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An analog barometer

A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, pressure systems and frontal boundaries.

Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument, but used for different purposes. An altimeter is intended to be used at different levels matching the corresponding atmospheric pressure to the altitude, while a barometer is kept at the same level and measures subtle pressure changes caused by weather and elements of weather. The average atmospheric pressure on the Earth's surface varies between 940 and 1040 hPa (mbar). The average atmospheric pressure at sea level is 1013 hPa (mbar).

Etymology

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The word barometer is derived from the Ancient Greek βάρος (báros), meaning "weight", and μέτρον (métron), meaning "measure".

History

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Evangelista Torricelli is usually credited with inventing the barometer in 1643,[1][2] although the historian W. E. Knowles Middleton suggests the more likely date is 1644 (when Torricelli first reported his experiments; the 1643 date was only suggested after his death).[3]Gasparo Berti, an Italian mathematician and astronomer, also built a rudimentary water barometer sometime between 1640 and 1644, but it was not a true barometer as it was not intended to move and record variable air pressure.[1][3] French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there is no evidence that he built a working barometer at that time.[1] In 1668, Robert Hooke's marine barometer, made by Henry Hunt, was noticed, and efforts were made to make it sea-worthy.[4]

Baliani's siphon experiment

[edit]
Siphon

On 27 July 1630, Giovanni Battista Baliani wrote a letter to Galileo Galilei explaining an experiment he had made in which a siphon, led over a hill about 21 m high, failed to work. When the end of the siphon was opened in a reservoir, the water level in that limb would sink to about 10 m above the reservoir.[5] Galileo responded with an explanation of the phenomenon: he proposed that it was the power of a vacuum that held the water up, and at a certain height the amount of water simply became too much and the force could not hold any more, like a cord that can support only so much weight.[5][6][7] This was a restatement of the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle, and which Galileo restated as resistenza del vacuo.

Berti's vacuum experiment

[edit]
Gasparo Berti's experiment
Main article: Gasparo Berti § Berti's vacuum experiment

Galileo's ideas, presented in his Discorsi (Two New Sciences), reached Rome in December 1638.[8] Physicists Gasparo Berti and father Raffaello Magiotti were excited by these ideas, and decided to seek a better way to attempt to produce a vacuum other than with a siphon. Magiotti devised such an experiment. Four accounts of the experiment exist, all written some years later.[8] No exact date was given, but since Two New Sciences reached Rome in December 1638, and Berti died before January 2, 1644, science historian W. E. Knowles Middleton places the event to sometime between 1639 and 1643.[8] Present were Berti, Magiotti, Jesuit polymath Athanasius Kircher, and Jesuit physicist Niccolò Zucchi.[7]

In brief, Berti's experiment consisted of filling with water a long tube that had both ends plugged, then standing the tube in a basin of water. The bottom end of the tube was opened, and water that had been inside of it poured out into the basin. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at an exact level, which happened to be 10.3 m (34 ft),[9] the same height limit Baliani had observed in the siphon. What was most important about this experiment was that the lowering water had left a space above it in the tube which had no intermediate contact with air to fill it up. This seemed to suggest the possibility of a vacuum existing in the space above the water.[7]

Evangelista Torricelli

[edit]
Evangelista Torricelli

Evangelista Torricelli, who was Galileo's amanuensis for the last three months of his life, interpreted the results of the experiments in a novel way. He proposed that the weight of the atmosphere, not an attracting force of the vacuum, held the water in the tube. In a letter to Michelangelo Ricci in 1644 concerning the experiments, he wrote:

Many have said that a vacuum does not exist, others that it does exist in spite of the repugnance of nature and with difficulty; I know of no one who has said that it exists without difficulty and without a resistance from nature. I argued thus: If there can be found a manifest cause from which the resistance can be derived which is felt if we try to make a vacuum, it seems to me foolish to try to attribute to vacuum those operations which follow evidently from some other cause; and so by making some very easy calculations, I found that the cause assigned by me (that is, the weight of the atmosphere) ought by itself alone to offer a greater resistance than it does when we try to produce a vacuum.[10]

It was traditionally thought, especially by the Aristotelians, that the air did not have weight; that is, that the kilometers of air above the surface of the Earth did not exert any weight on the bodies below it. Even Galileo had accepted the weightlessness of air as a simple truth. Torricelli proposed that rather than an attractive force of the vacuum sucking up water, air did indeed have weight, which pushed on the water, holding up a column of it. He argued that the level that the water stayed at—c. 10.3 m above the water surface below—was reflective of the force of the air's weight pushing on the water in the basin, setting a limit for how far down the water level could sink in a tall, closed, water-filled tube. He viewed the barometer as a balance—an instrument for measurement—as opposed to merely an instrument for creating a vacuum, and since he was the first to view it this way, he is traditionally considered the inventor of the barometer, in the sense in which we now use the term.[7]

Torricelli's mercury barometer

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Torricelli's mercury in glass tube experiment

Because of rumors circulating in Torricelli's gossipy Italian neighborhood, which included that he was engaged in some form of sorcery or witchcraft, Torricelli realized he had to keep his experiment secret to avoid the risk of being arrested. He needed to use a liquid that was heavier than water, and from his previous association and suggestions by Galileo, he deduced that by using mercury, a shorter tube could be used. With mercury, which is about 14 times denser than water, a tube only 80 cm was now needed, not 10.5 m.[11] Furthermore, Torricelli demonstrated that atmospheric pressure could support a column of mercury approximately 30 inches high.[12]

Blaise Pascal

[edit]
Blaise Pascal

In 1646, Blaise Pascal along with Pierre Petit, had repeated and perfected Torricelli's experiment after hearing about it from Marin Mersenne, who himself had been shown the experiment by Torricelli toward the end of 1644. Pascal further devised an experiment to test the Aristotelian proposition that it was vapours from the liquid that filled the space in a barometer. His experiment compared water with wine, and since the latter was considered more "spiritous", the Aristotelians expected the wine to stand lower (since more vapours would mean more pushing down on the liquid column). Pascal performed the experiment publicly, inviting the Aristotelians to predict the outcome beforehand. The Aristotelians predicted the wine would stand lower. It did not.[7]

First atmospheric pressure vs. altitude experiment

[edit]
Main article: Blaise Pascal § First atmospheric pressure vs. altitude experiment
Puy de Dôme
Florin Périer measuring the mercury level in a Torricelli barometer near the top of the Puy de Dôme
Florin Périer on the Puy de Dôme

However, Pascal went even further to test the mechanical theory. If, as suspected by mechanical philosophers like Torricelli and Pascal, air had weight, the pressure would be less at higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier, who lived near a mountain called the Puy de Dôme, asking him to perform a crucial experiment. Perier was to take a barometer up the Puy de Dôme and make measurements along the way of the height of the column of mercury. He was then to compare it to measurements taken at the foot of the mountain to see if those measurements taken higher up were in fact smaller. In September 1648, Perier carefully and meticulously carried out the experiment, and found that Pascal's predictions had been correct. The column of mercury stood lower as the barometer was carried to a higher altitude.[7]

