Barometer

A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer. The first makes use of a long narrow glass tube filled with mercury supported in a container of mercury, and the second makes use of an elastic disk whose size changes as a result of air pressure.

Mercury barometers

The principle of the mercury barometer was discovered by the Italian physicist Evangelista Torricelli in about 1643. That principle can be illustrated as follows: a long glass tube is sealed at one end and then filled with liquid mercury metal. The filled tube is then turned upside down and inserted into a bowl of mercury, called a cistern. When this happens, a small amount of mercury runs out of the tube into the cistern, leaving a vacuum at the top of the tube. Vacuums, by nature, exert very little or no pressure on their surrounding environment.

As atmospheric pressure pushes down on the surface of the mercury in the cistern, that mercury in turn pushes up with an equal pressure on the mercury in the glass tube. The height of the mercury in the tube, therefore, reflects the total pressure exerted by the surrounding atmosphere. Under normal circumstances, the column of mercury in the glass tube stands at a height of about 30 inches (76 centimeters) when measured at sea level.

In theory, a barometer could be made of any liquid whatsoever. Mercury is chosen, however, for a number of reasons. It is so dense that the column supported by air pressure is of a usable height. A similar barometer made of water, in comparison, would have to be more than 34 feet (100 meters) high. Mercury also has a low vapor pressure, meaning it does not evaporate very easily. Water has a greater vapor pressure. Because of this, the pressure exerted by water vapor at the top of the barometer would affect the level of the mercury in the tube and the barometric reading, a factor of almost no consequence with a mercury barometer.

Aneroid barometer

A major disadvantage of the mercury barometer is its bulkiness and fragility. The long glass tube can break easily, and mercury levels may be difficult to read under unsteady conditions, as on board a ship at sea. To resolve these difficulties, the French physicist Lucien Vidie invented the aneroid ("without liquid") barometer in 1843.

An aneroid barometer is a container that holds a sealed chamber from which some air has been removed, creating a partial vacuum. An elastic disk covering the chamber is connected to a needle or pointer on the surface of the container by a chain, lever, and springs. As atmospheric pressure increases or decreases, the elastic disk contracts or expands, causing the pointer to move accordingly.

One type of aneroid barometer has a pointer that moves from left to right in a semicircular motion over a dial, reflecting low or high pressure. The simple clocklike aneroid barometer hanging on the wall of many homes operates on this basis. Another type of aneroid barometer has the pointer resting on the side of a rotating cylinder wrapped with graph paper. As the cylinder rotates on its own axis, the pointer makes a tracing on the paper that reflects increases and decreases in pressure. A recording barometer of this design is known as a barograph.

Words to Know

Altimeter: An aneroid barometer used to measure altitude.

Barograph: An aneroid barometer modified to give a continuous reading of atmospheric pressures on graph paper.

Vapor pressure: The amount of pressure exerted by liquid molecules in the vapor state.

The altimeter. An important application of the aneroid barometer is the altimeter, an instrument used to measure one's distance above sea level. Atmospheric pressure is a function of altitude. The higher one is above sea level, the less the atmospheric pressure; the closer one is to sea level, the greater the atmospheric pressure. A simple aneroid barometer can be used to confirm these differences. If the barometer were mounted in an airplane, a balloon, or some other device that travels up and down in the atmosphere, one could determine the altitude by noting changes in atmospheric pressure.

[See also Atmospheric pressure ]

Atmospheric pressure

Aristotle, whose teachings sometimes otherwise inhibited the advancement of science, was right on target in his belief that the atmosphere surrounding the Earth had weight. Moreover, Aristotle stated that as air density decreased, it would be possible for an object to move faster. However, he did not believe in the concept of a vacuum because the absence of an atmosphere meant an object could move infinitely fast, and since infinite speed was not possible, a vacuum that allowed infinite speed was not considered possible either. Galileo disputed some of Aristotle's contentions. In 1638, Galileo published a book in which he asserted a vacuum was possible. But Galileo did not hold that air had a weight that could exert a pressure, even though his own experiments showed clearly that air exerted a force on objects. This was perhaps because he discounted everything Aristotle said, even when he happened to be right. Consequently, the thermometer Galileo invented was inaccurate because it did not take the effect of air pressure into account. Otto von Guericke became interested in air pressure because of Galileo's comments on the subject. In a public demonstration in 1657, Guericke became the first to use an air pump and create a vacuum, thus ending the debate on whether one could exist.

In 1643, in Florence, Italy, Evangelista Torricelli furthered Guericke's work. Filling a narrow tube with mercury and upending it in a bowl of mercury, Torricelli found that only a portion of the tube emptied. He correctly surmised that the atmospheric pressure upon the mercury in the bowl kept the tube from draining completely, and the vacant area at the top of the tube was a vacuum. He noticed the height of the column of mercury fluctuated from day to day, indicating that the atmospheric pressure changed. The barometer, a device to measure the pressure of the atmosphere, was born, yet that name wouldn't exist for another 20 years.

