barometer

Encyclopedic Entry

A barometer measures atmospheric pressure. Photograph by Senior Airman Andy Dunaway, U.S. Air Force

Storm Glass
A storm glass is a type of barometer used centuries ago. A storm glass is a sealed glass container with an open spout, partly filled with colored water. If the water level in the spout rises above the water level in the container, observers expect low pressure and stormy weather.

A barometer is a scientific instrument used to measure atmospheric pressure, also called barometric pressure. The atmosphere is the layers of air wrapped around the Earth. That air has a weight and presses against everything it touches as gravity pulls it to Earth. Barometers measure this pressure.

Atmospheric pressure is an indicator of weather. Changes in the atmosphere, including changes in air pressure, affect the weather. Meteorologists use barometers to predict short-term changes in the weather.

A rapid drop in atmospheric pressure means that a low-pressure system is arriving. Low pressure means that there isn’t enough force, or pressure, to push clouds or storms away. Low-pressure systems are associated with cloudy, rainy, or windy weather. A rapid increase in atmospheric pressure pushes that cloudy and rainy weather out, clearing the skies and bringing in cool, dry air.

A barometer measures atmospheric pressure in units of measurement called atmospheres or bars. An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit).

The number of atmospheres drops as altitude increases because the density of air is lower and exerts less pressure. As altitude decreases, the density of air increases, as does the number of atmospheres. Barometers have to be adjusted for changes in altitude in order to make accurate atmospheric pressure readings.

Types of Barometers

Mercury Barometer

The mercury barometer is the oldest type of barometer, invented by the Italian physicist Evangelista Torricelli in 1643. Torricelli conducted his first barometric experiments using a tube of water. Water is relatively light in weight, so a very tall tube with a large amount of water had to be used in order to compensate for the heavier weight of atmospheric pressure.

Torricelli’s water barometer was more than 10 meters (35 feet) in height, which rose above the roof of his home! This odd device caused suspicion among Torricelli’s neighbors, who thought he was involved in witchcraft. In order to keep his experiments more secretive, Torricelli deduced that he could create a much smaller barometer using mercury, a silvery liquid that weighs 14 times as much as water.

A mercury barometer has a glass tube that is closed at the top and open at the bottom. At the bottom of the tube is a pool of mercury. The mercury sits in a circular, shallow dish surrounding the tube. The mercury in the tube will adjust itself to match the atmospheric pressure above the dish. As the pressure increases, it forces the mercury up the tube. The tube is marked with a series of measurements that track the number of atmospheres or bars. Observers can tell what the air pressure is by looking at where the mercury stops in the barometer.

Aneroid Barometer

In 1844, the French scientist Lucien Vidi invented the aneroid barometer. An aneroid barometer has a sealed metal chamber that expands and contracts, depending on the atmospheric pressure around it. Mechanical tools measure how much the chamber expands or contracts. These measurements are aligned with atmospheres or bars.

The aneroid barometer has a circular display that indicates the present number of atmospheres, much like a clock. One hand moves clockwise or counterclockwise to point to the current number of atmospheres. The terms stormy, rain, change, fair, and dry are often written above the numbers on the dial face to make it easier for people to interpret the weather. Aneroid barometers slowly replaced mercury barometers because they were easier to use, cheaper to buy, and easier to transport since they had no liquid that could spill.

Some aneroid barometers use a mechanical tool to track the changes in atmospheric pressure over a period of time. These aneroid barometers are called barographs. Barographs are barometers connected to needles that make marks on a roll of adjacent graph paper. The barograph records the number of atmospheres on the vertical axis and units of time on the horizontal. A barograph’s tracking tool will rotate, usually once every day, week, or month. The spikes in the graph show when air pressure was high or low, and how long those pressure systems lasted. A severe storm, for instance, would appear as a deep, wide dip on a barograph.

Digital Barometers

Today’s digital barometers measure and display complex atmospheric data more accurately and quickly than ever before. Many digital barometers display both current barometric readings and previous 1-, 3-, 6-, and 12-hour readings in a bar chart format, much like a barograph. They also account for other atmospheric readings such as wind and humidity to make accurate weather forecasts. This data is archived and stored on the barometer and can also be downloaded onto a computer for further analysis. Digital barometers are used by meteorologists and other scientists who want up-to-date atmospheric readings when conducting experiments in the lab or out in the field.

The digital barometer is now an important tool in many of today’s smartphones. This type of digital barometer uses atmospheric pressure data to make accurate elevation readings. These readings help the smartphone’s GPS receiver pinpoint a location more accurately, greatly improving navigation.

Developers and researchers are also using the smartphone’s crowdsourcing capabilities to make more accurate weather forecasts. Apps like PressureNet automatically collect barometric measurements from each of its users, creating a vast network of atmospheric data. This data network makes it easier and faster to map out storms as they develop, especially in areas with few weather stations.