Types

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Water barometers

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Goethe's device

The concept that decreasing atmospheric pressure predicts stormy weather, postulated by Lucien Vidi, provides the theoretical basis for a weather prediction device called a "weather glass" or a "Goethe barometer" (named for Johann Wolfgang von Goethe, the renowned German writer and polymath who developed a simple but effective weather ball barometer using the principles developed by Torricelli). The French name, le baromètre Liègeois, is used by some English speakers.[13] This name reflects the origins of many early weather glasses – the glass blowers of Liège, Belgium.[13][14]

The weather ball barometer 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. The narrow spout 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.[15]

Mercury barometers

[edit]

A mercury barometer is an instrument used to measure atmospheric pressure in a certain location and has a vertical glass tube closed at the top sitting in an open mercury-filled basin at the bottom. Mercury in the tube adjusts until the weight of it 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 levels around 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. The tube has to be at least as long as the amount dipping in the mercury + head space + the maximum length of the column.

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

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.[1] He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".[16] Inspired by Torricelli, Otto von Guericke on 5 December 1660 found that air pressure was unusually low and predicted a storm, which occurred the next day.[17]

Fortin barometer

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters of mercury (mmHg). A torr was originally defined as 1 mmHg. The pressure is quoted as the level of the mercury's height in the vertical column. Typically, atmospheric pressure is measured between 26.5 inches (670 mm) and 31.5 inches (800 mm) of Hg. One atmosphere (1 atm) is equivalent to 29.92 inches (760 mm) 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.

In 2007, a European Union directive was enacted to restrict the use of mercury in new measuring instruments intended for the general public, effectively ending the production of new mercury barometers in Europe. The repair and trade of antiques (produced before late 1957) remained unrestricted.[18][19]

Fitzroy barometer

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Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes.

Fortin barometer

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Reservoir of a Fortin barometer

Fortin barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom (V in the diagram). This compensates for displacement of mercury in the column with varying pressure. To use a Fortin barometer, the level of mercury is set to zero by using the thumbscrew to make an ivory pointer (O in the diagram) just touch the surface of the mercury. The pressure is then read on the column by adjusting the vernier scale so that the mercury just touches the sightline at Z. Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport. This prevents water-hammer damage to the column in transit.

Sympiesometer

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Sympiesometer inscribed at bottom Improved sympiesometer and at top A R Easton, 53 Marischal Street, Aberdeen. Owned by descendants of the Aberdeen shipbuilding Hall family.

A sympiesometer is a compact and lightweight barometer that was widely used on ships in the early 19th century. The sensitivity of this barometer was also used to measure altitude.[20]

Sympiesometers have two parts. One is a traditional mercury thermometer that is needed to calculate the expansion or contraction of the fluid in the barometer. The other is the barometer, consisting of a J-shaped tube open at the lower end and closed at the top, with small reservoirs at both ends of the tube.

In 1778, Blondeau developed an iron tube barometer using narrow-bore musket barrels. This design resulted in a durable and polished instrument that resisted mercury corrosion and minimized breakage from the ship's movement.[4]

Marine Barometer

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The need for a practical marine barometer arose from the urgent necessity of weather prediction at sea, where sailors faced frequent, and often dangerous, changes in wind, calm, and storm conditions.[4] Traditional mercury barometers, though useful on land, proved unreliable on ships due to their susceptibility to the ship’s motion.[4] Oscillations caused the mercury to strike the top of the glass tube, leading to frequent breakage and making air pressure readings nearly impossible to interpret accurately during voyages.[4]

Roger North observed that many, including Robert Hooke, attempted to resolve these issues but often abandoned the endeavor due to technical limitations.[4] Nonetheless, Hooke remained persistent, proposing several adaptations including narrowing the open end of the siphon tube and exploring spiral tube designs.[4] His most notable contribution was the creation of a double thermometer marine barometer, also referred to as a manometer, which was presented to the Royal Society in 1668 and constructed by Henry Hunt.[4]

Hooke’s marine barometer marked a turning point in the development of nautical meteorological tools. It featured a compact, affordable design tailored for maritime use, becoming the first instrument specifically constructed for sailors.[4] The device combined a sealed spirit thermometer with an open air-based thermometer, calibrated to reflect barometric pressure changes through liquid displacement.[21][22] Hooke’s use of hydrogen-filled containers and colorful almond oil further enhanced visibility and responsiveness.[23] Notably, Edmund Halley tested this prototype on his South Atlantic voyage from 1698 to 1700 and praised its reliability in forecasting weather changes.[4][21] His endorsement led to greater interest and validation by the Royal Society.[4][21] Figure 8 below is from this report, depicting the Hooke Barometer, with detailed description in the writing.[21]

Figure 8 from Robert Hooke's Barometer Invention

Building on Hooke’s foundation, John Patrick sought to improve the design by replacing the water with mercury, advertising his version as a “new marine barometer.”[4] Though some criticized it for the difficulty of reading the mercury column due to shipboard vibrations, navigator Christopher Middleton employed it during expeditions to Hudson's Bay.[4] He consistently found it effective in forecasting storms, wind changes, and even the proximity of ice.[4]

A significant advancement occurred during Captain James Cook’s renowned voyages in the late 18th century.[4] As part of preparations for Cook’s second Pacific expedition (1772–1775), the Board of Longitude and the Royal Society commissioned the production of marine barometers.[4] Renowned instrument maker Edward Nairne was chosen to supply the equipment.[4] Contrary to expectations for spiral tubes, Nairne opted for straight, constricted tubes mounted on boards, coupled with a gimbaled suspension system to ensure vertical orientation and stability at sea.[4]

Nairne’s design represented a leap in functionality. The narrow bore significantly reduced mercury motion, enabling more accurate readings even in turbulent conditions.[4] These instruments proved so reliable that they were adopted not only by the Royal Navy but also by international expeditions.[4] The East India Company, Russian explorers, and French and Spanish navigators, including Jean-François de Galaup, comte de Lapérouse (voyage in 1785) and Alessandro Malaspina (voyage in 1789), incorporated variants of Nairne’s barometer into their voyages.[4]

Despite the widespread use of Nairne’s marine barometer, it was not without limitations.[24] Lapérouse lauded the device’s predictive capabilities but also noted inconsistencies in mercury behavior, highlighting the complexity of translating instrument readings into reliable forecasts.[24] In response to the fragility of glass tubes, other scientists, such as Le Roy, proposed alternate models like the folded Huygens barometer, designed for enhanced durability and reduced oscillation aboard ships.[24]

The marine barometer’s practical value was reaffirmed in 1801 when the Royal Society sent Captain Matthew Flinders on a three-year voyage from New Holland to New South Wales, equipped with one of Nairne’s barometers.[25] In his official correspondence, Flinders confirmed the instrument’s success and expressed appreciation for its stability and precision in recording atmospheric conditions.[25]

Throughout its evolution, the marine barometer transitioned from a theoretical invention to a critical navigational and meteorological tool. Its development not only reflected ingenuity in overcoming the challenges of shipboard instrumentation but also underscored its importance in the broader context of global exploration. These devices empowered mariners to make informed decisions, contributing to safer and more efficient voyages across the world's oceans.