Mathematician Blaise Pascal duplicated the experiments of Torricelli, and he expanded on them. In 1648, Pascal, who suffered from ill health, had his brother-in-law make measurements of air pressure at various altitudes on a mountain. As expected, the higher the altitude, the less pressure registered on the barometer. Obviously, the weight of the air at the surface of the Earth was greater because it has to support the atmosphere above it. Robert Boyle duplicated Torricelli's experiment as well. In 1660, Boyce placed his mercury-filled tube in a container and removed the surrounding air, creating a vacuum. As the air was removed, the column of mercury dropped. When completely evacuated, the mercury showed zero air pressure in the container. It was Boyle who coined the word "barometer" in 1665.

Today, it is known that the weight of Earth's atmosphere is more than five quadrillion tons. The weight of air pressing down on one's shoulders is about one ton, but we aren't flattened because we are supported on all sides by an equal amount of pressure. The normal barometric pressure at sea level, equal to one atmosphere (1 atm) of pressure equals 14.7 psi (pounds per square inch), or 760 mm (29.92 inches) of mercury.

See also Air masses and fronts; Weather forecasting

barometer , instrument for measuring atmospheric pressure. It was invented in 1643 by the Italian scientist Evangelista Torricelli, who used a column of water in a tube 34 ft (10.4 m) long. This inconvenient water column was soon replaced by mercury, which is denser than water and requires a tube about 3 ft (0.9 m) long. The mercurial barometer consists of a glass tube, sealed at one end and filled with pure mercury. After being heated to expel the air, it is inverted in a small cup of mercury called the cistern. The mercury in the tube sinks slightly, creating above it a vacuum (the Torricellian vacuum). Atmospheric pressure on the surface of the mercury in the cistern supports the column in the tube, which varies in height with variations in atmospheric pressure and hence with changes in elevation, generally decreasing with increases in height above sea level. Standard sea-level pressure is 14.7 lb per sq in. (1,030 grams per sq cm), which is equivalent to a column of mercury 29.92 in. (760 mm) in height; the decrease with elevation is approximately 1 in. (2.5 cm) for every 900 ft (270 m) of ascent. In weather forecasting, barometric readings are usually measured on electronically controlled instruments often tied to computers. The results are plotted on base maps so that analyses of weather-producing pressure systems can be made. At a given location a storm is generally anticipated when the barometer is falling rapidly; when the barometer is rising, fair weather may usually be expected. The aneroid barometer is a metallic box so made that when the air has been partially removed from the box the surface depresses or expands with variation of air pressure on it; this motion is transmitted by a train of levers to a pointer which shows the pressure on a graduated scale. A barograph is a self-recording aneroid barometer; an altimeter is often an aneroid barometer used to calculate altitude.

Atmospheric pressure

Earth's atmosphere consists of gases that surround the surface of the planet. Like any gas, which is made up of molecules that are constantly in motion, the atmosphere exerts a force or pressure on everything within it. This force, divided by the area over which it acts, is the atmospheric pressure. The atmospheric pressure at sea level—considered the mean atmospheric pressure—has an average value of 14.7 pounds per square inch or 29.92 inches of mercury (as measured by a barometer). This means that a one-inch-square column of air stretching from sea level to about 120 miles (200 kilometers) into the atmosphere would weigh 14.7 pounds.

Atmospheric pressure decreases with increasing altitude. The reason for this change with altitude is that atmospheric pressure at any point is a measure of the weight, per unit area, of the atmosphere above that point. Higher altitudes have a lower atmospheric pressure because there is less atmosphere weighing down from above. At an altitude of about 3.1 miles (5 kilometers), the atmospheric pressure is half of its value at sea level.

Atmospheric pressure is closely related to weather. Regions of pressure that are slightly higher or slightly lower than the mean atmospheric pressure develop as air circulates around Earth. The air rushes from regions of high pressure to low pressure, causing winds. The properties of the moving air (cool or warm, dry or humid) will determine the weather for the areas through which it passes. Knowing the location of high and low pressure areas is vital to weather forecasting, which is why they are shown on the weather maps printed in newspapers and shown on television.

