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Barometer History

Aneroid Barometers

An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule(cell) is made from an alloy of beryllium and copper. The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers and other devices which are displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. A barograph, which records a graph of atmospheric pressure, uses aneroid barometer mechanisms to move a needle on paper.

Mercury Barometers

A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more downward force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the downward force placed on the reservoir.

The first barometer of this type was devised in 1643 by Evangelista Torricelli. Torricelli had set out to create an instrument to measure the weight of air, or air pressure, and to study the nature of vacuums. He used mercury, perhaps on a suggestion from Galileo Galilei, because it is significantly denser than water. To create a vacuum with water takes a column over 30 feet long, while with mercury it takes less than three feet.

Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type can usually measure atmospheric pressures in the range between 28 and 31 inches of mercury.

Liquid Barometers

Water - Based Barometers

This concept of "decreasing pressure means bad weather" is the basis for a primitive weather prediction device called a weather glass or thunder glass. It can also be called a "storm glass" or a "Goethe thermometer" (the writer Goethe popularized it in Germany).

It consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level, where it is open to the atmosphere. When the air pressure is higher than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is lower than it was at the time the body was sealed, the water level in the spout will drop below the water level in the body.

The "Thunder Glass" also acts as a thermometer: increases in temperature will raise the water level in the spout.

Applications

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced. Isobars drawn on such a map link sites with the same pressure and give, in effect, a contour map of areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. If the barometer is falling then bad weather or some form of precipitation will fall, however if the barometer is rising then there will be nice weather or no precipitation.

Compensations

Temperature

The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. No such compensation is required for an aneroid barometer.

Altitude

As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude. A barometer is a good instrument to find out whether the temperature will drop or rise.

Barograph

A barograph is a recording aneroid barometer. It produces a paper or foil chart called a barogram that records the barometric pressure over time.

Barographs use one or more aneroid cells acting through a gear or lever train to drive a recording arm that has at its extreme end either a scribe or a pen. A scribe records on smoked foil while a pen records on paper. The recording material is mounted on a cylindrical drum which is rotated slowly by clockwork. Commonly, the drum makes one revolution per day, per week, or per month and the rotation rate can often be selected by the user.

Because the amount of movement that can be generated by a single aneroid is minuscule, up to seven aneroids (so called Vidie-cans) are often stacked "in series" to amplify their motion. It was invented in 1843 by the Frenchman Lucien Vidie (1805-1866).

As atmospheric pressure responds in a predictable manner to changes in altitude, barographs may be used to record elevation changes during an aircraft flight. Barographs were required by the FAI to record certain tasks and record attempts associated with sailplanes. A continuously varying trace indicated that the sailplane had not landed during a task, while measurements from a calibrated trace could be used to establish the completion of altitude tasks or the setting of records. Examples of FAI approved sailplane barographs included the Replogle mechanical drum barograph and the EW electronic barograph (which may be used in conjunction with GPS). Mechanical barographs are not commonly used for flight documentation now, having been displaced by GNSS Flight Recorders.

Nowadays, mechanical recording barographs have commonly been superseded by electronic weather instruments that use computer methods to record the barometric pressure. These are not only less expensive than mechanical barographs but they may also offer both greater recording length and the ability to perform further data analysis on the captured data including automated use of the data to forecast the weather.

Anemometer

An anemometer is a device for measuring the velocity or the pressure of the wind, and is one instrument used in a weather station. The term is derived from the Greek anemos meaning wind.

Anemometers can be divided into two classes: those that measure the velocity of the wind, and those that measure the pressure of the wind, but as there is a close connection between the pressure and the velocity, a suitable anemometer of either class will give information about both these quantities.

The first anemometer was invented by Leone Battista Alberti in the 15th century. It was later re-invented by Englishman Robert Hooke who is often mistakenly considered the inventor of the first anemometer.