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.
barometer
A barometer measures atmospheric pressure.

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.

accurate
Adjective

exact.

adjacent
Adjective

next to.

adjust
Verb

to change or modify something to fit with something else.

Noun

layer of gases surrounding Earth.

air pressure
Noun

force pressed on an object by air or atmosphere.

align
Verb

to put in a straight line.

Noun

the distance above sea level.

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.

Noun

layers of gases surrounding a planet or other celestial body.

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.

Noun

force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth.

Noun

an invisible line around which an object spins.

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.

Noun

an instrument that measures atmospheric pressure.

barometric pressure
Noun

atmospheric pressure as read by a barometer.

chamber
Noun

sealed compartment.

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

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.

Noun

number of things of one kind in a given area.

digital
Adjective

having to do with numbers (or digits), often in a format used by computers.

display
Verb

to show or reveal.

Noun

height above or below sea level.

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.

Noun

amount of water vapor in the air.

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.

Noun

art and science of determining an object's position, course, and distance traveled.

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.

Noun

liquid precipitation.

rapid
Adjective

very fast.

rotate
Verb

to turn around a center point or axis.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

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.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

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.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

weather station
Noun

area with tools and equipment for measuring changes in the atmosphere.

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.