Wheel barometers

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A wheel barometer uses a "J" tube sealed at the top of the longer limb. The shorter limb is open to the atmosphere, and floating on top of the mercury there is a small glass float. A fine silken thread is attached to the float which passes up over a wheel and then back down to a counterweight (usually protected in another tube). The wheel turns the point on the front of the barometer. As atmospheric pressure increases, mercury moves from the short to the long limb, the float falls, and the pointer moves. When pressure falls, the mercury moves back, lifting the float and turning the dial the other way.[26]

Around 1810 the wheel barometer, which could be read from a great distance, became the first practical and commercial instrument favoured by farmers and the educated classes in the UK. The face of the barometer was circular with a simple dial pointing to an easily readable scale: "Rain - Change - Dry" with the "Change" at the top centre of the dial. Later models added a barometric scale with finer graduations: "Stormy (28 inches of mercury), Much Rain (28.5), Rain (29), Change (29.5), Fair (30), Set fair (30.5), very dry (31)".

Natalo Aiano is recognised as one of the finest makers of wheel barometers, an early pioneer in a wave of artisanal Italian instrument and barometer makers that were encouraged to emigrate to the UK. He listed as working in Holborn, London c. 1785–1805.[27] From 1770 onwards, a large number of Italians came to England because they were accomplished glass blowers or instrument makers. By 1840 it was fair to say that the Italians dominated the industry in England.[28]

Vacuum pump oil barometer

[edit]

Using vacuum pump oil as the working fluid in a barometer has led to the creation of the new "World's Tallest Barometer" in February 2013. The barometer at Portland State University (PSU) uses doubly distilled vacuum pump oil and has a nominal height of about 12.4 m for the oil column height; expected excursions are in the range of ±0.4 m over the course of a year. Vacuum pump oil has very low vapour pressure and is available in a range of densities; the lowest density vacuum oil was chosen for the PSU barometer to maximize the oil column height.[29]

Aneroid barometers

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Aneroid barometer

An aneroid barometer is an instrument used for measuring air pressure via a method that does not involve liquid. Although Gottfried Wilhelm Leibniz first proposed the concept of an aneroid barometer around 1700, it was not until 1844 that French scientist Lucien Vidi successfully invented it.[1] The aneroid barometer uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper.[30] The evacuated capsule (or usually several capsules, stacked to add up their movements) 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 that a relative change can be seen. This type of barometer is common in homes and in recreational boats. It is also used in meteorology, mostly in barographs, and as a pressure instrument in radiosondes.

Barographs

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Main article: Barograph
Analogue recording barograph using five stacked aneroid barometer cells

A barograph is a recording aneroid barometer where the changes in atmospheric pressure are recorded on a paper chart.

The principle of the barograph is same as that of the aneroid barometer. Whereas the barometer displays the pressure on a dial, the barograph uses the small movements of the box to transmit by a system of levers to 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 using ink, held in a nib. The recording material is mounted on a cylindrical drum which is rotated slowly by a clock. Commonly, the drum makes one revolution per day, per week, or per month, and the rotation rate can often be selected by the user.

MEMS barometers

[edit]
The Galaxy Nexus has a built-in barometer

Microelectromechanical systems (or MEMS) barometers are extremely small devices between 1 and 100 micrometres in size (0.001 to 0.1 mm). They are created via photolithography or photochemical machining. Typical applications include miniaturized weather stations, electronic barometers and altimeters.[31]

A barometer can also be found in smartphones such as the Samsung Galaxy Nexus,[32] Samsung Galaxy S3-S6, Motorola Xoom, Apple iPhone 6 and newer iPhones, and Timex Expedition WS4 smartwatch, based on MEMS and piezoresistive pressure-sensing technologies.[33][34] Inclusion of barometers on smartphones was originally intended to provide a faster GPS lock.[35] However, third party researchers were unable to confirm additional GPS accuracy or lock speed due to barometric readings. The researchers suggest that the inclusion of barometers in smartphones may provide a solution for determining a user's elevation, but also suggest that several pitfalls must first be overcome.[36]

More unusual barometers

[edit]
Timex Expedition WS4 in Barometric chart mode with weather forecast function

There are many other more unusual types of barometer. From variations on the storm barometer, such as the Collins Patent Table Barometer, to more traditional-looking designs such as Hooke's Otheometer and the Ross Sympiesometer. Some, such as the Shark Oil barometer,[37] work only in a certain temperature range, achieved in warmer climates.

Applications

[edit]
See also: Surface weather analysis and Weather forecasting
Digital graphing barometer

Barometric pressure and the pressure tendency (the change of pressure over time) have been used in weather forecasting since the late 19th century.[38] When used in combination with wind observations, reasonably accurate short-term forecasts can be made.[39] 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, give a contour map showing areas of high and low pressure.[40] Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface brings clouds and sometimes precipitation.[41] The larger the change in pressure, especially if more than 3.5 hPa (0.1 inHg), the greater the change in weather that can be expected. If the pressure drop is rapid, a low pressure system is approaching, and there is a greater chance of rain. Rapid pressure rises, such as in the wake of a cold front, are associated with improving weather conditions, such as clearing skies.[42]

With falling air pressure, gases trapped within the coal in deep mines can escape more freely. Thus low pressure increases the risk of firedamp accumulating. Collieries therefore keep track of the pressure. In the case of the Trimdon Grange colliery disaster of 1882 the mines inspector drew attention to the records and in the report stated "the conditions of atmosphere and temperature may be taken to have reached a dangerous point".[43]

Aneroid barometers are used in scuba diving. A submersible pressure gauge is used to keep track of the contents of the diver's air tank. Another gauge is used to measure the hydrostatic pressure, usually expressed as a depth of sea water. Either or both gauges may be replaced with electronic variants or a dive computer.[44]

Compensations

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Temperature

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The density of mercury depends on temperature, so readings must be adjusted for the temperature of the instrument. For this purpose mercury thermometers may be mounted on barometers. Temperature compensation of an aneroid barometer can be accomplished by including a bi-metal element in the mechanical linkages. Inexpensive aneroid barometers sold for domestic use typically are manufactured to be accurate at room temperature, and have no provision for further adjustment for temperature.

Altitude

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A digital barometer with altimeter setting (for correction) displayed

As the air pressure decreases at altitudes above sea level (and increases below sea level) the uncorrected reading of the barometer will depend on its location. The reading is then adjusted to an equivalent sea-level pressure for purposes of reporting. For example, if a barometer located at sea level and under fair weather conditions is moved to an altitude of 1,000 feet (305 m), about 1 inch of mercury (~35 hPa) must be added on to the reading. The barometer readings at the two locations should be the same if there are negligible changes in time, horizontal distance, and temperature. If this were not done, there would be a false indication of an approaching storm at the higher elevation.

