Part 12

The mercurial barometer is a very delicate instrument and when once placed in the desired position should not be moved. Care should be taken that the room in which the barometer is placed is of nearly uniform temperature, for if the temperature at the top of the barometer is different than the temperature at the bottom, of course there will be an effect produced on the changes in the mercury column. All other barometers are set by the mercurial.

THE ANEROID BAROMETER (Fig. 42).

The aneroid barometer is so constructed that it contains no liquid whatever, and thus derives its name from the Greek compound word “aneroid,” meaning “without fluid.”

The essential parts of the instrument are a metallic case from which the air has been exhausted, and which contains a spring. The case of elastic metal is fastened to a base plate at the bottom and to the spring at the top. The pressure of the atmosphere causes the case to expand and contract, thus affecting the spring, which is connected to a needle or dial, causing the dial to move around on the scale on the face of the instrument and record the changes. The scale is marked off in inches from 28 to 31, and besides a brass hand or pointer, used to designate the changes in the atmospheric pressure, there is a small index hand to set over the needle so that the amount of change in a certain period is easily known on consulting the instrument.

Courtesy Julien Friez & Sons, Baltimore, Md.

Fig. 45 ]

The dial of the barometer is marked with the words “Fair,” “Change,” and “Rain,” etc., but these words have no significance, and should be disregarded. For instance, 29½ is marked “Change”; 30, “Fair”; 31, “Very dry”; 28½, “Rain.” If the barometer, which has been standing at 30.9, suddenly drops down to 29.9, this is positive indication that a storm is approaching, with strong winds, yet, according to the dial on the aneroid, the reading would be “Fair.” If the barometer were standing at 28 and rose to 29, this would actually indicate approach of cold, dry weather, and yet on the dial it reads “Rain.” This simply goes to show that the readings on the dial are of no significance whatsoever, and are not to be relied upon.

The aneroid is not as accurate an instrument as the mercurial, so should be checked up occasionally with the mercurial barometer.

Courtesy Taylor Instrument Companies Rochester, N. Y.

Fig. 46 ]

The aneroid type of barometer is also used in altitude work, but must be compensated before using.

This type of barometer possesses several advantages over the mercurial in that it is portable and therefore used for altitude work; at sea it is used because there is no fluid to become unsettled by the motion of the vessel; it is used also in observatory work because the action is quicker than the mercurial barometer action, and sudden changes likely to occur are indicated.

INDICATIONS FROM THE BAROMETER

A single observation reading of the barometer is of no significance. Readings must be taken at different intervals or the results will be misleading. The important thing about the barometer is to watch the rise and fall, particularly, whether it is gradual or rapid. From no single reading can you make an observation or a forecast. A rapid rise indicates that a strong wind is apt to blow. A rapid fall indicates that the weather will be unsettled, and that strong winds are apt to blow. Both indicate a change in the weather, depending upon many things, particularly, however, the direction from which the wind blows. If an observer stands with the wind blowing on his back, the area of low barometric pressure will be at his left, and that of high barometric pressure at his right. With low pressure in the west and high pressure in the east, the wind will be from the south; but with low pressure in the east and high pressure in the west, the wind will be from the north. The barometer rises for northerly winds, from northwest by the north to eastward, for dry, or less wet weather, for less wind, or for more than one of these changes—except on a few occasions, when rain, hail, or snow comes from the northward with strong wind. The barometer falls for southerly wind, from southeast, by the south, to the westward, for wet weather, for stronger wind, or for more than one of these changes, except on a few occasions, when moderate wind with rain or snow comes from the northward.

