What is a Barometer? - Definition, History & Function

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Can you think of a measuring device that is just as reliable as the forecast provided by your local meteorologist? Use this lesson to learn about this device, called the barometer. Find out the history behind its discovery and how it functions.

What Is a Barometer?

Before there were apps that could provide weather prediction at the click of a finger, there was the barometer. Long ago in the mid-17th century, an Italian scientist by the name of Evangelista Torricelli, decided to create a device to measure atmospheric pressure. This device is called the barometer.

A Barometer

The function of a barometer is to let you know whether or not atmospheric pressure is rising or falling. They are essential tools for weather forecasters and scientists when it comes to understanding the nature of storms or predicting changes in weather.

History and Mechanics of the Barometer

Dating back to the BC era, ancient philosophers such as Aristotle and Parmenides created the nature abhors a vacuum postulate to prove there was absolutely no possible way a vacuum could exist in nature. For almost 2000 years, scientists agreed with this theory.

Fast forward to early 17th century: Italian miners were puzzled by the fact that their water pumps would not raise water after a given height. Galileo and other scientists observed this issue and created a theory that some sort of suction raised the water, but that this vacuum force could only raise the water so high.

This of course was an exciting idea as for many years people believed no such vacuum could exist! Studying under Galileo, Torricelli decided to expand upon what his predecessor discovered. He already knew that the empty space in the tube might contribute to vacuum force. But what role did atmospheric pressure play in this process? This was truly an innovative way of thinking, as most scientists traditionally believed atmospheric air neither had weight nor could exert pressure.

In his experiment he took a tube and filled it with water. He then turned this filled tube upside down and placed it in a pool of water. As the water slowly drained he realized the only force acting on the water was air. This experimental tool was not only creating a vacuum, but a balance was being established between two different forces: (1) the force of the atmosphere air exerting on the water and (2) the force of the water inside the tube. He saw that the water would continue to drain until these two pressures were balanced.

How A Barometer Works According To Torricelli

This first experiment was quite large in design. Let's just say the the glass tube was almost 35 feet high. In an effort to make a more compact experiment, he decided to use mercury as opposed to water. Mercury (13.534g/cm3) was denser than water (1.00 g/cm3), causing it to fall farther than water in the tube. More importantly, the use of a dense element further supported Torricelli's explanation that atmospheric weight was a factor.

Barometer and Its Role in Weather Predictions

Another scientist by the name of Blaise Pascal decided to take Torricelli's mercury tube to a high altitude location and see what happens. He noted that the mercury level dropped when atmosphere pressure decreased according to decreasing altitude. Thus, the height level of mercury would rise or fall in relation to weather changes. This observation is what makes the barometer one useful weather forecaster. By observing changes in the height level of mercury, you can predict changes in the weather.

When tracking storms from a hurricane to a tornado, weather forecasters can use barometers to determine what type of storm is occurring and also predict future weather casts. For example, when the pressure is high this is an indication that the weather is fair. Low barometric pressure readings tell you to be on the lookout for a storm.

In a hurricane you may often hear a meteorologist state certain facts about the storm such as its wind speed, direction, and its atmospheric pressure in millibar. As a hurricane develops its atmospheric pressure falls. When the barometer reading drops this is a good indication that the storm is getting worse. The opposite is true when the barometer reading rises.

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