The air is made up of

molecules.

Air molecules are

Air molecules are

everywhere.

everywhere.

Like all matter, air molecules have

mass.

kg

kg

Gravity pulls the air

Gravity pulls the air

molecules toward the

molecules toward the

earth, giving them weight.

earth, giving them weight.

The weight of the air

The weight of the air

molecules all around us is

molecules all around us is

called the air pressure.

called the air pressure.

Air pressure

Air pressure

is equal in all

is equal in all

directions.

directions.

Pressure = force per unit

Pressure = force per unit

area

area

High altitudes = lower pressure

High altitudes = lower pressure

Low altitudes = higher

Low altitudes = higher

pressure

pressure

As

As

elevatio

elevatio

n goes

n goes

up

up

Barometri

Barometri

c

c

pressure

pressure

goes

goes

down.

down.

This is an inverse

This is an inverse

relationship.

relationship.

to

to

measure

measure

air

air

pressure.

pressure.

A

A

Barometer

Barometer

is

is

used

used

In 1643, Evangelista Torricelli

In 1643, Evangelista Torricelli

invented the barometer

invented the barometer

Maybe next I’ll

invent spaghetti-

flavored toothpaste!

Torricelli’s

Torricelli’s

barometer used a

barometer used a

glass column

glass column

suspended in a bowl

suspended in a bowl

of mercury. The

of mercury. The

pressure of the air

pressure of the air

molecules pushed

molecules pushed

the mercury up into

the mercury up into

the glass tube.

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

As

atmospheric

atmospheric

pressure

pressure

increases…

increases…

The mercury

The mercury

in the tube

in the tube

rises.

rises.

The Mercury Barometer

The Mercury Barometer

Good:

Good:

Bad:

Bad:

•

Simple to

Simple to

construct

construct

•

Highly accurate

Highly accurate

•

Glass tube is

Glass tube is

fragile

fragile

•

Mercury is

Mercury is

very

very

toxic

toxic

!

!

Is there a better way to measure air

Is there a better way to measure air

pressure?

pressure?

The

The

Aneroid

Aneroid

Barometer!!

Barometer!!

•

No fragile tubes!

No fragile tubes!

•

No toxic

No toxic

chemicals!

chemicals!

•

No batteries!

No batteries!

•

Never needs

Never needs

winding!

winding!

Get yours today!!

An aneroid

An aneroid

barometer uses a cell

barometer uses a cell

which has had most

which has had most

of the air removed.

of the air removed.

As the air pressure

As the air pressure

around the cell

around the cell

increases, it presses

increases, it presses

on the cell, which

on the cell, which

causes the needle

causes the needle

to move.

to move.

Television weather forecasters usually give

Television weather forecasters usually give

barometric pressure in inches of mercury.

barometric pressure in inches of mercury.

However, meteorologists measure atmospheric

However, meteorologists measure atmospheric

pressure in

pressure in

millibars.

millibars.

MILLIBARS

Since pressure changes with altitude…

Since pressure changes with altitude…

…

…

how does changing altitude affect a

how does changing altitude affect a

barometer?

barometer?

Two types of barometric pressure measurements:

Two types of barometric pressure measurements:

Station pressure

Station pressure

is the

is the

actual pressure at the

actual pressure at the

recording location. It is

recording location. It is

affected by the local

affected by the local

altitude.

altitude.

Sea level pressure

Sea level pressure

is

is

referenced to sea

referenced to sea

level, so it has the

level, so it has the

same altitude

same altitude

anywhere in the

anywhere in the

world.

world.

Most aneroid

Most aneroid

barometers have a

barometers have a

needle which can

needle which can

be set to

be set to

remember the

remember the

previous reading.

previous reading.

Why would this

Why would this

be important?

be important?

Changing Pressure

Changing Pressure

A

A

rising barometer

rising barometer

= increasing air

= increasing air

pressure.

pressure.

This usually means:

This usually means:

Changing Pressure

Changing Pressure

A

A

falling barometer

falling barometer

= decreasing air

= decreasing air

pressure.

pressure.

This usually means:

This usually means:

Where’d

the hula

dancer

go?