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, and weather front.
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).
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 , 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.
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.
It was traditionally thought, especially by the Aristotelianism, 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.
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.Jet Stream. Learning Lesson: Measure the Pressure – The "Wet" Barometer. Retrieved on 2019-01-21.
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".Strangeways, Ian. Measuring the Natural Environment. Cambridge University Press, 2000, p. 92. 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.
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 and of Hg. One atmosphere (1 atm) is equivalent to 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.
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.
Roger North observed that many, including Robert Hooke, attempted to resolve these issues but often abandoned the endeavor due to technical limitations. Nonetheless, Hooke remained persistent, proposing several adaptations including narrowing the open end of the siphon tube and exploring spiral tube designs. 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.
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. The device combined a sealed spirit thermometer with an open air-based thermometer, calibrated to reflect barometric pressure changes through liquid displacement. Hooke’s use of hydrogen-filled containers and colorful almond oil further enhanced visibility and responsiveness. Notably, Edmond Halley tested this prototype on his South Atlantic voyage from 1698 to 1700 and praised its reliability in forecasting weather changes. His endorsement led to greater interest and validation by the Royal Society. Figure 8 below is from this report, depicting the Hooke Barometer, with detailed description in the writing.
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.” 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. He consistently found it effective in forecasting storms, wind changes, and even the proximity of ice.
A significant advancement occurred during James Cook’s renowned voyages in the late 18th century. 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. Renowned instrument maker Edward Nairne was chosen to supply the equipment. 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.
Nairne’s design represented a leap in functionality. The narrow bore significantly reduced mercury motion, enabling more accurate readings even in turbulent conditions. These instruments proved so reliable that they were adopted not only by the Royal Navy but also by international expeditions. 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.
Despite the widespread use of Nairne’s marine barometer, it was not without limitations. Lapérouse lauded the device’s predictive capabilities but also noted inconsistencies in mercury behavior, highlighting the complexity of translating instrument readings into reliable forecasts. 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.
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. In his official correspondence, Flinders confirmed the instrument’s success and expressed appreciation for its stability and precision in recording atmospheric conditions.
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.
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 –1805. 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.
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.
A barometer can also be found in smartphones such as the Samsung Galaxy Nexus, This Is the Samsung Galaxy Nexus, Google's New Official Android Phone . Gizmodo.com (2011-10-18). Retrieved on 2011-11-15. 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. Inclusion of barometers on smartphones was originally intended to provide a faster GPS lock. Galaxy Nexus barometer explained, Sam Champion not out of a job. Engadget (2011-10-20). Retrieved on 2011-12-03. 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.
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".
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.
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 Clock face 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.)
1013 hPa |
1013 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.
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.
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
(Sea Level) 0 | ||||
1,000 | ||||
2,000 | ||||
3,281 | ||||
5,000 | ||||
6,562 | ||||
10,000 | ||||
16,404 | ||||
20,000 | ||||
25,000 | ||||
29,029* | ||||
32,808 | ||||
50,000 | ||||
65,617 | ||||
* Elevation of Mount Everest, the highest point on earth |
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