How The Barometer Changed Weather Forecasting

In 1643, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish, and watched the mercury settle at a height of about 76 centimeters (30 inches), leaving an empty space above it. He had built the first barometer. In doing so, he proved something Aristotle's followers had insisted was impossible for nearly two thousand years: a vacuum could exist, and the air itself has weight that presses on everything beneath it. Two centuries later, that same principle became the backbone of the world's first organized storm-warning system.

The plumbing problem that started it all

Torricelli was not trying to predict weather. He was trying to solve a practical headache that had been vexing well-diggers, miners, and fountain engineers across northern Italy: suction pumps refused to lift water more than about 10 meters (33 feet), no matter how well they were built or how tightly they were sealed.

At the time, the standard explanation invoked horror vacui, the old Aristotelian idea that "nature abhors a vacuum" and would do almost anything to prevent one from forming, including pulling water up a pipe. Galileo Galilei had already grown suspicious of that explanation. He had noted the 10-meter (33-foot) limit himself and suspected something more concrete was at work, but he died in January 1642 before he could resolve it. Torricelli, his student and eventual successor as court mathematician in Florence, picked up the problem.

Torricelli's insight was that the atmosphere was not passive. It was a physical fluid with weight, piled miles deep over every square meter of the Earth's surface, and that weight was doing the pushing, not some abstract natural aversion to empty space. He reasoned that if air pressure was pushing water up an evacuated pipe, then a denser liquid should rise to a proportionally shorter height under that same push. Mercury is roughly 13.6 times denser than water, so he predicted it would rise only about a fourteenth as far, somewhere around 76 centimeters (30 inches) instead of 10 meters (33 feet).

He built the experiment with his colleague Vincenzo Viviani in 1643: a glass tube about one meter (roughly 3 feet) long, sealed at one end, filled completely with mercury, and inverted into an open dish of mercury while keeping a finger over the opening. When the finger came away, the mercury column dropped only slightly, settling at close to the predicted 76 centimeters, and a gap appeared at the sealed top of the tube (Leybold, "Torricelli and the mercury barometer"; PMC, "Torricelli and the Ocean of Air"). That gap became known as "Torricelli's vacuum," and it was authentic empty space, not air that had somehow squeezed through solid glass.

Torricelli described the discovery in a 1644 letter using a phrase that has outlived nearly everything else he wrote: "We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight" (Britannica, "Evangelista Torricelli"). It remains one of the more quietly startling sentences in the history of science, because it reframes something invisible and seemingly weightless as a literal ocean pressing down on every living thing.

Why the mercury stops where it does

The physics behind Torricelli's tube is a balance of two forces acting on the same puddle of mercury in the open dish. Atmospheric pressure pushes down on the mercury in the dish, and that push transmits through the liquid and up into the tube, holding the column aloft against gravity, which is constantly trying to pull it back down.

A mercury column stops rising or falling exactly at the height where the weight of that column equals the push of the atmosphere on the dish below it. Think of it as a tug-of-war rope stretched between two teams of exactly equal strength. The rope (the mercury) does not move once the forces balance, and its resting position tells you precisely how hard each team is pulling. In a barometer, the height of the mercury is a direct, quantitative readout of how hard the atmosphere above the dish is pulling down.

This is also why the vacuum at the top of the tube matters so much for the experiment's validity. If any air had leaked into that space, it would have pushed back down on the mercury from above, shrinking the column and giving a falsely low reading. Torricelli's careful sealing and filling technique, done entirely underneath a bath of mercury so no air pocket could sneak in, is what made the number reliable enough to become a physical constant.

Why is standard atmospheric pressure defined as 760 millimeters of mercury?

That figure is the average height a mercury column reaches at sea level under typical atmospheric conditions, later adopted as the reference value for "one standard atmosphere." Mercury's density, about 13.6 times that of water, keeps the column short enough (about 76 centimeters, or 30 inches) to fit in a benchtop instrument rather than the 10-meter (33-foot) tube water would require.