Vocabulary

Term	Part of Speech	Definition	Encyclopedic Entry
accurate	Adjective	exact.
adjacent	Adjective	next to.
adjust	Verb	to change or modify something to fit with something else.
air	Noun	the layer of gases surrounding Earth.	Encyclopedic Entry: air
air pressure	Noun	force pressed on an object by air or atmosphere.
align	Verb	to put in a straight line.
altitude	Noun	the distance above sea level.	Encyclopedic Entry: altitude
analysis	Noun	process of studying a problem or situation, identifying its characteristics and how they are related.
aneroid barometer	Noun	tool that determines atmospheric pressure by measuring how much a metal chamber expands or contracts.
app	Noun	(application) specialized program downloaded onto a mobile device.
archive	Verb	to keep records or documents.
associate	Verb	to connect.
atmosphere	Noun	layers of gases surrounding a planet or other celestial body.	Encyclopedic Entry: atmosphere
atmosphere (atm)	Noun	(atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.
atmospheric pressure	Noun	force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth.	Encyclopedic Entry: atmospheric pressure
axis	Noun	an invisible line around which an object spins.	Encyclopedic Entry: axis
bar	Noun	(b) unit of measurement for pressure; 1 bar is about equal to the atmospheric pressure at sea level.
barograph	Noun	barometer that tracks changes in atmospheric pressure over time.
barometer	Noun	an instrument that measures atmospheric pressure.	Encyclopedic Entry: barometer
barometric pressure	Noun	atmospheric pressure as read by a barometer.
chamber	Noun	sealed compartment.
cloud	Noun	visible mass of tiny water droplets or ice crystals in Earth's atmosphere.	Encyclopedic Entry: cloud
compensate	Verb	to make up for a loss or injury, usually in money, goods, or services.
complex	Adjective	complicated.
conduct	Verb	to transmit, transport, or carry.
contract	Verb	to shrink or get smaller.
crowdsourcing	Noun	technique that enlists the public to assist with a specialized task.
data	Plural Noun	(singular: datum) information collected during a scientific study.
decrease	Verb	to lower.
deduce	Verb	to reach a conclusion based on clues or evidence.
density	Noun	number of things of one kind in a given area.	Encyclopedic Entry: density
digital	Adjective	having to do with numbers (or digits), often in a format used by computers.
display	Verb	to show or reveal.
elevation	Noun	height above or below sea level.	Encyclopedic Entry: elevation
Evangelista Torricelli	Noun	(1608-1647) Italian physicist.
exert	Verb	to force or pressure.
expand	Verb	to grow or get larger.
forecast	Verb	to predict, especially the weather.
GPS receiver	Noun	device that gets radio signals from satellites in orbit above Earth in order to calculate a precise location.
graph paper	Noun	paper marked with small boxes, or intersecting horizontal and vertical lines.
gravity	Noun	physical force by which objects attract, or pull toward, each other.
horizontal	Adjective	left-right direction or parallel to the Earth and the horizon.
humidity	Noun	amount of water vapor in the air.	Encyclopedic Entry: humidity
indicate	Verb	to display or show.
instrument	Noun	tool.
interpret	Verb	to explain or understand the meaning of something.
invent	Verb	to create.
low-pressure system	Noun	weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.
measurement	Noun	process of determining length, width, mass (weight), volume, distance or some other quality or size.
mercury	Noun	chemical element with the symbol Hg.
mercury barometer	Noun	tool that determines atmospheric pressure by measuring how much mercury moves in a glass tube.
metal	Noun	category of elements that are usually solid and shiny at room temperature.
meteorologist	Noun	person who studies patterns and changes in Earth's atmosphere.
navigation	Noun	art and science of determining an object's position, course, and distance traveled.	Encyclopedic Entry: navigation
network	Noun	series of links along which movement or communication can take place.
observer	Noun	someone who watches, or observes.
physicist	Noun	person who studies the relationship between matter, energy, motion, and force.
predict	Verb	to know the outcome of a situation in advance.
pressure	Noun	force pressed on an object by another object or condition, such as gravity.
previous	Adjective	earlier, or the one before.
rain	Noun	liquid precipitation.	Encyclopedic Entry: rain
rapid	Adjective	very fast.
rotate	Verb	to turn around a center point or axis.
sea level	Noun	base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.	Encyclopedic Entry: sea level
smartphone	Noun	mobile telephone with additional features, such as a web browser or music playing device.
storm	Noun	severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.
storm glass	Noun	glass container filled with water or another liquid that responds to changes in atmospheric pressure.
suspicion	Noun	doubt or mistrust.
temperature	Noun	degree of hotness or coldness measured by a thermometer with a numerical scale.	Encyclopedic Entry: temperature
transport	Verb	to move material from one place to another.
vast	Adjective	huge and spread out.
vertical	Noun	up-down direction, or at a right angle to Earth and the horizon.
weather	Noun	state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.	Encyclopedic Entry: weather
weather station	Noun	area with tools and equipment for measuring changes in the atmosphere.
wind	Noun	movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.
witchcraft	Noun	changing of everyday events using supernatural or magical powers.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writer

Andrew Turgeon

Editor

Caryl-Sue, National Geographic Society

Producer

Caryl-Sue, National Geographic Society

User Permissions

For information on user permissions, please read our Terms of Service.

If you have questions about licensing content on this page, please contact natgeocreative@ngs.org for more information and to obtain a license.

If you have questions about how to cite anything on our website in your project or classroom presentation, please visit our FAQ page.

Media

Some media assets (videos, photos, audio recordings and PDFs) can be downloaded and used outside the National Geographic website according to the Terms of Service. If a media asset is downloadable, a download button appears in the lower right hand corner () of the media viewer. If no button appears, you cannot download or save the media.