Barometric Pressure

Barometric pressure is a measure of atmospheric pressure as recorded by a scientific instrument known as a barometer. Barometric pressure readings have been made since the eighteenth century and remain useful today in forecasting weather. These readings allow meteorologists to identify the areas of high and low pressure that are integral to weather events.

Barometric Pressure Defined

Barometric pressure, also known as atmospheric pressure, measures the amount of pressure the atmosphere exerts on the surface of the Earth at a given point in time. Barometric pressure is the accumulated weight of the air, influenced by the force of gravity, above the point being measured.

When the temperature is cold and the air is dense, the air pressure will be higher than when the temperature is warm, and the air is less dense. This occurs because the denser, heavier air exerts more downward pressure than warmer, lighter air.

An active interest in atmospheric pressure can be traced to Galileo Galilei and Evangelista Torricelli in the seventeenth century. Galileo wondered how long a straw could remain useful when moving fluid up that straw. The question was based on actual observations of pumps that brought water from beneath the ground. The pumps were effective up to about 10 meters (m). Beyond that height, the pumps failed. Torricelli began exploring this phenomenon.

Having observed that water could not reach a height of more than 10 m, Torricelli calculated that if atmospheric pressure were the cause, mercury, with a greater density, would not be able to attain a height of more than 74 centimeters (cm). Through experimentation, he had removed the air from the top of a glass straw by pumping it out; he correctly concluded that the force of the atmosphere had caused the water to rise to a height of 10 m and the mercury to rise to a height of 74 cm.

Torricelli noticed that the height of the column of mercury changed from day to day. He also noticed that if he traveled to areas that ranged in altitude, the column of mercury changed in height depending upon the altitude. He theorized that the atmosphere's weight was changing but could not identify how or why.

Rene Descartes was next to investigate atmospheric pressure. In 1647, he added numbers to a Torricelli tube so that the readings from day to day could be accurately recorded. Blaise Pascal posited that air became thinner as one went higher in the atmosphere and proved this in 1648. In 1666, Robert Boyle was the first to describe the Torricelli tube as a barometer.

By the eighteenth century, mercury barometers were an important part of the equipment of oceangoing vessels, in large part because of the work of Edmund Halley and John Locke in proving a correlation between barometer readings and weather conditions. To this day, barometric pressure readings play a vital role in predicting the weather. They help track the flow of air from high-pressure areas to low-pressure areas.

Barometer Types and Measuring Barometric Pressure

All barometers determine the weight of the atmosphere and the weight of gravity pressing down upon them.

Widely used in the eighteenth century, mercury barometers are seldom used outside a lab today. Mercury barometers rely upon columns of mercury in glass tubes marked with a scale, and the height of the mercury column is noted. As the pressure of the atmosphere becomes greater, the column of mercury rises, and the high pressure is noted. As the atmosphere's pressure decreases, the mercury column falls, and low pressure is noted. By recording readings at the same time and in the same place each day, the readings can be compared to note the trends in pressure. These trends can be used to forecast the weather.

Aneroid barometers were widely used in the mid-nineteenth century but did not include mercury. Instead, they include a bellows that contracts and expands with the pressure of the atmosphere. When the pressure is great, the pin measuring the barometric pressure moves to a higher position and indicates high pressure. When the pressure is less, the pin measuring the barometric pressure moves to a lower position and indicates low pressure. As with the mercury barometer, the trend in readings can be used to forecast the weather.

In use today, electronic barometers rely upon internal sensors and electronic circuits to measure and display atmospheric pressure. These barometers are very accurate and often can display graphs of recent readings. Their digital displays generally include other readings, such as temperature, which are of use. In the twenty-first century, digital Micro-Electro-Mechanical Systems (MEMS) barometers measure pressure through tiny integrated circuits that combine electronic and mechanical elements. MEMS barometers can be found in many forms of technology, including smart devices, weather stations, airplanes, drones, global positioning systems, and medical devices. They use either capacitive or piezoresistive sensors. Precision digital barometers also use MEMS technology but are highly calibrated and, therefore, more accurate. 

Influence of Gravity on Barometric Pressure

Gravity is involved in barometric pressure readings because it is part of the force of the atmosphere. If a person envisions themselves in what Torricelli called an ocean of air, that person is at the bottom of that “ocean.” The pressure is not felt because it comes from all sides and effectively cancels itself out. However, gravity plays a part in the force of the atmosphere.

If a person goes to a location 1,524 m (5,000 ft) above sea level, that individual will find that the atmosphere is always lighter than it is at a location at or below sea level. Because of this, adjustments are made to bring all readings to sea level. In this way, forecasters can note meaningful differences between locations that vary in altitude. It is also useful to track changes in barometric pressure at one location because this allows the forecaster to note the trends for that specific location.

Barometric Pressure and the Weather

Barometric pressure readings are integral to weather forecasting. Because of atmospheric pressure, air flows from high-pressure areas into low-pressure areas, causing a weather event.

The air flowing into low-pressure areas is doing so because of the force of gravity as it pulls upon the denser air. The air flowing from the high-pressure area does so through winds. These winds are subject to the Earth's rotation and the Coriolis force. Named for the nineteenth-century French engineer and mathematician Gaspard-Gustave Coriolis, the Coriolis force, or effect, describes the effect of an inertial force and demonstrates that an object appears to deviate from its path when viewed in a coordinate system.

In truth, the object is not deflected; however, the wind does travel counterclockwise around a low-pressure area in the Northern Hemisphere and clockwise around a high-pressure area in the Northern Hemisphere. (The reverse is true in the Southern Hemisphere.) The weather conditions experienced will be the result of the winds, temperatures, humidity, and other factors in play. By noting the trend in the barometric pressure, forecasters have an idea of what type of weather is developing and approaching and can tailor their observations to those expectations when creating their forecasts.

Principal Terms

altimeter: a scientific instrument that measures the altitude of an object above a fixed level

aneroid barometer: a device that uses an aneroid capsule composed of an alloy of beryllium and copper to measure changes in external air pressure

atmospheric pressure: force exerted on a surface by the weight of air above that surface; measured in force per unit area

barograph: a graph that records atmospheric pressure in time

barometer: a device for measuring atmospheric pressure; some are water-based, some use mercury or an aneroid cell, and some create a line graph of atmospheric pressure

high-pressure area: region in which the atmospheric pressure is greater than that in the areas around it; represented by H on weather maps

low-pressure area: region where the atmospheric pressure is lower than that in surrounding areas; represented by L on weather maps

mercury barometer: glass tube of a minimum of 84 centimeters (33 inches), closed at one end, with a mercury-filled pool at the base; the weight of the mercury creates a vacuum at the top of the tube; mercury adjusts its level to the weight of the mercury in the higher column

meteorology: the study of changes in temperature, air pressure, moisture, and wind direction in the troposphere; the interdisciplinary scientific study of the atmosphere

water-based barometer: also known as a storm glass or Goethe barometer, a device with a glass container and a sealed body half full of water; it also has a spout that fills with more or less water depending upon atmospheric conditions and their forces

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