3368 proportion for parts in the usual manner ----- with such Tables. 3553 Cor. C = 3 × 1 = -3 Cor. D +10 ---- Height 3560 feet.

The illustration (Fig. 33) shows the instrument with the telescopic tube drawn out for use, and the thermometer surrounded by the vapour of boiling water. The lamp is protected from wind by a perforated japanned tin case covered with wire gauze. When the boiler is charged and the lamp ignited the mercury ascends, and the point at which it becomes stationary shows the temperature, which will give the elevation in feet above the sea-level on reference to the table supplied by the optician from whom the instrument is purchased.

A highly-refined automatic arrangement is adopted at some observatories called a Barograph, which, by the aid of photography, becomes a self-recording mercurial barometer. It is simpler in its arrangement than the thermograph, and includes a clock of superior construction, causing a cylinder bearing photographic paper to make one complete revolution in forty-eight hours. A double combination of achromatic lenses brings to a focus rays passing through a slit placed in front of the mercurial column, behind which is a strong gaslight or paraffin lamp, the rays of which are condensed upon the slit by a combination of two plano-convex lenses.

Although a barometer is an instrument artificially constructed by man, it should not be forgotten that when once made the column of mercury is placed in a passive or quiescent state in direct relation with the great forces of nature, so that its indications become to some extent natural phenomena. This is aptly illustrated by what is called the “daily fluctuation” of the barometer which occurs in all countries, though the hours and extent vary with the latitude, diminishing as the latitude increases, according to a definite law. The phenomena does not admit of a satisfactory explanation, but is doubtless connected with the daily variations of temperature and of vapour in the air. The mercury falls naturally (so to speak) from nine or ten to between three and four p.m.; it then rises till between nine and ten p.m. It falls again about four a.m., and rises again about ten a.m. It is usually highest at nine a.m. and nine p.m., and lowest at three a.m. and three p.m.

These natural elevations and depressions of the mercury should be allowed for in reading the barometer, as any rise or fall in opposition to the natural rise and fall possesses for that reason increased importance. For instance, fine weather may be expected if the mercury rises between nine a.m. and three p.m.; in like manner rain may be expected should a fall take place between three p.m. and nine p.m.

It will be inferred from the preceding facts that there are certain hours better suited for “taking a reading” than others. When one observation only is made daily, noon is the best time, two observations should be made at nine a.m. and nine p.m., and for three the best hours are nine a.m. (maximum), noon (mean), and three p.m. (minimum).

The opinion generally entertained that a high barometer is an indication of fine weather, and a low one a warning of bad weather, is open to exception, and an increased value would attach to the indications of the instrument in proportion as the following points are noted and allowed for:—

1. The actual height of the mercury. 2. Whether it is rising or falling. 3. The rate of rise and fall. 4. Whether the rise or fall has been long continued.

The state of the barometer foretells coming weather, and when the present weather disagrees with the barometer a change will soon take place. A fall of half a tenth, or more, in an hour is a sure warning of a storm, a rapid rise is a warning of unsettled weather.

The barometer is generally lowest with wind from the S.W., and highest with wind N.E., or with a calm. N.E. and S.W. may be called the wind’s poles, and the difference of height due to direction only from one of these bearings to another amounts to about half an inch.

BAROMETER PRECAUTIONS.

If vacuum suspected, cause mercury to strike top of tube. A clear metallic “click” indicates a good vacuum. A dull “thud” indicates air or moisture. In latter case return to optician, but if unable Incline very gently until nearly inverted, when Air if present will ascend in a bubble into the cistern. Suspend barometer in good light out of sunshine. Let no heat of fire or lamp affect it. Let no sudden changes of temperature affect it. It must hang absolutely vertically. Note temperature of attached thermometer before reading barometer. Then adjust mercury in cistern to touch ivory point. Then adjust vernier and take reading quickly. Ascertain height above sea-level according to direction.

The Storm Glass (Fig. 36) is a glass bottle, ten inches long, containing a mixture of camphor, nitre, sal-ammoniac, alcohol, and water. As “temperature affects the mixture much,” an arrangement has recently been designed in which the stem of a thermometer is immersed in the fluid, as shown at Fig. 37, thus imparting a higher value to its indications. The late Admiral Fitzroy says—

“Since 1825, we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, preferably, in the outer air, the chemical mixture in a so-called storm glass varies in character with the direction of the wind—not its force.”

The quarter from which the wind or storm is blowing is indicated by the substance adhering more closely to the bottom of the glass opposite to the point whence the wind or tempest arises.

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