Blaise Pascal takes the barometer up a mountain

Torricelli's tube proved a vacuum could exist and that air had weight, but it did not immediately settle every objection. Some contemporaries argued that whatever filled the space above the mercury was some exotic substance, not true nothingness, and that the column's behavior might reflect properties of that substance rather than atmospheric pressure from below.

Blaise Pascal, the French mathematician and philosopher, devised a decisive test. If atmospheric pressure was really what held the mercury up, then carrying a barometer to a higher altitude, where there is less air overhead pressing down, should cause the column to drop. In September 1648, Pascal's brother-in-law Florin Périer carried a Torricelli-style barometer up the Puy de Dôme, a volcanic peak in central France roughly 1,465 meters (4,806 feet) tall, while a second barometer stayed at the base as a control.

The mercury column at the summit measured noticeably lower than the one left at the base, and it kept dropping in stages as Périer climbed, exactly as Pascal had predicted. The experiment did two things at once: it confirmed that atmospheric pressure decreases with altitude, and it closed the door on rival explanations by showing the effect was tied directly to how much air sat overhead. Later, this altitude sensitivity became the working principle behind aircraft altimeters, which are essentially barometers calibrated to read altitude instead of pressure.

From curiosity to storm predictor

For roughly a century after Pascal's mountain experiment, the barometer remained mostly a scientific instrument, a way to study the atmosphere rather than to forecast it. Sailors, farmers, and naturalists slowly built up an informal, empirical body of knowledge: a falling reading tended to precede rough weather, and a rising one tended to precede calm skies. Nobody yet had a physical mechanism to explain why, only accumulated experience.

The mechanism became clear as scientists mapped out how air behaves when it warms, cools, rises, and sinks. A region of falling pressure typically means air is rising. As that rising air climbs, it cools, and the moisture it carries condenses into clouds and eventually precipitation, which is why a dropping barometer so often precedes rain or storms. A region of rising pressure usually means air is sinking, which warms it and suppresses cloud formation, producing the clear skies associated with "high pressure." Reading a barometer, in other words, is really an indirect way of reading whether the air column above you is going up or coming down.

By the early 1800s, barometers were common enough on ships and in scientific households that the correlation between falling pressure and incoming storms was widely, if informally, understood. What was missing was a systematic, networked way to collect readings from many locations at once and turn scattered individual measurements into an organized warning system that could reach people before a storm arrived.

Robert FitzRoy and the birth of public forecasting

Robert FitzRoy, the Royal Navy officer who had commanded HMS Beagle during Charles Darwin's famous 1831 to 1836 voyage, became the person who turned scattered barometric observation into an organized public service. In 1854, FitzRoy was appointed to head a new statistical department within the Board of Trade, an office that would eventually become the United Kingdom's Meteorological Office (Met Office, "Robert FitzRoy and the early Met Office").

The turning point came after the Royal Charter Gale of October 1859, a storm that sank the steam clipper Royal Charter off the coast of Wales and killed more than 450 people, among the deadliest storms in British maritime history. In its aftermath, FitzRoy developed a national storm warning service, which began operating in February 1861 and issued its first recorded gale warning that same month (Met Office, "Robert FitzRoy and the early Met Office"). To reach coastal communities without telegraph access, FitzRoy also worked with the instrument makers Negretti and Zambra to design a simplified "fishery barometer," complete with plain-language rhymes printed on the dial to help sailors and fishing crews interpret pressure changes without technical training.

On August 1, 1861, FitzRoy published what is widely regarded as the first public weather forecast to appear in a British newspaper, printed in The Times (Met Office archive; JSTOR Daily, "Robert FitzRoy and the Laws of Storms"). His forecasts combined barometric pressure trends, wind direction, and cloud observations, filtered through decades of personal seafaring experience. They were imperfect and occasionally wrong, drawing criticism and even ridicule from parts of the press, but they represented something genuinely new: an attempt to warn ordinary people about weather that had not happened yet, based on instrument readings rather than folklore or superstition.