Aneroid barometers have a mechanical adjustment 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. Setting an aneroid barometer is similar to resetting an analog clock that is not at the correct time. Its dial is rotated so that the current atmospheric pressure from a known accurate and nearby barometer (such as the local weather station) is displayed. No calculation is needed, as the source barometer reading has already been converted to equivalent sea-level pressure, and this is transferred to the barometer being set—regardless of its altitude. Though somewhat rare, a few aneroid barometers intended for monitoring the weather are calibrated to manually adjust for altitude. In this case, knowing either the altitude or the current atmospheric pressure would be sufficient for future accurate readings.

The table below shows examples for three locations in the city of San Francisco, California. Note the corrected barometer readings are identical, and based on equivalent sea-level pressure. (Assume a temperature of 15 °C.)

Location Altitude
(feet)		 Uncorrected Patm
(inches Hg)		 Corrected Patm
(inches Hg)		 Altitude
(metres)		 Uncorrected Patm
(hPa)		 Corrected Patm
(hPa)
City Marina (Sea Level) 0 29.92 29.92 0 1013 hPa 1013 hPa
Nob Hill 348 29.55 29.92 106 1001 hPa 1013 hPa
Mt. Davidson 928 28.94 29.92 283 980 hPa 1013 hPa

In 1787, during a scientific expedition on Mont Blanc, De Saussure undertook research and executed physical experiments on the boiling point of water at different heights. He calculated the height at each of his experiments by measuring how long it took an alcohol burner to boil an amount of water, and by these means he determined the height of the mountain to be 4775 metres. (This later turned out to be 32 metres less than the actual height of 4807 metres). For these experiments De Saussure brought specific scientific equipment, such as a barometer and thermometer. His calculated boiling temperature of water at the top of the mountain was fairly accurate, only off by 0.1 kelvin.[45]

Based on his findings, the pressure altimeter was developed as a specific application of the barometer. In the mid-19th century, this method was used by explorers.[46]

Equation

[edit]

When atmospheric pressure is measured by a barometer, the pressure is also referred to as the "barometric pressure". Assume a barometer with a cross-sectional area A, a height h, filled with mercury from the bottom at Point B to the top at Point C. The pressure at the bottom of the barometer, Point B, is equal to the atmospheric pressure. The pressure at the very top, Point C, can be taken as zero because there is only mercury vapour above this point and its pressure is very low relative to the atmospheric pressure. Therefore, one can find the atmospheric pressure using the barometer and this equation:[47][clarification needed]

Patm = ρgh

where ρ is the density of mercury, g is the gravitational acceleration, and h is the height of the mercury column above the free surface area. The physical dimensions (length of tube and cross-sectional area of the tube) of the barometer itself have no effect on the height of the fluid column in the tube.

In thermodynamic calculations, a commonly used pressure unit is the "standard atmosphere". This is the pressure resulting from a column of mercury of 760 mm in height at 0 °C. For the density of mercury, use ρHg = 13,595 kg/m3 and for gravitational acceleration use g = 9.807 m/s2.

If water were used (instead of mercury) to meet the standard atmospheric pressure, a water column of roughly 10.3 m (33.8 ft) would be needed.

Standard atmospheric pressure as a function of elevation:

Note: 1 torr = 133.3 Pa = 0.03937 inHg

Patm / kPa Altitude
(m)		 Patm / inHg Altitude
(ft)
101.325 (Sea Level) 0 29.92 (Sea Level) 0
97.71 305 28.86 1,000
94.21 610 27.82 2,000
89.88 1,000 26.55 3,281
84.31 1,524 24.90 5,000
79.50 2,000 23.48 6,562
69.68 3,048 20.58 10,000
54.05 5,000 15.96 16,404
46.56 6,096 13.75 20,000
37.65 7,620 11.12 25,000
32.77 8,848 * 9.68 29,029*
26.44 10,000 7.81 32,808
11.65 15,240 3.44 50,000
5.53 20,000 1.63 65,617
* Elevation of Mount Everest, the highest point on earth

See also

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The tower barometer on one of the towers of the main building of the Moscow State University

References

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Further reading

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Patents

[edit]
Table of Pneumaticks, 1728 Cyclopaedia
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Barometer

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A barometer is an instrument designed to measure , which is the force exerted by the weight of the air above a given point on Earth's surface. The device operates on the principle that causes a or mechanical element to rise or fall to a height proportional to that , providing a quantifiable reading typically in units such as millibars, inches of mercury, or hectopascals. Invented in 1644 by Italian physicist , the original mercury barometer consisted of a filled with mercury, inverted into a reservoir of the same liquid, where the height of the mercury column—often around 30 inches (760 mm) at standard sea-level —directly indicates air variations. This invention not only quantified but also laid the foundation for understanding patterns, as barometric tendency—whether atmospheric pressure is rising or falling—serves as a key indicator, with falling pressure often signaling approaching storms while rising pressure suggests fair . Over time, barometers evolved into several types to suit different applications and safety concerns, particularly due to mercury's toxicity. The mercury barometer remains a precise standard for calibration in and scientific research, featuring a closed glass tube partially filled with mercury connected to an open reservoir, where vacuum above the column allows to balance the liquid height. In contrast, the aneroid barometer, invented in 1843 by French physicist Lucien Vidie, uses a flexible metal capsule evacuated of air that expands or contracts with changes, linked to a pointer on a dial for easy reading without liquids; this portable design became widely used in altimeters, where it measures altitude by correlating decreases with elevation gain. Less common are liquid barometers using or oil for educational or low-pressure environments, though they require longer tubes due to the fluids' lower compared to mercury. Barometers play a critical role in meteorology by enabling the tracking of pressure systems on weather maps, where isobars (lines of equal pressure) help forecast fronts, cyclones, and high-pressure ridges associated with clear skies. In aviation and mountaineering, they inform altitude calculations and flight planning, as standard pressure levels (e.g., 1013.25 millibars at sea level) are assumed for safe navigation. Modern digital barometers, often integrated into smartphones and weather stations, employ electronic sensors like piezoresistive strain gauges for real-time data, enhancing global monitoring through networks like those operated by the National Weather Service. Despite these advances, traditional barometers continue to serve as benchmarks for accuracy in laboratories and observatories.

Introduction

Definition and Function

A barometer is a scientific instrument designed to measure atmospheric pressure, which represents the force exerted per unit area by the weight of the air column above a specific location on Earth's surface. This pressure arises from the gravitational pull on the atmosphere, varying with factors such as location and environmental conditions. Measurements from barometers are typically expressed in units including hectopascals (hPa), inches of mercury (inHg), or millibars (mbar), with standard sea-level pressure around 1013 hPa or 29.92 inHg. The primary function of a barometer is to detect and quantify variations in , which signal shifts in patterns, altitude, or other atmospheric dynamics. Rising pressure often indicates clear, stable , while falling pressure may precede storms or , aiding meteorologists in short-term . Additionally, since atmospheric pressure decreases with increasing —roughly by 1 inHg per 1,000 feet—barometers facilitate altitude estimation in , , and applications. At its core, a barometer incorporates a pressure-sensitive medium, such as a column or a deformable diaphragm, that physically responds to external air by changing height, volume, or shape, allowing for precise readings. Invented in the , this instrument revolutionized the understanding and monitoring of atmospheric conditions.

