GENERAL BAROMETER INDICATIONS

Courtesy Taylor Instrument Companies, Rochester, N.Y.Fig.47Like the hygrometer, this instrument measures the “relative humidity.”

Fig. 48GILBERT HYGROMETER

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:

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

Courtesy Julien Friez & Son Baltimore, Md.Fig. 49U. S. STANDARD RAIN GAUGE

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.

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.

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.

Fig. 50GILBERT RAIN GAUGE

Maximum and minimum thermometers are used to record the daily maximum and minimum temperatures. Fig.46shows 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.

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.

Courtesy Julien Friez & Sons, Baltimore, Md.Fig. 51TIPPING BUCKET RAIN GAUGE

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.

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.

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.

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.

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

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 thehumidity by consulting a table (see table on pages58–59), 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.49–50showing the standard Weather Bureau station instrument and the Gilbert hygrometer.

Fig.49shows the U. S. Standard Weather Bureau Station Rain Gauge, Fig.50the Gilbert Rain Gauge and Fig.51the 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.51shows 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 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-tenthof an inch of rain on the ground; if 10 inches of rain in the tube, it signifies one inch of rain on the ground. Inother words, divide the figure recorded on the measuring stick by tenfor 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.

A bucket and a spring balance are used. The bucket is filled with snow, but not packed down too hard, and weighed. The reading of the index hand on the spring balance gives the density of the snow. The depth of the snow in the vicinity of the spot from which the bucket was filled is obtained and this figure is multiplied by the density, thus giving the water equivalent of the snow collected. For instance, if the reading of the balance was .16, and the depth of the snow was 7 inches, multiply .16 by 7, and the result, 1.12, is the water equivalent of the snow.

The first thermometer scale to give satisfaction was devised in 1714 by Fahrenheit. He determined the fixed points on the thermometer in a very novel manner. Having been born at Dantzig, he took for the zero point on his scale the lowest temperature observed by him at Dantzig, which he found was that produced by mixing equal quantities of snow and sal-ammoniac. The space between this point and that to which the mercury rose at thetemperature of boiling water he divided into 212 parts. He determined, with his thermometer, that the atmospheric pressure governed the boiling point of water. Today the Fahrenheit thermometer is used extensively, and has for its freezing point 32° and for its boiling point 212°.

Another scale that has not become too well known, because of the fact that it did not meet with public favor, was devised by a Frenchman, named Reaumur, in 1730, and bears his name. He determined the freezing point of the scale at 0° and the boiling point of water at 80°.

Another Frenchman, named Anders Celsius, devised a scale with the boiling point of water at 0° and the freezing point at 100°. In 1743 a Frenchman, named Christin, living at Lyons, France, reversed the points, and today the scale is known as the Centigrade scale, and, together with the Fahrenheit scale, is used almost exclusively wherever thermometers are required.

Centigrade degrees into Fahrenheit: multiply by 9, divide the product by 5 and add 32.

Fahrenheit degrees into Centigrade: subtract 32, multiply by 5, and divide by 9.

Reaumur degrees into Fahrenheit: multiply by 9, divide by 4, and add 32.

Fahrenheit degrees into Reaumur: subtract 32, multiply by 4, and divide by 9.

Reaumur degrees into Centigrade: multiply by 5 and divide by 4.

Centigrade degrees into Reaumur: multiply by 4 and divide by 5.

The following is a list of the Weather Bureau Stations of the United States, and from any of these offices, preferably the one nearest you, you will be able to obtain the weather reports and weather map (see Fig.52), indicating many things of interest, and from which you will be able to make a careful study of the weather.

You will notice that on this map different lines are drawn: First, the Isobar lines—these are solid lines drawn through places which have the same barometric pressure. Second, the Isotherm lines—these are dotted lines drawn through places having the same temperature.

The Weather Bureau Maps are gotten out on the same day all over the country, and the preparation of them is quite interesting.

