The first part of this book may not appeal to you, if you are of a scientific trend of mind, but it is quite essential that you possess a knowledge of the fundamentals treated in the earlier pages in order to thoroughly understand the weather instruments we will now describe. These instruments are the scientific means of forecasting what the weather is going to be. They definitely indicate certain things, and from these indications you are going to be able to draw conclusions and become a scientist or meteorologist. The success that you attain will depend upon the accuracy of the instruments and the care you use in reading them. You will be able to rig up a Weather Bureau of your own, and the use of these instruments will interest anyone in a study of the weather.

THE WEATHER VANE

To make a forecast, it is essential from what we have already written, to know the direction of the wind, and to determine the direction we must have a weather vane. It is real important that the vane should be sensitive to the slightest movement of the wind and give actual wind directions. At the same time it must possess the property of steadiness, so that when it is set up it will be rigid.

Fig. 34 shows the standard weather vane used at all United States Weather Bureau Stations and Fig. 35 shows the Gilbert Weather Vane.

Fig. 35. The Gilbert weather vane consists of a metal arrow pointer and a metal rod eight inches long and five thirty-seconds of an inch in diameter. The rod is fastened by means of a few staples to the side of a pole, or whatever is to be used as a support for the vane. About three inches from the top of the rod is a collar with set screw, which is tightened, and the vane itself is then placed on the rod, the rod passing through the small angles A and B, between the sides of the vane. It will be found that the vane will swing freely on this support, and by constructing two crosspieces with letters N, S, E, and W at each end of the pieces, of course having N pointing directly north, the vane will swing around and show the direction of the wind.

The standard United States Weather Bureau type hardly needs explanation, as the illustration clearly shows all parts. It is the old, reliable, standard iron, combined wind vane and anemometer support complete, twenty feet high; iron contact box near base, improved roller bearings for six-foot vane; latter, with electrical contacts shown enlarged at the right. The vane is fastened securely to the roof of the building and held in a perfectly vertical position.

It is essential to know the velocity of the wind. This is determined by means of an instrument called the anemometer.

Fig. 36. The Standard U. S. Weather Bureau Station Anemometer.

This is the well-known standard Robinson Anemometer, now in universal use throughout the world for the registration of wind velocity, but of the latest improved construction. It records electrically the miles or kilometers, etc., of wind movements on a register. The standard pattern as furnished to Weather Bureau stations is made of brass, highly polished and finished, aluminum (or copper reinforced) cups, steel spindle with hard steel bearings, a ten-mile or kilometer indicator, electrical contacts, etc.

The four hollow hemispherical cups are mounted upon cross-arms at right angles to each other, with the open sections vertical and facing the same way around the circumference. The cross-arms are on a vertical axis, which has at its lower end an endless screw. This axis is supported so as to turn with as little friction as possible. The endless screw is in gear with a wheel which moves two dials registering the number of revolutions of the cups. The mechanisms are mounted in a suitable metal case with glass front, as shown in the illustration, well protected from the weather, the whole being designed for outdoor use.

The center of the cups moves with a velocity about one-third that of the wind which puts them in motion. The cups are four inches in diameter. The distance from center of cup to center of rotation or axis is 6.72 inches. Assuming that the wind-travel is exactly three times that of the center of the cup, the dials are marked to register miles of wind travel, five hundred revolutions of the cups corresponding to a mile.

The ratio of wind-travel to travel of cup is in reality variable, depending on the velocity of the wind. It is less for high than low velocities. It varies also with the dimensions of the instrument, being different for every different length of arm and diameter of cup.

On account of the great interference offered by buildings and other obstructions to the free movement of the wind, its velocity is much less in the vicinity of these obstructions than beyond; therefore, in selecting the location for an anemometer, preference should be given to the more elevated points in the vicinity of the station, and some rigid support should be used to raise the instrument as far as practicable above the immediate influence of the office building itself. The support must be set up so that the anemometer on top or on the cross-arm is as nearly vertical as possible.

The illustration shows clearly the appearance of an approved Weather Bureau pattern combined support for wind instruments, similar to the one installed at our plant.

Fig. 37. The Gilbert Anemometer.