Etymology

The term "barometer" was coined in the 1660s by the Anglo-Irish physicist and chemist , derived from the words baros (βάρος), meaning "," and metron (μέτρον), meaning "measure," to denote an instrument for gauging the or of the air. This nomenclature aptly captured the device's conceptual foundation in quantifying atmospheric heaviness, a notion emerging from early 17th-century vacuum experiments that demonstrated air's tangible . Related terminology in barometry also draws from classical roots. "" stems from "atmosphere," a word introduced in the late from Greek atmos (ἀτμός), signifying "vapor" or "," combined with sphaira (σφαῖρα), meaning "," originally referring to the vaporous envelope enveloping the . Additionally, the pressure unit "," adopted in 1949, honors the Italian physicist , whose 1644 mercury tube experiments laid the groundwork for , with one torr defined as the pressure exerted by a 1 mm column of mercury at . The term "barometer" gained prominence in the scientific lexicon following Boyle's publications, particularly his 1665 publication New Experiments and Observations Touching Cold, where he popularized the device and its name amid burgeoning interest in pneumatics and hydrostatics across Europe. By the late 17th century, it had become standard in natural philosophy texts, supplanting earlier descriptive phrases like "weather glass" and facilitating precise discourse on atmospheric phenomena in works by contemporaries such as Christiaan Huygens and Gottfried Wilhelm Leibniz.

History

Early Experiments

In the early , Italian natural philosophers began to question the long-held Aristotelian doctrine of horror vacui—the idea that nature abhors a —through empirical observations that inadvertently revealed the effects of . These experiments, conducted amid debates over and , provided crucial groundwork for understanding air's weight, though they were not initially designed as pressure measurements. One pivotal observation came from Baliani, a Genoese patrician, who in 1630 attempted to construct a siphon to convey over a hill approximately 21 meters high. The device failed, as would not rise beyond a certain height in the longer leg of the , leading Baliani to correspond with . Galileo explained the limitation by invoking a partial in the , suggesting that could only support a up to about 11 meters (roughly 18 braccia), beyond which the column would break. Baliani interpreted this as evidence that air actively pushed upward, rather than the being pulled by aversion to . Building on such ideas, Gasparo Berti conducted a more deliberate experiment around 1640–1641, erecting a lead tube about 11 meters long on the wall of a tower in Rome. He filled the tube with water, sealed its upper end, and then submerged and opened the lower end in a large basin of water. Upon inversion, much of the water flowed out, but a column remained suspended at a height of approximately 10 meters, with an empty space forming above it—demonstrating the creation of a partial vacuum. Berti viewed this as proof of air's tangible weight pressing on the basin's surface to balance the column, challenging horror vacui by showing that a void could exist without collapse. Accounts of the setup were later documented by contemporaries like Emmanuel d'Aguilon, though the experiment received limited immediate attention. These Italian discoveries, emerging accidentally from efforts to improve water transport and devices, predated the formal of the barometer and signified a pivotal empirical shift toward recognizing as a measurable . They influenced subsequent investigations, including Evangelista Torricelli's work in the 1640s.

Torricelli's Invention

, an Italian physicist and mathematician born in 1608, served as Galileo's assistant following the latter's death in 1642 and built upon contemporary inquiries into suction pumps and the limits of ascent. Influenced by Gasparo Berti's earlier experiment around 1640, which demonstrated a above a tall column of rising to approximately 10 meters in a sealed tube, Torricelli hypothesized that air possesses weight and exerts capable of supporting such columns. This idea challenged prevailing notions of a "horror vacui" and positioned atmospheric weight as the driving force behind fluid behavior in pumps. In 1643, Torricelli devised the first mercury barometer to test his hypothesis, employing a roughly 1 meter long, sealed at one end, filled completely with mercury, and inverted into an open dish of the same liquid while covering the open end to prevent spillage. Upon releasing the cover, the mercury within the tube descended, stabilizing at a height of about 76 centimeters above the dish's surface at , thereby establishing this measurement as the standard equivalent. This simple yet revolutionary apparatus marked the initial practical use of mercury for , leveraging the metal's higher to create a more compact and manageable device compared to water-based setups. The mechanism of Torricelli's barometer relied on , where the weight of the atmosphere pressing down on the mercury in the dish balanced the column's height, with a —now known as Torricelli's vacuum—forming at the tube's upper end due to the absence of external there. Torricelli astutely recognized that the mercury level fluctuated slightly from day to day, providing the first of natural variations in and underscoring the instrument's potential to quantify these changes. This insight transformed the barometer from a mere demonstrator of air's weight into a foundational tool for meteorological and scientific observation. Torricelli detailed his invention and its implications in a pivotal letter dated June 11, 1644, addressed to his colleague Michelangelo Ricci, famously declaring, “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.” This correspondence, initially private, disseminated the concept across European scientific circles and cemented the barometer's role in advancing understanding of atmospheric phenomena.

Developments by Pascal and Others

Following Evangelista Torricelli's invention of the mercury barometer in 1643, Blaise Pascal advanced the understanding of atmospheric pressure through systematic experiments in the mid-1640s. Between 1646 and 1648, Pascal collaborated closely with his brother-in-law Florin Périer to test the device under varying conditions. Périer conducted the landmark altitude experiment on September 19, 1648, ascending the Puy de Dôme, a dormant volcano in central France rising approximately 1,465 meters above the surrounding plain. Starting at the base in the cloister of the Minimes du Puy, where the mercury column measured about 27 inches, Périer carried the barometer to the summit and nearby sites, observing the level drop by roughly 3 inches at the peak—a progressive decrease of approximately 8.5 mm per 100 meters of elevation gain. This demonstrated that atmospheric pressure diminishes with height, as the weight of the air above decreases. These results confirmed and extended the principles outlined in Pascal's 1647 publication Expériences Nouvelles Touchant le Vide (New Experiments Concerning the Void), refuting Aristotelian theories positing a perfect vacuum or "horror vacui" as impossible. The work established the barometer as a reliable tool for measuring atmospheric variations and laid groundwork for hydrostatic principles. In , Robert built on these insights with improvements to the barometer around 1660. Boyle introduced the J-shaped or barometer, a more portable design with a bent tube that allowed easier filling and transport while maintaining accuracy for measurements; this facilitated his experiments on the elasticity of air, later known as . Denis Papin contributed further refinements in the late 1600s, focusing on eliminating residual air in barometers to enhance precision. In a 1686 paper presented to the Royal Society, Papin described methods for creating air-free mercury columns, reducing errors from trapped gases and improving the instrument's reliability for scientific observations. By the , efforts toward standardization addressed inconsistencies in scale markings and environmental corrections. Barometers varied due to non-uniform inch definitions across , prompting instrument makers to adopt consistent calibrations; many integrated thermometers using Daniel Gabriel Fahrenheit's scale (developed in the early 1700s) to compensate for effects on mercury , enabling more accurate readings in meteorological applications.