The Royal Charter Gale that motivated FitzRoy's storm warning service killed more than 450 people in a single event off the Welsh coast in October 1859. It remains a grim but concrete illustration of the stakes behind turning barometric data into an early-warning network rather than leaving it as a scientific curiosity.

How aneroid barometers ditched the mercury

Mercury barometers are accurate but bulky, fragile, and mildly hazardous, mercury being a toxic heavy metal that most modern instrument makers now avoid handling in liquid form. The French scientist Lucien Vidi solved this with the aneroid barometer, patented in 1844, which replaced the liquid column entirely with a sealed, flexible metal capsule that has had some of its internal air removed (the word "aneroid" literally means "without liquid").

As atmospheric pressure rises, it compresses the capsule slightly. As pressure falls, the capsule expands slightly, since there is less external push holding it in. A mechanical linkage of small levers and springs inside the case amplifies that tiny, almost imperceptible flex into a visible sweep of a needle across a printed dial. The whole device behaves something like a miniature mechanical lung, expanding and contracting in response to the invisible push of the sky overhead, translating that motion into a number a sailor or farmer could read at a glance.

Aneroid barometers were lighter, more portable, and safer than mercury tubes, and they could be built into compact wall-mounted household instruments or ship's barometers with the same fishery-style rhymes FitzRoy had championed. They also paved the way for the altimeter, which is functionally an aneroid barometer with a dial calibrated in altitude rather than pressure, since both instruments respond to the same underlying physical quantity.

How does a falling barometer predict a storm?

A falling reading signals that a low-pressure system is approaching, one associated with rising, cooling air that condenses into clouds and precipitation. The speed of the drop matters nearly as much as the number itself: a rapid fall of several hectopascals within a few hours often precedes a strong storm, while a slow, shallow decline usually points to a milder shift in conditions.

Comparing the three eras of barometer design

Type	Era introduced	Mechanism	Strengths	Limitations
Mercury (Torricelli)	1643	Liquid mercury column balanced against atmospheric push in a sealed glass tube	Extremely accurate, defined the physical reference standard for "one atmosphere"	Heavy, fragile, uses toxic liquid mercury, impractical to carry
Aneroid (Vidi)	1844	Sealed, partially evacuated flexible metal capsule linked to a mechanical needle	Portable, no liquid, safe for household and maritime use, basis for altimeters	Needs periodic mechanical calibration, less inherently precise than mercury
Digital / electronic	Mid-to-late 1900s onward	Piezoresistive or capacitive pressure sensor converting flex into an electrical signal	Compact, integrates with smartphones and weather stations, logs data automatically	Depends on battery power and sensor calibration, no direct physical reference

Each generation of barometer traded some of the physical directness of Torricelli's original design for portability, safety, and automation, while relying on the same underlying atmospheric-pressure principle.

What people still get wrong about barometers

A common misconception is that a barometer measures weather directly. It does not. It measures atmospheric pressure, a single variable that correlates strongly with certain weather patterns but does not, by itself, guarantee any specific outcome. A perfectly accurate barometer paired with no other data can still produce a poor forecast, because temperature, humidity, wind direction, and moisture content all shape what actually happens in the sky.

Another frequent error is expecting a single reading to mean much on its own. What matters more is the trend over three to six hours. This is why traditional ship's barometers, including the fishery barometers FitzRoy distributed, included a small movable marker that a sailor would reset at each reading, so the next check would immediately show whether pressure had risen, fallen, or held steady, rather than requiring the reader to remember the last number from memory.

A third confusion involves the phrase "Torricelli's vacuum." It sounds abstract, but it refers to a genuinely empty region, not a low-pressure pocket of some special substance. Establishing that ordinary physical vacuums could exist and be created on a laboratory bench, rather than only in the theoretical imagination of philosophers, was itself one of the more disruptive scientific claims of the 17th century, directly contradicting Aristotelian physics that had gone largely unchallenged for two millennia.

