Part 4

The glass mercury tube is enclosed in a brass case. About two inches from the top of the case is an opening extending down the front and back for a distance of about eight inches. On each side of this opening is a graduated scale, one side being in inches and the other graduated in centimeters. The opening is fitted with a sliding vernier scale graduated in millimeters, thus permitting the reading of changes in the height of the mercury column most accurately, as the sliding vernier may be adjusted to the level of the mercury by means of a thumb screw fitted on the side of the case. The cistern containing the mercury is of glass, with a soft leather or chamois bottom and an adjusting screw, used to raise or lower the level of the mercury, so that it just comes in contact with a small ivory point, inserted in the top of the cistern, and which is used to mark the zero of the scale. Observations of the changes in the atmospheric pressure should be taken at regular intervals, and it is necessary to adjust the height of the mercury in the cistern before each observation. This is done by bringing the ivory point in contact with the level of the mercury and then bringing the vernier scale absolutely level with the top of the column of mercury in the tube, and then take the reading.

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.

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.

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 ─────┴───┴───┴───┴───┴───┴───┴───┴───┴───┴────┴────┴────┴────┴────┴────┴────┴────

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.

A very slow fall from a high point is usually connected with wet and unpleasant weather without much wind.

The following table of the United States Weather Bureau gives a summary of the wind and barometer indications:

Barometer Reduced to Sea Level Wind Character of Weather Indicated Direction

─────────────────────────────────────────────────────────────────────── 30.10 to 30.20 and steady SW to NW Fair with slight temperature changes for 1 to 2 days 30.10 to 30.20 and rising SW to NW Fair, followed within 2 days rapidly by warmer and rain 30.10 to 30.20 and falling SW to NW Warmer, with rain in 24 to 36 slowly hours 30.10 to 30.20 and falling SW to NW Warmer, with rain in 18 to 24 rapidly hours 30.20 and above and stationary SW to NW Continued fair, with no decided temperature change 30.20 and above and falling SW to NW Slowly rising temperature and slowly fair for two days 30.10 to 30.20 and falling S to SE Rain within 24 hours slowly 30.10 to 30.20 and falling S to SE Wind increasing in force, with rapidly rain within 12 to 24 hours 30.10 to 30.20 and falling SE to NE Rain in 12 to 18 hours slowly 30.10 to 30.20 and falling SE to NE Increasing wind, with rain rapidly within 12 hours 30.10 and above and falling E to NE In summer, with light winds, slowly rain may not fall for several days. In winter, rain within 24 hours 30.10 and above and falling E to NE In summer, rain probably rapidly within 12 to 24 hours. In winter, rain or snow, with increasing wind will often set in, when the barometer begins to fall and the wind sets in from the NE 30 or below and falling slowly SE to NE Rain will continue 1 or 2 days 30 or below and falling SE to NE Rain, with high wind, followed rapidly within 24 hours by clearing and cooler 30 or below and rising slowly S to SW Clearing within a few hours and continued fair for several days 29.80 or below and falling S to E Severe storm of wind and rain rapidly or snow imminent, followed within 24 hours by clearing and colder 29.80 or below and falling E to N Severe northeast gales and rapidly heavy rain or snow, followed in winter by a cold wave 29.80 or below and rising Going to Clearing and colder rapidly W

A sudden fall indicates a sudden shower or high winds, or both.

A stationary barometer indicates a continuance of existing weather conditions. (_Note_: Tap the barometer slightly on the face. If the hands move a trifle, it indicates that there is the tendency to rise or fall, depending upon the direction of movement of the hands.)

Northeasterly winds precede storms that approach from the southwest; that is, in New England and the Middle States and the Ohio Valley. Southeasterly winds precede storms that approach from the Lake region.

THERMOMETERS

For information regarding the manufacture of thermometers, we recommend P. R. Jameson’s book, “Weather and Weather Instruments,” published by the Taylor Instrument Companies of Rochester, N. Y.

Thermometers are of great importance to us in determining weather.

LOCATION OF THERMOMETERS

1. They must be properly exposed.

2. A good circulation of air around them is necessary.

3. They must be properly protected from the rays of the sun.

_Note_: If these instructions are not carefully followed out, errors are apt to occur, and you will be misled.

For a change of wind towards northerly directions, a thermometer falls. For a change of wind toward southerly directions a thermometer rises.