The Gilbert Anemometer consists of a case containing a spindle passing through a worm gear, which turns a toothed gear. This gear in its rotary motion makes a contact with a brass brush, which is connected electrically with a flashlight. The cross-arms, with cups attached, is placed on the spindle, and as the wind blows, it revolves the cups, causing the contact. The velocity of the wind is determined by counting the flashes for fifteen seconds, thus giving you the number of miles per hour. For instance, if light flashes eight times in fifteen seconds, this signifies that the wind is blowing eight miles an hour.

Fig. 38. How to Connect the Gilbert Anemometer.

By referring to the diagram, you will see that one wire which should be the annunciator wire, or even a small electric light wire, is connected from the wire at the anemometer case directly to one side of the lamp socket. Another piece of the same size wire connects the other side of the lamp socket to one terminal of your switch. The second terminal of the switch should be connected to an outer post of one dry battery. The inner post of this same dry battery should be connected to the outer post of the second dry battery. Complete the circuit by connecting the inner post of the second dry battery to any one of the screws at the bottom of the anemometer case. The lamp used should be a small flashlight battery lamp for use on two and a half to three volts. Be sure in making the connections that the ends of your wire are scraped free from insulation and dirt. This can be done by cutting off the insulation with a knife and then rubbing the copper wire bright by a piece of sandpaper or emery cloth, or even a file. The switch should be left open when you are not taking readings, in order to prolong the life of your batteries. By unloosening the little screw in the hub of the anemometer vanes, you can remove them and also take off the brass cap on the anemometer case. This should be taken apart once or twice a month, and some machine oil used around the bearings to keep them from wearing out too quickly.

Fig. 39. The standard sunshine recorder is designed for recording the duration of sunshine electrically, continuously, and automatically, on a register. The instrument is essentially a differential air thermometer in the form of a straight glass tube with cylindrical bulbs at each end, enclosed in a protecting glass sheath, with suitable platinum wire electrodes fused in at the center, the whole mounted in a metal socket on an adjustable support.

The base is secured to the support on the roof so that the glass tube points north and south, with the blackened bulb toward the south and lowermost, then the tube is inclined at such an angle that the instrument will begin and cease to record sunshine with the proper degree of cloudiness. This inclination should be approximately 45° from the vertical. The machine should be adjusted at an hour when the sun is wholly obscured.

In temperate and cold climates, slightly different adjustments will be found necessary at different seasons of the year.

Fig. 40. The Gilbert Sunshine Recorder consists of a metal case, cylindrical in form, with a piece of metal turned up on the ends, dividing the cylinder in half. On each side of the case are small holes through which the sun casts its rays and records its movement and duration on a small piece of blue print paper inside the cylinder, one piece of paper being in each compartment. When the blue print paper is dipped in water, it becomes entirely bleached, with the exception of the path made by the sun, which shows up in a blue line.

The sunshine recorder should be set up so that the ends point directly north and south. The holes pierced in the sides of the case are nearer one end than the other. The end that the holes are nearest should be toward the south. It should be held firmly in place.

THE BAROMETER

The barometer is used for measuring the pressure of the atmosphere. The principle of this instrument was first discovered by Torricelli, a pupil of Galileo, the great Italian philosopher and scientist, in 1643. Many and various types of instruments have been made, but the two most generally used, especially where accurate indications are desired, are the mercurial and aneroid barometers. Either of these instruments are quite sensitive to changes in the weight or pressure of the earth’s atmosphere, and from their variations we are able to draw conclusions relative to changes in the weather. Figs. No. 41 and 42 illustrate the standard mercurial and aneroid barometers used most extensively today. A description of these barometers will serve to make the photographs clearer to the readers of this text.

THE MERCURIAL BAROMETER (Fig. 41)

The mercurial barometer in use today is practically the same as that invented by Torricelli. Of course, many changes have been made in the case containing the tube of mercury, adding to its attractiveness, but the principle remains the same.

The standard mercurial barometer consists of a straight glass tube about thirty-two or thirty-three inches in length, hermetically sealed at one end. The tube is of half-inch bore and is filled with chemically pure mercury, which has been boiled in the tube to insure the total exclusion of all air and moisture. After the tube has been filled, the open end is immersed in a cistern of mercury. Upon immersion the mercury drops in the tube to a height of 29.92 inches at sea level, or until counterbalanced by the weight of the surrounding atmosphere pressing upon the surface of the mercury in the cistern. The space in the top of the tube is a perfect vacuum and is called the Torricellian vacuum.

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.