Why this instrument still matters

Before instrumented forecasting, predicting weather relied on almanacs, folklore, and observations of animal behavior, none of which offered any reliable way to anticipate conditions more than a few hours ahead or across any real distance. Barometric networks, especially once combined with the telegraph, changed the available warning window from "the storm is visibly here" to "the storm is roughly a day away and heading in your direction," a shift that mattered enormously for agriculture, shipping, fishing communities, and eventually aviation.

The barometer also became foundational to atmospheric science as a discipline. Once meteorologists could collect simultaneous pressure readings across many locations, they could plot isobars, lines connecting points of equal pressure, and begin to map the large-scale structure of high- and low-pressure systems, fronts, and cyclones. Modern forecasting now layers satellite imagery, radar, and enormous computer models on top of that foundation, but the underlying logic, reading pressure gradients to infer how air is moving, still traces back to Torricelli's basic insight that the atmosphere has weight and pushes on everything beneath it.

FitzRoy's own story carries a sobering coda. Despite pioneering public forecasting, he struggled with the scientific establishment's skepticism toward predicting an inherently chaotic system, and he died in 1865, only a few years after publishing his first forecasts. The Met Office he helped found, however, endured and grew, and the storm-warning principle he built around barometric telegraphy remains recognizable in the severe weather alerts issued by national weather services today.

The takeaway

Torricelli set out to solve a plumbing puzzle involving suction pumps and well water, and in the process discovered that the atmosphere has measurable, quantifiable weight, a finding that within roughly two centuries turned into humanity's first organized early-warning system for storms. The instrument's enduring lesson has less to do with mercury or metal capsules than with method: a single, honest physical measurement, tracked consistently over time and shared across a network of observers, can outperform guesswork by giving people hours or days of lead time that, before 1643, simply did not exist.

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Read next:

• How the thermometer changed science
• How the seismograph made earthquakes measurable
• The Millikan oil drop experiment explained

FAQ

Q: Who invented the barometer?

Evangelista Torricelli built the first mercury barometer in 1643, working with his colleague Vincenzo Viviani, while investigating why suction pumps could not lift water above roughly 10 meters (33 feet). His experiment showed that atmospheric pressure, not "nature's abhorrence of a vacuum," was responsible for the limit.

Q: How does a falling barometer predict a storm?

A falling reading indicates approaching low pressure, which is associated with rising, cooling, moisture-laden air that condenses into clouds and precipitation. The faster the drop, the more likely a significant storm is approaching within hours to a day.

Q: What is an aneroid barometer?

It is a barometer that uses a sealed, partially evacuated metal capsule instead of a liquid column. Atmospheric pressure changes flex the capsule, and a mechanical linkage converts that flex into needle movement on a dial. It was patented by Lucien Vidi in 1844.

Q: Why is standard pressure 760 millimeters of mercury?

That is the height a column of mercury reaches at sea level under average atmospheric pressure, a value later adopted as the reference for "one standard atmosphere." Mercury's high density, about 13.6 times that of water, keeps the column short enough to be practical in a benchtop instrument.

Q: Did barometers alone create modern forecasting?

No. Barometric pressure was the first reliable quantitative input, turned into a public storm-warning service by Robert FitzRoy in the 1860s following the deadly Royal Charter Gale of 1859. Modern forecasting adds temperature, humidity, wind, and satellite data processed through computer models, but it is still built on pressure-gradient principles that trace back to Torricelli's original instrument.

Q: What did Blaise Pascal's Puy de Dôme experiment prove?

In 1648, Pascal had his brother-in-law carry a barometer up the Puy de Dôme in France while a second barometer stayed at the base. The mercury column dropped noticeably at higher altitude, confirming that atmospheric pressure decreases with elevation and that the space above the mercury in Torricelli's tube was a genuine vacuum, not some undetected substance.