[See also Atmosphere, composition and structure; Barometer; Weather; Weather forecasting ]

barometer Instrument for measuring atmospheric pressure. There are two basic types. A mercury barometer has a vertical column of mercury, whose length alters with changes in atmospheric pressure. An aneroid barometer has a chamber containing a partial vacuum, and the chamber changes shape with shifts in pressure. Barometers are used in weather forecasting to predict local weather changes: a rising barometer (increasing pressure) indicates dry weather; a falling barometer indicates wet weather. In a barograph, the pointer of an aneroid barometer is replaced by a pen that traces variations in pressure on a revolving cylindrical chart. A barometer can also be used in an altimeter to measure altitude by indicating changes in atmospheric pressure. See also bar

atmospheric pressure The downward force exerted by the weight of the overlying atmosphere, expressed per unit area in a given horizontal cross-section. Pressure varies throughout the atmosphere, owing to the distribution of mass; there are small diurnal variations of partly tidal and partly thermal origin, as well as bigger changes associated with the passage of depressions and anticyclones. Atmospheric pressure is measured in millibars (mb), 1 mb being equal to 100 kilopascals (kPa). Measurements are usually made with a mercury barometer. The overall global average pressure at sea level is 1013.25 mb, but as air is readily compressible, pressure decreases exponentially with altitude.

atmospheric pressure Downward force exerted by the weight of the overlying atmosphere, expressed per unit area in a given horizontal cross-section. Pressure varies throughout the atmosphere, owing to the distribution of mass; there are small diurnal variations of partly tidal and partly thermal origin, as well as bigger changes associated with the passage of depressions and anticyclones. Atmospheric pressure is measured in millibars (mb), 1 mb being equal to 100 kilopascals (kPa). Measurements are usually made with a mercury barometer. The overall global average pressure at sea level is 1013.25 mb, but as air is readily compressible, pressure decreases exponentially with altitude.

mercury (quicksilver, symbol Hg) Liquid, metallic element. The chief ore is cinnabar (a sulphide), from which it is extracted by roasting. It is the only metal that is liquid at normal temperatures. The silvery element is poisonous, causing kidney and brain damage, as well as birth defects. Mercury is used in barometers, thermometers, laboratory apparatus, mercury-vapour lamps, and mercury cells. Mercury compounds are used in pharmaceuticals and in dentists' amalgams (alloys) to fill cavities in teeth. Properties: at.no. 80; r.a.m. 200.59: r.d. 13.6; m.p. 238.87°C; (237.97°F); b.p. 356.58°C (673.84°F); most common isotope Hg²⁰² (29.8%).

Mercury Smallest of the four inner planets, and the planet closest to the Sun. It has no known satellite. Very little was known about Mercury's surface until the Mariner 10 probe made three close approaches to the planet in 1974 and 1975, and returned pictures of nearly half the surface. These showed a heavily cratered, lunar-like world marked by valleys and ridges. Radar mapping of Mercury's polar regions in 1991 and 1992 revealed what may be water-ice on the floors of craters permanently in shadow. There is a very tenuous atmosphere, mainly of helium and sodium, and a weak magnetic field.

http://lpl.arizona.edu/nineplanets/nineplanets/mercury.html; http://wr.usgs.gov

barometer An instrument for the measurement of atmospheric pressure. The usual type is a mercury barometer, in which the atmosphere's pressure on a small reservoir of mercury supports a column of mercury in a vacuum tube, the open end of which is below the surface of the mercury in the reservoir. The column is on average about 76 cm (30 inches) high. Readings must be corrected to compensate for pressure variation due to gravitational anomalies and for thermal expansion or contraction of the mercury; therefore correction to a standard temperature is necessary. See also aneroid barometer; Fortin barometer; and Kew barometer.

barometer Instrument for the measurement of atmospheric pressure. The usual type is a mercury barometer, in which the atmosphere's pressure on a small reservoir of mercury supports a column of mercury in a vacuum tube the open end of which is below the surface of the mercury in the reservoir. The column is on average about 76 cm (30 inches) high. Readings must be corrected to compensate for pressure variation due to gravitational anomalies and for thermal expansion or contraction of the mercury; therefore correction to a standard temperature is necessary. See also ANEROID BAROMETER; FORTIN BAROMETER; and KEW BAROMETER.

ba·rom·e·ter / bəˈrämitər/ • n. an instrument measuring atmospheric pressure, used esp. in forecasting the weather and determining altitude. ∎  something that reflects changes in circumstances or opinions: furniture is a barometer of changing tastes. DERIVATIVES: bar·o·met·ric / ˌbarəˈmetrik/ adj. bar·o·met·ri·cal adj. ba·rom·e·try / -ˈrämitrē/ n.

atmospheric pressure Pressure exerted by the atmosphere because of its gravitational attraction to the Earth (or other body), measured by barometers and usually expressed in units of mercury. Standard atmospheric pressure at sea level is 760mm (29.92in) of mercury. The column of air above each cm² of Earth's surface weighs c.1kg (2.2lb), or c.6.7kg (14.7lb) above each in².

barometer, an instrument that measures atmospheric pressure either by a column of mercury or, in the case of the aneroid barometer, by a thin metal cylinder partially compressed by the atmosphere.

Mike Richey

barometer XVII. f. Gr. báros weight + métron -METER.
Hence barometric XIX, barometrical XVII. So barograph XIX.

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