At 7:40 A. M. simultaneous readings are taken at all weather bureau stations of the country. On the coast, where the time is three hours different than at New York, the readings are taken at 4:40, so that the hour corresponds at all places. At 8:00 A. M. the various stations telephone their findings to the Western Union Office located in their city and immediately the messages are transmitted by Western Union to a central district office, or circuit center as it is called. For New England, the circuit center is Boston. All messages are received at this office, and from here transmitted to the next office, which is New York, and from New York to the next center, until the news is transmitted to the coast. The wires are open from 8:00 until 9:30 A. M. The western offices follow the same procedure until the weather indications are received by all stations. Immediately the preparation of the map is begun and they are mailed to interested parties by the Weather Bureau Stations of the United States.

Figs.52,53and54show three maps, typifying storms traveling from the west to the east, and by studying them on successive days you can at once grasp the importance of studying the weather from these maps.

Fig.53shows a storm of low pressure and how this area of low pressure is progressing and moving from the west to the east. Particular notice should be taken of how fast the storm travels, that is, the distance it goes each day, and the direction it is going and the results.

Fig. 52

Fig. 53

Fig. 54

The arrows denote the direction of the wind, and you will notice they point to the region of low barometric pressure. In the regions of high barometric pressure the winds are in the opposite direction. This readily explains to you why it is that you can expect changes in weather conditions when the wind changes.

From the markings and printed matter on each map, information is secured regarding observations of the barometer, thermometer, wind velocity, direction of the wind, kind of clouds, and their movements, and the amount of precipitation (rain or snow), in different localities.

Clear, partly cloudy, cloudy, rain or snow indications are symbolized. The shaded area designates places or areas where precipitation has occurred during the preceding twelve hours.

Low barometric pressure, or the storm centers, are indicated on the map by the word “low.” High barometric pressure centers are indicated by the word “high.” Note how they move in an easterly direction; how they are progressive. They can be compared to a series of waves, which we will call atmospheric waves. The crest of the wave may be likened to the “highs” and the troughs to the “lows.”

Usually the winds are southerly or easterly and therefore warmer in advance of a “low.” When the “lows” progress east of a place, the wind generally shifts to westerly and the temperature lowers. The westward advance of the “lows” is preceded by precipitation, and almost always in the form of rain or snow, following which the weather is generally clear. Note how a “low” is followed by a “high,” and so on as they move along eastwardly.

If the Isotherms run nearly parallel, that is, east and west, there will most likely be no change in the temperature. Southerly to east winds prevail west of the nearly north and south line, passing through the middle of a “high” and also east of a like line passing through the middle of a “low.”

To the west of a nearly north and south line passing through the middle of a “low,” northerly to westerly winds prevail. We will find the same condition prevailing to the east of a line passing through the center of a “high.”

Fig. 55

When we find an absence of decidedly energetic “lows” and “highs,” this is an indication of the continuance of existing weather. We can expect this state of the atmosphere until later maps show a beginning of a change, usually first appearing in the west.

The storms of the United States follow, however, year after year, a series of tracks, not likely to change suddenly, and not irregular, but related to each other by very well-defined laws.

The United States Weather Bureau has made a very intensive study of the positions of the tracks of the storms. Fig.55shows the mean tracks and the movement of storms from day to day. This map indicates that generally there are two sets of lines running west and east, one set over the northwestern boundary, the Lake region, and the St. Lawrence Valley, the other set over the middle Rocky Mountain districts and the Gulf States. Each of these is double, with one for the “highs” and one for the “lows.” Furthermore, there are lines crossing from the main tracks to join them together, showing how storms pass from one to the other. On the chart, the heavy lines all belong to the tracks of the “highs,” and the lighter lines to the track of the “lows.”

A “high” reaching the California coast may cross the mountains near Salt Lake City (follow the track on the map), and then pass directly over the belt of the Gulf States, turning northeastward and reaching the Virginia coast; or it may move farther northward, cross the Rocky Mountains in the State of Washington, up the Columbia River Valley, then turn east, and finally reach the Gulf of St. Lawrence. These tracks are located where they are by the laws of general circulation of the atmosphere and the outline of the North American continent. This movement of the “highs” from the middle Pacific coast to Florida or to the Gulf of St. Lawrence is confined to the summer half of the year, that is, from April to September. In the winter months, on the other hand, the source of the “highs” is different, though they reach the same terminals.

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