Atmospheric pressure and Torricelli’s experiment. Theory and practice with simulations

The historic discovery and units of measurement

The realization that air has weight and exerts pressure was not always obvious in the history of science. For centuries, the Aristotelian idea that nature abhors a vacuum prevailed, until a series of experiments in the 17th century demonstrated the true physical nature of the atmosphere and made it possible to establish methods for quantifying it.

Torricelli’s mercury experiment

In 1643, the Italian physicist Evangelista Torricelli conducted a historic experiment that changed physics forever. Torricelli filled a one-meter-long glass tube with mercury, sealed one end, and placed it upside down in a container filled with the same liquid metal. Instead of emptying completely, the mercury in the tube dropped only to an exact height of 760 millimeters at sea level, leaving an empty space at the top. Torricelli correctly understood that the mercury did not fall any further because the weight of the outside air pushed against the surface of the liquid in the container, balancing the weight of the column of mercury inside the tube. This milestone gave rise to the first barometer in history and to the unit of measurement known as the millimeter of mercury (mmHg).

The pascal and other equivalents in the international system

The unit officially adopted by the International System for measuring pressure is the pascal (Pa), named in honor of the French scientist Blaise Pascal, who expanded on Torricelli’s studies. A pascal is simply defined as a force of one newton applied to an area of one square meter (1 N/m²). Since the pascal is a very small unit, meteorology uses the hectopascal (hPa), which is equivalent to 100 pascals. As for the bar, this unit is exactly equal to 100,000 pascals, while the millibar (mbar) is one-thousandth of a bar, that is, 100 pascals. Thus, one hectopascal and one millibar are exactly the same, and normal atmospheric pressure at sea level can be expressed as 1 atmosphere, 760 mmHg, 1013.25 millibars, or 1013.25 hectopascals.

How pressure varies with altitude and temperature

Atmospheric pressure is neither a static value nor uniform across the entire planet. As a gaseous fluid, air is highly compressible, which means that its distribution and the force it exerts change drastically in response to one’s position relative to the Earth’s crust and to changes in the surrounding environment’s energy.

The vertical behavior of air density

The variable that most affects atmospheric pressure is altitude. As we ascend (for example, when climbing a mountain or traveling by plane), pressure decreases rapidly. This occurs for two physical reasons: first, because as we gain altitude, the column of air above us becomes progressively smaller and therefore weighs less; second, because the air in the lower layers is highly compressed by the atmosphere’s own weight, whereas at high altitudes the gas molecules are more widely spaced. As a result, air density decreases and pressure drops non-linearly; in fact, at an altitude of about 5,500 meters, the pressure has already been reduced to half its value at sea level.

The effect of temperature on gas pressure

The temperature of the Earth’s surface is the other key factor that affects local air pressure. When the Sun heats an area of the surface, the air in contact with the ground absorbs that heat; its molecules move faster and expand, decreasing its density. As it becomes lighter, this warm air tends to rise toward the upper layers, leaving a relative void below that results in an area of low atmospheric pressure. Conversely, when the air cools, it becomes denser, heavier, and more concentrated, causing it to sink toward the ground and exert greater force on the surface, creating an area of high pressure.

The essential instruments for measuring pressure

To study the atmosphere and predict changes in the weather, science needs to measure air pressure constantly and with extreme precision. The devices used for this task are called barometers, and their technological evolution has gone hand in hand with advances in fluid physics and electronics.

The mechanical operation of the mercury barometer

The mercury barometer is a direct evolution of Torricelli’s original design and has been the standard of precision in laboratories for centuries. Its operation is purely mechanical: it consists of a calibrated glass tube filled with mercury inverted over a basin. When the external atmospheric pressure increases, it pushes the mercury in the basin upward, causing the column inside the tube to expand and rise. If the air pressure decreases, the weight of the column prevails and the mercury level drops. Although their accuracy is legendary, the use of these barometers has been greatly restricted in recent decades due to the toxicity of mercury in the event of breakage.

The aneroid barometer and modern digital sensors

To avoid the dangers and inconvenience of carrying heavy liquids, the aneroid barometer was invented, which uses no fluid at all. This instrument contains a vacuum-sealed metal capsule with highly elastic walls. When the external air pressure changes, the capsule compresses or expands microscopically. A system of mechanical levers and gears amplifies this movement and transmits it to a needle that indicates the value on a graduated dial.

Today, professional weather stations and our own cell phones use digital barometers based on electronic sensors (MEMS microchips), which measure pressure changes by detecting variations in the electrical resistance of a semiconductor material as it deforms under the force of the air.

The relationship between atmospheric pressure and weather phenomena

Differences in atmospheric pressure between different regions of the Earth are the primary driver of climate and weather conditions. The global pressure map is constantly changing due to the Sun’s uneven heating, which creates distinct zones that meteorologists identify to predict the weather for the coming days.

High-pressure systems and weather stability

A high-pressure area, also known as an anticyclone, forms when cold air from the upper layers of the atmosphere—which is denser and heavier—descends slowly and continuously toward the Earth’s surface. As it descends, this air naturally warms due to compression, which prevents water vapor from condensing to form clouds. For this reason, the presence of an anticyclone in a region is synonymous with stable weather, clear skies, no precipitation, and generally calm or very light winds.

Storms and wind formation in the atmosphere

In contrast, a low-pressure area, known as a storm or cyclone, occurs when warm, moist air near the surface expands, becomes lighter, and rises rapidly into the upper layers of the atmosphere. As it rises, this air cools, causing the water vapor it carries to condense, leading to the formation of large cloud masses, rain, and storms. Wind is generated precisely by the interaction between these two systems: nature always seeks physical equilibrium, so air naturally moves from high-pressure areas (anticyclones) toward low-pressure areas (storms). The greater the pressure difference between these two areas, the higher the resulting wind speed.