The aneroid barometer was invented about 1843. The name aneroid simply means the movement is without fluid. Mercury barometers preceded the aneroids. In those days barometers were quite long and the scales measured in inches of mercury. The system stayed with the more modern aneroid barometers. Readings on a barometer are frequently expressed in inches of mercury. The words “change", “fair" on the dial of the barometer are relative. Readings of 30.0 inches or higher suggest dry settled weather. Readings below 29.0 may suggest more unsettled weather.
Barometers have an indicating hand that responds to changes in atmospheric pressure, and a set hand that is manually set to measure change in the indicating hand.
Some barometers are marked “compensated," which means that the accuracy of the instrument is not influenced by changes in temperature.
Barometric readings taken literally do not represent today’s weather, but weather that will come in about a day.
Barometers function equally well inside and outside of the house. The atmospheric pressure is the same throughout. Many barometers have an altitude adjustment on the back. People who live in especially high altitudes may find that their barometers have “maxed" out and are unable to register significant atmospheric change.
In order to forecast the weather the weather bureau and the meteorological departments of various countries have built extensive organizations. The functions of these organizations have been determined by international conferences. Every discovery and improvement that is made in the science of meteorology is examined by all interested countries, so anxious are all engaged in the work to get better results
Weather forecasts 100% correct are not yet possible. That they will reach this perfection there is no doubt; nor is there any doubt that long distance weather forecasts, issued three to six months ahead will come.
In the present highly specialized science of meteorology many instruments are employed. First among them is the barometer. Barometer readings are the basis of all weather forecasts. Observations of wind, temperature, sunshine and precipitation are of secondary importance.
We don’t suggest that installing a barometer will enable you to compete with the weather bureau in issuing weather forecasts. The meteorologist operating a weather bureau issues a forecast that will cover his entire district. He receives observations from all parts, so he knows just how the different weather systems are interacting. In Washington DC, at the Weather Bureau they receive weather observations from all districts and can thus issue forecasts for the entire country.
The individuals with their own barometers can not forecast for a large area. There is no necessity for it. All they want to know is the weather prospect for their own locality. They can determine this with a barometer.
The barometer was invented some centuries ago by Torricelli. Its earliest form was a glass tube closed at one end and filled with mercury, then reversed and the open end put into a small open reservoir of mercury. This style of barometer is often used today in physics classes.
Mercurial barometers of all styles have been used ever since Torricelli’s time. For testing and laboratory work mercurial barometers are desirable. (note: this was stated in 1926)
The aneroid barometer was invented about 1871. As its name suggests, it is an instrument made without fluid (mercury). Instead of using a column of mercury a vacuum chamber is employed.
This method revolutionized barometer making. A mercurial barometer must be about 36" long you it is easier to see why there are instances where the size of the instrument is a drawback to its use. The requirements of surveying and engineering work utilize barometers determine the contour, or varying elevation of the country. It would be quite a task to carry around a 36" barometer for this purpose. Hi grade aneroid barometers are made in a size suitable for carrying in the pocket.
In the case of house barometers the aneroid is preferable to the mercurial type. The latter is easily to damaged, and is prone to misalignment if not handled correctly. The house barometer should never be other than the aneroid type either indicating or recording.
SIGNIFICANCE OF FIGURES ON DIALS
As the earliest barometers were principally mercurial instruments, it was usual to express atmospheric pressure in terms of the length of the column of mercury. That practice has never varied and today all barometers aneroid and mercurial are scaled in inches of mercury, or the equivalent in the metric system.
If we say that the barometer reading is 30.15 inches high we mean that the atmospheric pressure will support a column of mercury 30.15 inches high. All aneroid barometers are calibrated with mercurial standard instruments, and their dials are figured in inches, tenths of an inch, and hundredths of an inch. Do not confuse these figures with pounds per square inch. They mean inches as measured on a mercury column.
The words rain, change and fair on the dials of aneroid barometers are simply relative. It does not follow for instance that rain is unlikely unless the barometer has been indicating rain. We do know that normally when the barometer reading is 30.0 inches or higher the coming weather is likely to be dry and settled. We know also that a barometer reading of 29.0 inches or lower signifies stormy weather. The weather words have been put at average points. For instance the word change is put at readings that are usually associated with variable or unsettled weather.
Barometers are sometimes made for people who prefer them. Most people select barometers with weather words on the dial and for them Taylor often provided lists of weather forecasts.
TAKING A READING
The indicating hand is the darkened hand responding readily to any change in atmospheric pressure which ordinarily is followed by a change in existing weather conditions.
The bright hand is the set hand and is actuated by a brass knob, usually brass that passes through the glass. This hand should be placed coincident with the indicating hand at each reading of the barometer. This will be shown by the dark or indicating hand being either to the right, or the left of the set hand. dependent on the change in atmospheric pressure. Rising to the right and falling to the left.
Barometers should not be used out of doors. They work quite well indoors, and only deteriorate when exposed to the elements. They should be kept in a place where they are not exposed to the direct rays of the sun, or to any other source of heat.
Some aneroid barometers are marked “compensated" on the dial. They are first quality instruments, and the movement is compensated in such a way that changes of temperature do not affect the accuracy of the readings.
Remember that barometers indicate coming, and not present weather conditions.
Occasionally people using weather barometers may not completely understand the effect of altitude on the readings of their instruments.
Since the pressure at the base of a column is greater than at a point half way up because of the greater weight of the mass it has to support. Air pressure will vary with each altitude and these differences must be taken into consideration when taking observations of atmospheric pressures for forecasting. Weather forecasts have to be made from barometers corrected to some common base and that base is sea level. In the case of mercurial barometers it’s not possible to do any such setting. Appropriate corrections must be applied to all mercurial barometer readings in order to correct such listings to sea level.
For every 900 feet above sea level a barometer will show a difference of approximately one inch in pressure from readings taken at sea level.
On most weather barometers the setting is done by changing the position of the hand. The hand is set forward or clockwise, 0.1" for every 90 feet above sea level.
This should be accomplished by removing the bezel, glass and hand; resetting the hand to the new point and then making any fine adjustments by means of the screw at the back of the case. For elevations up to 200’ this correction can by made by means of this screw only.
The ideal barometer is one that can be easily adjusted to register at any elevation the equivalent pressure at sea level.
Occasionally it is desirable to check the aneroid. The easiest and best way is to consult the local weather resources to obtain the actual pressure reading uncorrected for altitude.