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Schematic drawing of a simple mercury barometer with vertical mercury column and reservoir at base

Goethe's device

A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

The first barometer is thought to have been built unintentionally by Gasparo Berti, sometime between 1640 and 1643.^[1] Evangelista Torricelli, who is generally credited with inventing the barometer, went on to build the first instrument with mercury in 1643^[2].

This concept of "decreasing atmospheric pressure predicts stormy weather" was invented by Lucien Vidie and 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 barometer" (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 lower 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 higher, the water level in the spout will drop below the water level in the body. A variation of this type of barometer can be easily made at home.^[3]

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 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 force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury the scale for reading the height of the mercury is adjusted to compensate for this effect.

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^[2] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and 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. In addition, the mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.

A barograph, which records a graph of some atmospheric pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil or to move a pen upon paper, both of which are attached to a drum moved by clockwork.^[4]

Barographs may be calibrated for altitude and this type is often used to preserve a record of baloon and glider flights.

See also: Surface weather analysis

See also: Weather forecasting

Barograph using five stacked aneroid barometer cells.

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. When used in combination with wind observations, reasonably accurate short term forecasts can be made.^[5] Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map showing 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 deteriorating weather or some form of precipitation will fall, however if the barometer is rising then there will be nice weather or no precipitation.

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. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use seldom go to the trouble.

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.

Table of Pneumaticks, 1728 Cyclopaedia

US patent 2194624, "Diaphragm pressure gauge having temperature compensating means", granted 1940-03-26, assigned to Bendix Aviat Corp 

• U.S. Patent 2,472,735  : C. J. Ulrich : "Barometric instrument"
• U.S. Patent 2,691,305  : H. J. Frank : Barometric altimeter"
• U.S. Patent 3,273,398  : D. C. W. T. Sharp : "Aneroid barometer"
• U.S. Patent 3,397,578  : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
• U.S. Patent 3,643,510  : F. Lissau : "Fluid displacement pressure gauges"
• U.S. Patent 4,106,342  : O. S. Sormunen : "Pressure measuring instrument"
• U.S. Patent 4,238,958  : H. Dostmann : "Barometer"
• U.S. Patent 4,327,583  : T. Fijimoto : "Weather forecasting device"

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Barometer

1. ^ Drake, Stillman (1970). "Berti, Gasparo". Dictionary of Scientific Biography 2. New York: Charles Scribner's Sons. 83-84. ISBN 0684101149. 
2. ^ ^{a b} The Invention of the Barometer
3. ^ JetStream. Learning Lesson: Measure the Pressure - The "Wet" Barometer. Retrieved on 2007-05-05.
4. ^ Glossary of Meteorology. Barograph. Retrieved on 2007-05-05.
5. ^ USA Today. Using winds and a barometer to make forecasts. Retrieved on 2007-05-05.

• Middleton, W.E. Knowles. (1964). The history of the barometer. Baltimore: Johns Hopkins Press. New edition (2002), ISBN 0801871549.

• Barograph
• Microbarometer
• Surface weather analysis
• Weather forecasting
• storm glass
• Robert FitzRoy's Meteorology

Look up Barometer in
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• Various Barometer Designs