Air

pressure

Air

pressure

is

the

weight

of

the

atmosphere

in

a

certain

location.

Low

pressure

is

created

when

warm

air

starts

rising.

When

the

warm

air

has

risen,

it

cools

and

forms

clouds,

which

usually

means

a

storm

is

coming.

High

pressure

occurs

when

cool

air

sinks

and

warms

near

the

surface

of

the

Earth.

High

pressure

usually

means

fine

weather.

Air

pressure

is

measured

with

a

barometer.

In

1644,

Evangelista

Torricelli

invented

the

first

barometer.

He

set

the

end

of

a

long

tube

in

a

bowl

of

mercury.

When

there

was

high

pressure

it

pushed

down

on

the

mercury

in

the

bowl,

forcing

it

up

the

tube.

When

there

was

low

pressure,

the

mercury

in

the

tube

dropped

down.

Later,

a

scale

was

added

to

the

tube

so

the

air

pressure

could

be

measured.