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.
History
Although
Evangelista Torricelli is universally credited with inventing the barometer in 1643, two other noteworthy efforts must be cited. Historical documentation also suggests
Gasparo Berti, an Italian mathematician and astronomer, unintentionally built a
water barometer sometime between 1640 and 1643.
French scientist and philosopher
Rene Descartes described the design of an experiment on atmospheric pressure determination as early as 1631, but there is no evidence that he built a working barometer at that time.
Types
Water-based barometers
The concept that 'decreasing atmospheric pressure predicts stormy weather' was postulated by
Lucien Vidie -- and it is the basis for a weather prediction device called a 'storm glass' or '
Goethe barometer' (who 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.
[JetStream. Retrieved on 2007-05-05.]Mercury barometers
A mercury barometer has a glass tube of at least 33
inches (about 84
cm) in height, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury actually creates a vacuum in the top of the tube. 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[ ]. 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 normally measure atmospheric pressures between 28 and 31 inches of mercury.
Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers.
Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes.
On
June 5,
2007, a
European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.
Aneroid barometers
Old aneroid barometer
Modern aneroid barometer
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 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.
Barographs
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.
Applications
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. 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.
Typically if the barometer is falling, deteriorating
weather or some form of precipitation is indicated; however, if the barometer is rising, it is likely 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. 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.
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.
Patents
Table of Pneumaticks, 1728 Cyclopaedia
- : C. J. Ulrich : "Barometric instrument"
- : H. J. Frank : Barometric altimeter"
- : D. C. W. T. Sharp : "Aneroid barometer"
- : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
- : F. Lissau : "Fluid displacement pressure gauges"
- : O. S. Sormunen : "Pressure measuring instrument"
- : H. Dostmann : "Barometer"
- : T. Fijimoto : "Weather forecasting device"
See also