Principle of Operation

Hydrostatic Equilibrium

In a barometer, the height of the liquid column achieves , where the downward force due to the weight of the balances the upward force exerted by on the surface. This equilibrium occurs because the pressure at the base of the column in the tube equals the acting on the open , with the space above the column being a exerting no pressure. The force balance in the system can be described as follows: PP pushes the liquid up the sealed tube until the hydrostatic pressure generated by the column's counteracts it exactly. The hydrostatic pressure at the base is given by P=ρghP = \rho g h, where ρ\rho is the of the liquid, gg is the acceleration due to gravity, and hh is the of the column. At equilibrium, this equals the external , so h=Pρgh = \frac{P}{\rho g}, assuming the pressure is zero. This relation holds under the assumption of a static, incompressible where remains constant. Torricelli's original design illustrates this like an inverted manometer, with the tube filled with (typically mercury) and inverted into a ; the rises until the forms above it, preventing further ascent as the column's weight balances the external . This ideal model assumes an incompressible and neglects effects, which can slightly depress the meniscus in narrower tubes and require corrections for precise measurements.

The Barometric Formula

The barometric formula describes the variation of with altitude in a planetary atmosphere, providing a mathematical model essential for interpreting barometer measurements beyond . It arises from applying the principle of to the atmosphere, where the downward force of on air parcels is balanced by the . The derivation begins with the hydrostatic equilibrium equation, which states that the change in pressure with height is given by dPdh=−ρg\frac{dP}{dh} = -\rho g, where PP is atmospheric pressure, hh is altitude, ρ\rho is air density, and gg is gravitational acceleration. Substituting the ideal gas law, ρ=PMRT\rho = \frac{P M}{R T}, where MM is the molar mass of air, RR is the universal gas constant, and TT is temperature, yields dPdh=−MgRTP\frac{dP}{dh} = -\frac{M g}{R T} P. Assuming an isothermal atmosphere (constant TT), this differential equation integrates to the exponential form: P(h)=P0exp⁡(−MghRT),P(h) = P_0 \exp\left( -\frac{M g h}{R T} \right), where P0P_0 is the reference pressure at sea level (h=0h = 0). This equation predicts an exponential decay of pressure with height, with a scale height H=RTMgH = \frac{R T}{M g} typically around 8 km for Earth's troposphere under standard conditions. The isothermal assumption simplifies the model but overlooks the actual temperature decrease with altitude, known as the environmental lapse rate, approximately 6.5 K/km in the lower atmosphere. For a linear temperature profile T(h)=T0−ΛhT(h) = T_0 - \Lambda h, where Λ\Lambda is the lapse rate and T0T_0 is sea-level temperature, the barometric formula becomes a power-law approximation: P(h)≈P0(1−ΛhT0)MgRΛ.P(h) \approx P_0 \left(1 - \frac{\Lambda h}{T_0}\right)^{\frac{M g}{R \Lambda}}. This form, with Λ=0.0065\Lambda = 0.0065 K/m, better matches observations up to about 11 km. In practice, the barometric formula enables the conversion of barometer readings to altitude (altimetry) or to standard sea-level pressure equivalents, crucial for , , and . For instance, measured pressures at elevated sites are adjusted using the to estimate true altitude or normalize data.

Types

Mercury Barometers

Mercury barometers are instruments that measure using a column of liquid mercury in a . The basic design features a closed , typically about 80 cm long and narrow in , filled with mercury and inverted into an open containing more mercury, creating a partial above the column. supports the mercury column against , with the of the column directly proportional to the . There are two primary configurations: the U-tube type, where mercury is in both arms of a U-shaped tube and differences cause level changes, and the type, which uses a single and a vertical tube for straightforward . Mercury's high density of 13.6 g/cm³ enables a compact design, as standard atmospheric pressure of 1013.25 hPa supports a column height of exactly 760 mm (76 cm) at 0°C, far shorter than the over 10-meter column required for water-based alternatives due to water's lower density. Historical subtypes include the Fortin barometer, which incorporates an adjustable cistern with a leather diaphragm and thumbscrew to precisely set the mercury level to a fixed zero datum before each reading, enhancing accuracy in portable applications. Another variant is the Fitzroy barometer, a cistern-style instrument integrated with a storm glass—a sealed tube containing a chemical solution that changes appearance to aid qualitative weather predictions alongside pressure readings. These barometers offer high accuracy, typically to within ±0.1 hPa, through direct hydrostatic measurement without mechanical intermediaries, making them a longstanding for , though their use is now limited due to concerns and supplemented by electronic standards. However, their use has declined due to mercury's , which poses risks from vapor or spills, and the instrument's fragility from components susceptible to breakage. Following the in 2013, many countries have phased out mercury barometers in favor of non-toxic options to reduce environmental and hazards.

Water and Other Liquid Barometers

Water barometers operate on the same principle as early mercury designs but employ as the measuring fluid due to its lower , necessitating a much taller column to balance . In the 1640s, Italian scientist Gasparo Berti constructed one of the first known water barometers by filling a 13-meter-long lead tube with water and inverting it into a , observing a form at the top and a height that varied with conditions. At standard atmospheric pressure, a water barometer requires a column approximately 10.3 meters tall, making it suitable primarily for educational demonstrations or measurements in low-pressure environments like high altitudes where shorter columns suffice. To address the impractical height of water barometers, alternatives using other liquids with densities between water and mercury allow for more portable designs. Alcohol, with its low density and visibility, has been used in simple laboratory barometers, though its high volatility leads to rapid evaporation and requires frequent recalibration. Oils, such as mineral or oil, offer better stability due to lower , enabling compact setups for educational or experimental use; for instance, a 12.4-meter oil barometer was built at in 2013 as a demonstration of fluid in a controlled setting. A notable hybrid variant, the sympiesometer, patented by Scottish instrument maker Alexander Adie in 1818, combines oil with compressed air in a sealed tube to create a shorter, more robust instrument ideal for marine applications. In this design, compresses air above an oil , raising or lowering the level against a scale, reducing spill risks compared to open-tube barometers and allowing gimbaled mounting on ships for stability during rough seas. Sympiesometers provided non-toxic operation with clear visibility of level changes, making them popular on 19th-century vessels for weather monitoring without the hazards of heavier fluids. Marine adaptations often featured wheel barometers with cycling dials, which displayed pressure trends via a rotating mechanism linked to the liquid column, aiding naval officers in quick readings amid motion. These designs emphasized durability, non-toxicity, and ease of observation, with liquids like oil ensuring safe handling in humid, salty environments. Despite these innovations, water and other liquid barometers face significant limitations, including the need for excessive vertical space and susceptibility to evaporation or boiling of the fluid at the low-pressure vacuum top, which can distort readings over time. Today, they are rarely employed outside of educational demonstrations, where their visual clarity helps illustrate atmospheric pressure concepts without relying on more compact alternatives.