MAXIMUM AND MINIMUM THERMOMETERS

Maximum and minimum thermometers are used to record the daily maximum and minimum temperatures. Fig. 46 shows a typical maximum and minimum thermometer used for giving the extremes of temperature. One side of the thermometer has a scale reading, beginning at the top, from 60° below zero to 140° above zero. This is the scale used when determining the coldest temperature reached during a day. The other side of the thermometer has a scale marked from 70° below zero, beginning at the bottom and reading up, to 130° above zero. On this side the maximum heat reached during the day is recorded. There is a small metal piece in the tubes, one on each side, and as the mercury pushes ahead or recedes, the small index is left at the lowest point reached in one tube and at the highest point reached in the other. The small metal piece is drawn back to the level of the mercury by means of a small magnet.

WHEN MAXIMUM TEMPERATURE IS REACHED

You can generally look for maximum temperature between three and four o’clock in the afternoon. At this time the sun has reached its highest altitude.

WHEN THE MINIMUM TEMPERATURE IS REACHED

This usually occurs a little while before sunrise. It is important in weather observing to make a record of the highest temperature of the day and the lowest temperature of the night. Continuous observation, as the reader will appreciate, is practically impossible for such a record.

THE THERMOMETER FOR HUMIDITY IN THE AIR

Moisture or dampness in the air, as shown by an instrument called the hygrometer, increases before rain, fog, or dew.

Before describing the hygrometer, a definition of a few of the terms used in conjunction with the instrument will be found useful.

ABSOLUTE HUMIDITY

The amount of vapor actually present in the atmosphere is termed the absolute humidity, expressed usually either in the expansive force that the vapor exerts or in its weight in grains per cubic foot of air.

RELATIVE HUMIDITY

The absolute humidity divided by the amount of vapor that might exist if the air were saturated gives a ratio that is called the relative humidity.

DEW POINT

The temperature at which moisture begins to be condensed on a cold vessel or other container and becomes visible is called the dew point.

HOW HYGROMETERS ARE MADE

The most generally used hygrometer consists of two ordinary thermometers, the bulb of one being covered with a piece of muslin and kept constantly moistened with water by means of a wick or cotton thread communicating with a container of water. The difference in the readings of the two thermometers, the wet and the dry, is observed, and knowing this, it is very easy to determine the humidity by consulting a table (see table on pages 58–59), which has been prepared for this purpose. These instruments are, according to the increase in price, equipped with a table, and the container is held in a wire frame, as you will see from the Figs. 49–50 showing the standard Weather Bureau station instrument and the Gilbert hygrometer.

Fig. 49 shows the U. S. Standard Weather Bureau Station Rain Gauge, Fig. 50 the Gilbert Rain Gauge and Fig. 51 the U. S. Standard Weather Bureau Station Rain Gauge, Tipping Bucket Type.

The Gilbert Weather Station is equipped with the Tipping Bucket Type Rain Gauge. Fig. 51 shows the apparatus clearly, complete and mounted ready for use. The brass bucket seen in position through the open door is adjusted to tip for each hundredth inch of rainfall collected in the twelve-inch diameter receiver at the top, and this rainfall is electrically recorded at any convenient distance on a register. After any desired period the water may be drawn off and check measurements made by means of the brass measuring tube and graduated cedar stick shown in the figure.

THE GILBERT RAIN GAUGE (Fig. 50).

(_a_) Tube.

(_b_) Funnel.

(_c_) Measuring stick.

The essential parts of the Gilbert Rain Gauge consists of a metal tube twelve inches long, having a diameter of 1⁵⁄₁₆ inches (inside) and a funnel-shaped top, the neck of which fits snugly into the open end of the metal tube. The outside diameter of the neck of the funnel is a trifle less than 1⁵⁄₁₆ inches. The area of the circle formed at the top of the tube is one-tenth the area of the funnel circle. A measuring stick is provided to measure the rainfall collected in the tube.

To determine the amount of rainfall on the surface of the ground, the rain collected in the tube should be measured at regular intervals, usually twelve hours apart. For every inch of rain collected in the tube, as denoted by the measuring stick, it means that there is one one-tenth of an inch of rain on the ground; if 10 inches of rain in the tube, it signifies one inch of rain on the ground. In other words, divide the figure recorded on the measuring stick by ten for actual rainfall.

It is well to put some sort of a shelter around the gauge, so that it will be protected from strong winds. The shelter is usually placed at a distance from the tube equal to the height of the tube. With the Gilbert rain gauge it is well to erect the shelter at a distance of about three feet from the tube. It is essential that the gauge be held in an upright position, so it should be fastened to the roof.

Snow is measured by melting the quantity collected in the gauge and follow the same procedure as in rainfall measurements.

There is another very common method, called ground measurement. There are many instances where ground measurements are inaccurate:

1. When snow and rain are mixed or alternate.

2. When melting accompanies snowfall.

3. When snow is already upon the ground.

4. When the amount of fall is very small.

5. When drifting is very bad.

6. When the snow is blown about after the storm and before measurements have been made.