RELATIVE HUMIDITY TABLES

Per Cent Fahrenheit Temperatures

Difference in Degrees Between Wet and Dry Bulb Thermometers

═════╤═══╤═══╤═══╤═══╤═══╤═══╤═══╤═══╤═══╤════╤════╤════╤════╤════╤════╤════╤════ Read-│1.0│2.0│3.0│4.0│5.0│6.0│7.0│8.0│9.0│10.0│11.0│12.0│13.0│14.0│15.0│16.0│17.0 ing │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ of │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Dry │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Bulb │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Ther-│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ mom- │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ eter │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────┼───┼───┼───┼───┼───┼───┼───┼───┼───┼────┼────┼────┼────┼────┼────┼────┼──── 32│ 90│ 79│ 69│ 60│ 50│ 41│ 31│ 22│ 13│ 4│ │ │ │ │ │ │ 33│ 90│ 80│ 71│ 61│ 52│ 42│ 33│ 24│ 16│ 7│ │ │ │ │ │ │ 34│ 90│ 81│ 72│ 62│ 53│ 44│ 35│ 27│ 18│ 9│ 1│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 35│ 91│ 82│ 73│ 64│ 55│ 46│ 37│ 29│ 20│ 12│ 4│ │ │ │ │ │ 36│ 91│ 82│ 73│ 65│ 56│ 48│ 39│ 31│ 23│ 14│ 6│ │ │ │ │ │ 37│ 91│ 83│ 74│ 66│ 58│ 49│ 41│ 33│ 25│ 17│ 9│ 1│ │ │ │ │ 38│ 91│ 83│ 75│ 67│ 59│ 51│ 43│ 35│ 27│ 19│ 12│ 4│ │ │ │ │ 39│ 92│ 84│ 76│ 68│ 60│ 52│ 44│ 37│ 29│ 21│ 14│ 7│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 40│ 92│ 84│ 76│ 68│ 61│ 53│ 46│ 38│ 31│ 23│ 16│ 9│ 2│ │ │ │ 41│ 92│ 84│ 77│ 69│ 62│ 54│ 47│ 40│ 33│ 26│ 18│ 11│ 5│ │ │ │ 42│ 92│ 85│ 77│ 70│ 62│ 55│ 48│ 41│ 34│ 28│ 21│ 14│ 7│ │ │ │ 43│ 92│ 85│ 78│ 70│ 63│ 56│ 49│ 43│ 36│ 29│ 23│ 16│ 9│ 3│ │ │ 44│ 93│ 85│ 78│ 71│ 64│ 57│ 51│ 44│ 37│ 31│ 24│ 18│ 12│ 5│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 45│ 93│ 86│ 79│ 71│ 65│ 58│ 52│ 45│ 39│ 33│ 26│ 20│ 14│ 8│ 2│ │ 46│ 93│ 86│ 79│ 72│ 65│ 59│ 53│ 46│ 40│ 34│ 28│ 22│ 16│ 10│ 4│ │ 47│ 93│ 86│ 79│ 73│ 66│ 60│ 54│ 47│ 41│ 35│ 29│ 23│ 17│ 12│ 6│ 1│ 48│ 93│ 87│ 80│ 73│ 67│ 60│ 54│ 48│ 42│ 36│ 31│ 25│ 19│ 14│ 8│ 3│ 49│ 93│ 87│ 80│ 74│ 67│ 61│ 55│ 49│ 43│ 37│ 32│ 26│ 21│ 15│ 10│ 5│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 50│ 93│ 87│ 81│ 74│ 68│ 62│ 56│ 50│ 44│ 39│ 33│ 28│ 22│ 17│ 12│ 7│ 2 51│ 94│ 87│ 81│ 75│ 69│ 63│ 57│ 51│ 45│ 40│ 35│ 29│ 24│ 19│ 14│ 9│ 4 52│ 94│ 88│ 81│ 75│ 69│ 63│ 58│ 52│ 46│ 41│ 36│ 30│ 25│ 20│ 15│ 10│ 6 53│ 94│ 88│ 82│ 75│ 70│ 64│ 58│ 53│ 47│ 42│ 37│ 32│ 27│ 22│ 17│ 12│ 7 54│ 94│ 88│ 82│ 76│ 70│ 65│ 59│ 54│ 48│ 43│ 38│ 33│ 28│ 23│ 18│ 14│ 9 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 55│ 94│ 88│ 82│ 76│ 71│ 65│ 60│ 55│ 49│ 44│ 39│ 34│ 29│ 25│ 20│ 15│ 11 56│ 94│ 88│ 82│ 77│ 71│ 66│ 61│ 55│ 50│ 45│ 40│ 35│ 31│ 26│ 21│ 17│ 12 57│ 94│ 88│ 83│ 77│ 72│ 66│ 61│ 56│ 51│ 46│ 41│ 36│ 32│ 27│ 23│ 18│ 14 58│ 94│ 89│ 83│ 77│ 72│ 67│ 62│ 57│ 52│ 47│ 42│ 38│ 33│ 28│ 24│ 20│ 15 59│ 94│ 89│ 83│ 78│ 73│ 68│ 63│ 58│ 53│ 48│ 43│ 39│ 34│ 30│ 25│ 21│ 17 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 60│ 94│ 89│ 84│ 78│ 73│ 68│ 63│ 58│ 53│ 49│ 44│ 40│ 35│ 31│ 27│ 22│ 18 61│ 94│ 89│ 84│ 79│ 74│ 68│ 64│ 