Aneroid Barometer

Background

Earth's atmosphere weighs about 6.5 × 1021 (5.98 × 1024). Spread out across Earth's entire surface area, it exerts an air (barometric) pressure of about 14.7 pounds per square inch (psi) (101 kilopascals [kPa]) at sea level. While that is the average, the actual barometric pressure varies greatly from place to place and from one moment to the next. The barometric pressure at the summit of Mount Everest, is one third of the barometric pressure at sea level. The greatest barometric pressure extremes ever recorded at sea level were 15.7 psi (108 kPa) during a very cold winter in Siberia and 13.5 psi (87 kPa) recorded in the eye of a Pacific Ocean typhoon. Barometric pressure differences are important because they are the basic creators of weather. The sun is the major factor in causing pressure variations in the atmosphere. Hot equatorial air rises and flows north. As it moves Coriolis forces in the northern hemisphere bend it to the west in the tropics and to the east in the temperate zones, setting up cells of clockwise and counterclockwise atmospheric flow. The changing atmospheric pressures that come with these flows can be used to predict the weather. In fact, prior to the advent of the radio, the only tool sailors had to predict the weather was the barometer, which told them which way the air pressure was changing. A rising barometric pressure was a sign of improving weather. A falling barometer was a sign to batten down the hatches and hope for the best.

History

Many people do not realize that atmospheric pressure exists since it cannot be felt. Its existence was discovered by the Italian scientist Evangelista Torricelli. Torricelli made his discovery during an attempt to help silver miners, who were having trouble keeping their mines dry. The only pump available to the miners were suction pumps, which could only raise water 32 ft (9.8 m). Torricelli deduced the reason the pump could not raise water more than that was because the weight of the atmosphere was only heavy enough to support a column of water 32 ft (9.8 m) high. Torricelli's insight was that if a see-saw were arranged such that half of it was under a vacuum and half of it was under atmospheric pressure, 32 ft (9.8 m) of water would have to be placed on the vacuum side of the see-saw to balance the atmospheric pressure acting on the other side. The miners' pumps were like a see-saw trying to balance more than 32 ft (9.8 m) of water. To test his theory, Torricelli took a glass tube about 4 ft (1.2 m) long, sealed it at one end, and filled it with mercury. Holding his thumb over the open end, he upended the tube into a bowl of mercury. His theory was that, since mercury is 13.5 times more dense than water, barometric pressure would only be high enough to support a column of mercury 2.4 ft (0.73 m) high (the maximum height the suction pumps could pull water divided by 13.5). In actuality, the atmosphere supported a column of mercury 2.5 ft (0.76 m) high. The extra distance was because the vacuum at the top of the glass tube was almost perfect—Torricelli was also the first person to create a vacuum—and the seals in the miners' pumps were not. It is not clear who noticed that barometers could be used to forecast the weather, though it is possible it was Ferdinand dei Medici, Grand Duke of Tuscany. While mercury barometers, even to this day, are the most accurate barometers, they are not without drawbacks. Trying to read a mercury barometer on board a ship caught in a hurricane is not easy. The idea for a mercury-free barometer (an aneroid barometer) first occurred to Gottfried Leibniz (coinventor of calculus) around 1700. Metallurgy was not sufficiently advanced in 1700 to realize Leibniz's idea. The French inventor Lucien Vidie developed the first practical aneroid barometer in 1843. Aneroid barometers are the most common barometers in use today. They are the circular, brass, clock-like instruments with a sweep indicator pointing to the current barometric pressure. They are commonly seen in weather stations and on board boats. Aneroid barometers function by measuring the expansion and contraction of a hollow metal capsule.