Aneroid Barometers

Aneroid barometers operate using a mechanical sensing element known as an aneroid capsule, which is a thin, sealed metal box partially evacuated to create a near-vacuum inside. This capsule, typically made from a beryllium-copper alloy for flexibility and durability, features a corrugated diaphragm that expands or contracts in response to changes in . The slight deformation of the diaphragm is amplified through a series of levers, springs, and gears connected to a pointer on a dial, allowing the instrument to display readings directly without the need for columns. The aneroid barometer was invented in 1843 by French physicist Lucien Vidie, who patented the device as a fluidless alternative to mercury barometers. These innovations addressed the limitations of liquid-based instruments, such as spillage risks and the need for leveling. Subtypes of aneroid barometers include barographs, which incorporate a recording mechanism where the pointer traces pressure variations onto a rotating driven by , providing a continuous graphical record of trends over time. Another variant is the portable , a compact aneroid device calibrated to indicate altitude based on pressure differences from , often used by mountaineers and pilots for tracking. Aneroid barometers offer key advantages, including their compact size, absence of liquids that could leak or freeze, and resistance to shocks, making them suitable for mobile applications. They achieve typical accuracy of about 1 hPa, sufficient for most practical uses, and were widely employed in instrumentation before the advent of electronic sensors. Temperature compensation mechanisms, such as bimetallic strips, are often integrated to minimize errors from .

Digital and MEMS Barometers

Digital and barometers utilize micro-electro-mechanical systems () technology to produce compact sensors capable of precise measurement. These devices incorporate miniaturized silicon diaphragms that deform under pressure variations, with deflection detected via piezoresistive or capacitive mechanisms integrated into the silicon structure. Piezoresistive sensors measure strain-induced resistance changes in embedded resistors, while capacitive variants detect alterations in spacing for higher sensitivity and lower power use; Bosch Sensortec's BMP series exemplifies this, transitioning from piezoresistive designs in earlier models like the BMP180 to capacitive in advanced ones such as the BMP581, fabricated using above-polymer sensing membrane (APSM) techniques for enhanced reliability. Key digital features include standardized interfaces like and SPI for seamless integration, alongside on-chip temperature sensors that enable real-time compensation for thermal effects on pressure readings. These sensors achieve absolute accuracy of ±1 hPa and relative precision down to ±0.12 hPa, supporting applications such as indoor and fitness tracking. In consumer devices, they facilitate altimetry; for example, Apple integrated a Bosch BMP280 barometer in the starting in 2014 to provide floor-level detection and elevation data. Advancements through 2025 emphasize integration into IoT ecosystems for , with sensors like the BMP585 offering low noise (0.2 Pa RMS) and ultra-low power (1.2 µA average) for battery-operated nodes in smart cities and stations. The digital barometer market, encompassing MEMS-based units, is forecasted to expand from USD 1.03 billion in 2022 to USD 1.46 billion by 2032, fueled by adoption in wearables and connected health devices. Smartphone-embedded barometers also power apps that analyze pressure trends for personal health insights, such as prediction by alerting users to impending drops that correlate with attack triggers. Among variants, novelty types like the Goethe barometer—a sealed glass vessel with colored liquid that rises or falls in a narrow tube to visually signal pressure shifts—serve as decorative curiosities, while storm glasses employ chemical solutions whose crystallization patterns purportedly forecast weather, though both lack the quantitative accuracy of electronic designs.

Calibration and Corrections

Temperature Compensation

Temperature variations affect barometer readings by causing in the sensing elements, which alters the measured . In mercury barometers, rising temperatures expand the mercury, decreasing its and effectively increasing the column for a given ; this gross effect is approximately 0.14 mm per °C for a standard 760 mm column, based on mercury's cubical expansion coefficient of 181.8 × 10^{-6} per °C. The scale material, typically with a linear expansion coefficient of 18.4 × 10^{-6} per °C, expands less than the mercury, resulting in a net overestimate of that requires correction; the standard temperature correction formula is Ct=h×(s−m)t1+mtC_t = h \times \frac{(s - m)t}{1 + m t}, where hh is the observed reading, ss and mm are the scale and mercury coefficients, and tt is the temperature deviation from 0°C. These effects interact with the principle by changing the ρ\rho, thus requiring adjustments to maintain accurate representation. In aneroid barometers, temperature induces errors by expanding the metal diaphragm or capsule, which reduces its tension and shifts the response; compensation is achieved using bimetallic strips that counteract this expansion through differential of two metals with differing coefficients. These strips are integrated into the mechanical linkage to ensure the pointer deflection remains stable across temperatures from -10°C to 50°C, with errors not exceeding 0.3 hPa for temperature changes of 30 . , where the diaphragm does not fully return to its original shape after cycles, should not exceed 0.3 hPa after a 50 hPa change. Digital and barometers employ electronic compensation via onboard thermistors that measure simultaneously with , applying algorithms to correct for thermal effects on the sensor material, such as silicon diaphragms. For instance, these algorithms model nonlinearity and temperature dependence using coefficients derived from factory , achieving accuracies of ±0.15 hPa over 0°C to 50°C. In Fortin barometers, historical cistern adjustments involve setting the mercury level to a fixed index at the measurement , but full compensation still requires applying the standard mercury density corrections referenced in authoritative tables. Best practices for temperature compensation include calibrating barometers at standard reference temperatures of 0°C for metric systems or 62°F for imperial mercury instruments, using precision thermometers to measure both mercury/scale and ambient conditions within ±0.5°C. Standard coefficients, such as those for mercury expansion, are provided by the National Institute of Standards and Technology (NIST), ensuring to fundamental physical constants. Regular verification against a reference standard mitigates residual errors from or incomplete compensation.

Altitude and Latitude Adjustments

Atmospheric pressure decreases exponentially with increasing altitude due to the reduced weight of the overlying air column, as described by the barometric formula. This necessitates adjustments to barometer readings taken at elevated locations to obtain equivalent sea-level values, enabling consistent comparisons across different sites. In aviation, such corrections are applied to compute QNH, the sea-level pressure that would yield the observed station pressure at the measurement altitude, ensuring accurate altimeter indications above mean sea level. The acceleration due to gravity, which influences the hydrostatic balance in barometers, varies with owing to the Earth's shape and rotational effects. This variation causes to decrease by approximately 0.5% from the poles to the , leading to systematic errors in readings if uncorrected, particularly in mercury barometers where column height is inversely proportional to local . Corrections standardize readings to a reference , typically 45°32'40" N, using the International Gravity Formula of 1967: g=9.780327(1+0.0053024sin⁡2ϕ−0.0000058sin⁡22ϕ) m/s2g = 9.780327 \left(1 + 0.0053024 \sin^2 \phi - 0.0000058 \sin^2 2\phi \right) \, \mathrm{m/s^2} where ϕ\phi is the latitude in degrees. For precise altimetry, hypsometers employ barometric principles by measuring the boiling point of water, which correlates with local pressure and thus altitude, offering accuracies suitable for geodetic surveys. Modern digital barometers often integrate with GPS receivers through data fusion techniques, such as Kalman filtering, to hybridize pressure-based altitude estimates with satellite-derived positions, mitigating errors from transient weather variations. The International Civil Aviation Organization (ICAO) standard sea-level pressure of 1013.25 hPa serves as the baseline for these adjustments in the International Standard Atmosphere model, though deviations in temperature can introduce secondary errors by altering the pressure lapse rate.