59│ 54│ 50│ 45│ 40│ 36│ 32│ 28│ 24│ 20 62│ 94│ 89│ 84│ 79│ 74│ 69│ 64│ 60│ 55│ 50│ 46│ 41│ 37│ 33│ 29│ 25│ 21 63│ 95│ 90│ 84│ 79│ 74│ 70│ 65│ 60│ 56│ 51│ 47│ 42│ 38│ 34│ 30│ 26│ 22 64│ 95│ 90│ 85│ 79│ 75│ 70│ 66│ 61│ 56│ 52│ 48│ 43│ 39│ 35│ 31│ 27│ 23 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 65│ 95│ 90│ 85│ 80│ 75│ 70│ 66│ 62│ 57│ 53│ 48│ 44│ 40│ 36│ 32│ 28│ 25 66│ 95│ 90│ 85│ 80│ 76│ 71│ 66│ 62│ 58│ 53│ 49│ 45│ 41│ 37│ 33│ 29│ 26 67│ 95│ 90│ 85│ 80│ 76│ 71│ 67│ 62│ 58│ 54│ 50│ 46│ 42│ 38│ 34│ 30│ 27 68│ 95│ 90│ 85│ 81│ 76│ 72│ 67│ 63│ 59│ 55│ 51│ 47│ 43│ 39│ 35│ 31│ 28 69│ 95│ 90│ 86│ 81│ 77│ 72│ 68│ 64│ 59│ 55│ 51│ 47│ 44│ 40│ 36│ 32│ 29 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 70│ 95│ 90│ 86│ 81│ 77│ 72│ 68│ 64│ 60│ 56│ 52│ 48│ 44│ 40│ 37│ 33│ 30 71│ 95│ 90│ 86│ 82│ 77│ 73│ 69│ 64│ 60│ 56│ 53│ 49│ 45│ 41│ 38│ 34│ 31 72│ 95│ 91│ 86│ 82│ 78│ 73│ 69│ 65│ 61│ 57│ 53│ 49│ 46│ 42│ 39│ 35│ 32 73│ 95│ 91│ 86│ 82│ 78│ 73│ 69│ 65│ 61│ 58│ 54│ 50│ 46│ 43│ 40│ 36│ 33 74│ 95│ 91│ 86│ 82│ 78│ 74│ 70│ 66│ 62│ 58│ 54│ 51│ 47│ 44│ 40│ 37│ 34 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 75│ 96│ 91│ 87│ 82│ 78│ 74│ 70│ 66│ 63│ 59│ 55│ 51│ 48│ 44│ 41│ 38│ 34 76│ 96│ 91│ 87│ 83│ 78│ 74│ 70│ 67│ 63│ 59│ 55│ 52│ 48│ 45│ 42│ 38│ 35 77│ 96│ 91│ 87│ 83│ 79│ 75│ 71│ 67│ 63│ 60│ 56│ 52│ 49│ 46│ 42│ 39│ 36 78│ 96│ 91│ 87│ 83│ 79│ 75│ 71│ 67│ 64│ 60│ 57│ 53│ 50│ 46│ 43│ 40│ 37 79│ 96│ 91│ 87│ 83│ 79│ 75│ 71│ 68│ 64│ 60│ 57│ 54│ 50│ 47│ 44│ 41│ 37 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 80│ 96│ 91│ 87│ 83│ 79│ 76│ 72│ 68│ 64│ 61│ 57│ 54│ 51│ 47│ 44│ 41│ 38 82│ 96│ 92│ 88│ 84│ 80│ 76│ 72│ 69│ 65│ 62│ 58│ 55│ 52│ 49│ 46│ 43│ 40 84│ 96│ 92│ 88│ 84│ 80│ 77│ 73│ 70│ 66│ 63│ 59│ 56│ 53│ 50│ 47│ 44│ 41 86│ 96│ 92│ 88│ 85│ 81│ 77│ 74│ 70│ 67│ 63│ 60│ 57│ 54│ 51│ 48│ 45│ 42 88│ 96│ 92│ 88│ 85│ 81│ 78│ 74│ 71│ 67│ 64│ 61│ 58│ 55│ 52│ 49│ 46│ 43 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 90│ 96│ 92│ 89│ 85│ 81│ 78│ 75│ 71│ 68│ 65│ 62│ 59│ 56│ 53│ 50│ 47│ 44 92│ 96│ 92│ 89│ 85│ 82│ 78│ 75│ 72│ 69│ 65│ 62│ 59│ 57│ 54│ 51│ 48│ 45 94│ 96│ 93│ 89│ 86│ 82│ 79│ 75│ 72│ 69│ 66│ 63│ 60│ 57│ 54│ 52│ 49│ 46 96│ 96│ 93│ 89│ 86│ 82│ 79│ 76│ 73│ 70│ 67│ 64│ 61│ 58│ 55│ 53│ 50│ 47 98│ 96│ 93│ 89│ 86│ 83│ 79│ 76│ 73│ 70│ 67│ 64│ 61│ 59│ 56│ 53│ 51│ 48 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 100│ 96│ 93│ 90│ 86│ 83│ 80│ 77│ 74│ 71│ 68│ 65│ 62│ 59│ 57│ 54│ 52│ 49 ─────┴───┴───┴───┴───┴───┴───┴───┴───┴───┴────┴────┴────┴────┴────┴────┴────┴────

Courtesy Taylor Instrument Companies, Rochester, N.Y.

Fig. 47

Like the hygrometer, this instrument measures the “relative humidity.” ]

Fig. 48

GILBERT HYGROMETER ]

GENERAL BAROMETER INDICATIONS

A gradual but steady rise indicates settled fair weather.

A gradual but steady fall indicates unsettled or wet weather.

A very slow rise from a low point is usually associated with high winds and dry weather.

A rapid rise indicates clear weather with high winds.