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Use your Air Pressure Guide Notes with these slides. Barometric Pressure. B ar ometric Pressure. i. What is barometric pressure?. The air is made up of molecules. . Air molecules are all around us. kg. Like all matter, air molecules have mass.

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B ar ometric Pressure

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Use your Air Pressure Guide Notes with these slides.

Barometric Pressure

Barometric Pressure


What is barometric pressure?

The air is made up of molecules.


Air molecules are all around us


Like all matter, air molecules have mass.

Gravity pulls the air molecules toward the earth, giving them weight. The weight of the air molecules all around us is called the air pressure.

Weather changes as air pressure changes.

Three factors change air pressure in the atmosphere. Stay alert to learn what they are.

Now answer #3 and #4. Why does Earth have a blanket of air on the outside? Why doesn’t the moon have an atmosphere?

Who feels the most pressure?

  • Imagine a pile of bodies. The bottom person feels all the weight of the all the bodies stacked above it, while the second one up feels slightly less weight. The higher in the stack, the less pressure one would feel.

Where is the most weight?

  • in a stack of bodies?

  • in a stack of books

  • in a stack of air?

Same answer for each—at the bottom. But air is a fluid, so there is a difference. Guess what it is (#9).

  • Stack of air

  • Stack of bodies or books


(Psst! The arrows are a hint.)

#9 Answer:

Air pressure (fluid pressure) is from all directions, not just downward.

Pressure = force per unit area

Air pressure at sea level is ~ 14.7 lbs/ in2

Barometric pressure goes down.

As elevation goes up

What makes air pressure change?

This is an inverse relationship.

We can graph it!

High altitudes = lower pressure

Low altitudes = higher pressure.... See?

#11, 12, 13 Check:

  • What is altitude?

  • In what units is it measured?

  • What is air pressure?

  • In what units is it measured?

#15: Since pressure changes with altitude…

…how does changing altitude affect a barometer?

#15 Answer

Air inside a ball pushes against the sides.

The more air we put in a ball, the more the molecules push.

So the pressure increases as we add air.

Inflating a ball

What makes air pressure change?

#17—Temperature change

  • As air temperature increases, the molecules move more rapidly.

  • The air expands, becomes less dense, and rises.

  • This rising air is exerting less force on the ground and pressure lowers!

But what about this?

Does pressure lower when the ball gets hotter?

Back to the atmosphere Here comesWater Vapor

  • As water vapor moves into an area, the light molecules of water force out the heavier oxygen, nitrogen, and other gases.

  • This results in a drop in air pressure

  • On a weather map, you may notice these symbols L H

Review: What three factors change air pressure?(You just learned this.)

  • 1

  • 2

  • 3

  • Δ altitude

  • Δ temperature

  • Δ water vapor

#21 What does all this have to do with weather?

  • You can predict weather based on changing air pressure.

  • How? See the next two slides.

Changing Pressure

  • A rising barometer reading

    = increasing air pressure.

  • This usually means clear weather.

Changing Pressure

A falling barometer = decreasing air pressure.

This usually means wet weather.

(Does the barometer really fall?)

I have an umbrella and you don’t.

A Barometer

is used

to measure air pressure.

How can I represent the downward force of air?

In 1643, Evangelista Torricelli invented the barometer

Torricelli’s barometer used a glass column suspended in a bowl of mercury. The pressure of the air molecules pushed the mercury up into the glass tube.

The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.

As atmospheric pressure increases…

The mercury in the tube rises.

That’s why it is called a mercury barometer. (#22)

The Mercury Barometer



  • Simple to construct

  • Highly accurate

  • Glass tube is fragile

  • Mercury is very toxic!

Is there a better way to measure air pressure?


The Aneroid Barometer!!

  • No fragile tubes!

  • No toxic chemicals!

  • No batteries!

  • Never needs winding!

Get yours today!!


An aneroid barometer uses a cell which has had most of the air removed (hence the name).

As the air pressure around the cell increases, it presses on the cell, which causes the needle to move.

Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists measure atmospheric pressure in millibars or bars.

#25 Two types of barometric pressure measurements:

Station pressure is the actual pressure at the recording location. It is affected by the local altitude.

Sea level pressure is referenced to sea level, so it has the same altitude anywhere in the world.

Most aneroid barometers have a needle which can be set to remember the previous reading.

#26 Why would this be important?

Finish your guided notes!

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