Applications

Weather Prediction

Barometers play a central role in weather prediction by measuring , which serves as a key indicator of impending weather changes. Falling typically signals the approach of low-pressure systems associated with storms, , and , while rising often precedes high-pressure systems bringing clear skies and stable conditions. For instance, a steady decrease in pressure over several hours can forecast wet , with rapid drops commonly preceding rain or thunderstorms in mid-latitudes. These trends arise because low-pressure areas draw in moist air, promoting formation and instability, whereas high-pressure zones subside air, inhibiting . Historically, barometers enabled early systematic , notably through the efforts of in the 1860s. As head of the British Meteorological Department, FitzRoy distributed barometers to coastal communities and fishing fleets starting in 1858, allowing users to monitor pressure for local risks following devastating events like the 1859 gale. By 1860, he issued the first using telegraphed barometer readings from coastal stations, and in 1861, he expanded to general two-day forecasts published in newspapers, interpreting pressure falls as harbingers of gales. This approach empowered sailors to avoid dangers independently and laid the groundwork for organized . In modern , barometers form the backbone of global observation networks coordinated by the (WMO), where surface stations report data as part of the Integrated Global Observing System. Synoptic stations, operating hourly or more frequently, equip automatic weather systems with digital barometers to capture at resolutions of 0.1 hPa, feeding into models. These readings integrate with variables like , , and to refine forecasts; for example, data from dense station arrays delineate isobars on weather maps, revealing gradients that drive storm tracks and fronts. Moreover, synoptic observations include pressure tendency—the net change in atmospheric pressure over the preceding three-hour period, along with its characteristic trend—which enables meteorologists to evaluate the intensification or weakening of pressure systems and thereby improves predictions of fronts, storms, and other phenomena through synoptic analysis and short-term forecasting. Digital barometer networks enhance nowcasting—short-term forecasts up to 6 hours—by providing real-time, high-resolution data for immediate alerts on convective events like thunderstorms. Arrays of sensors in urban and rural areas achieve uncertainties as low as 0.3 hPa, enabling precise tracking of pressure perturbations in models. However, limitations persist in capturing microscale variations, such as those from local or urban heat islands, where below 1 km may miss subtle gradients, and instrument drift over time can introduce errors without regular .

Altimetry and Navigation

Barometers play a crucial role in altimetry by measuring and converting it to altitude estimates through the , which models the exponential decrease in with height in the atmosphere. This principle assumes , where at a given altitude hh is given by p(h)=p0exp⁡(−MghRT)p(h) = p_0 \exp\left(-\frac{M g h}{R T}\right), with p0p_0 as sea-level , MM as of air, gg as , RR as the , and TT as . The (ISA) defines baseline conditions for these calculations, including a sea-level of 15°C and of 1013.25 hPa, enabling consistent altitude determinations across applications. In , barometric altimeters integrated into pitot-static systems provide pilots with real-time altitude data by sensing from external ports on the . The QNH setting adjusts the subscale to local , yielding indicated altitude above mean for , while QNE uses the standard 1013.25 hPa (29.92 inHg) for above the standard datum plane during high-altitude flight. Non-standard atmospheric conditions, such as deviations or errors, introduce inaccuracies, with approximately 30 feet of altitude error per 1 hPa discrepancy near . For navigation, historical marine barometers enabled sailors from the onward to monitor trends for storm avoidance, as falling pressures signaled approaching gales, allowing course adjustments to safer waters. In modern contexts, hybrid GPS-barometer systems enhance precision for activities like by combining barometric altitude data, which offers high relative accuracy over short vertical changes, with GPS-derived positioning to correct for absolute elevation errors up to 20-25 meters. These devices calibrate barometric readings against GPS fixes periodically, improving elevation profiles for navigation in remote areas. Safety in altimetry and relies on regular instrument to mitigate drift and ensure reliability; the U.S. (FAA) mandates altimeter tests and inspections under 14 CFR §91.411 every 24 calendar months for instrument-equipped aircraft, verifying accuracy within ±20 feet at and addressing or friction effects. Drift corrections involve comparing readings against known altitudes or reference barometers, with adjustments for environmental factors like to maintain operational during flight.

Modern Uses in Technology and Health

In modern , microelectromechanical systems () barometers have become integral to smartphones, enabling precise indoor positioning through floor detection and () navigation. These sensors measure changes to estimate altitude with accuracies up to 1 meter, complementing GPS limitations in enclosed spaces and facilitating features like automatic floor switching in mapping apps. For instance, algorithms using multiple barometer readings can detect floor levels in multi-story buildings with high reliability, enhancing in indoor navigation systems. In the (IoT) ecosystem, support smart home and building applications, including monitoring for anomalies in HVAC systems. Devices like the ENS220 exemplify this integration, offering low-power operation suitable for continuous IoT deployment in energy-efficient buildings. Industrially, barometers provide precision monitoring in manufacturing processes, such as vacuum chambers where maintaining low-pressure environments is critical for fabrication and applications. High-accuracy models, like those from Vaisala's BAROCAP , ensure stable measurements in controlled settings, supporting in automated production lines. In stations, NIST-traceable barometers deliver reliable atmospheric data for tracking and , with transducers from Systems offering stability in harsh outdoor conditions. In health applications, barometer data is leveraged by apps to track fluctuations, aiding in the prediction of onset or flare-ups for sensitive individuals. Research indicates that drops in barometric pressure correlate with increased frequency, as observed in patient diary studies from and . Similarly, changes in pressure have been linked to heightened joint pain in sufferers, potentially due to tissue expansion in lower-pressure conditions, with correlations confirmed in weather-sensitive cohort analyses. Apps such as Barometer Reborn allow users to log pressure trends alongside symptoms, enabling personalized alerts for conditions like sleep disturbances tied to pressure shifts. As of 2025, integrations with AI in apps use barometric data for in weather-sensitive conditions, enhancing approaches. Emerging integrations extend barometer use to wearables and drones, where real-time pressure data enhances and altitude stabilization. In wearables, barometers combined with accelerometers improve human motion tracking, though from environmental factors poses calibration challenges. For drones, these sensors support precise altimetry during flights, but battery life remains a key limitation in portable systems, constraining continuous operation to under an hour in many designs.

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