1

2

The air is made up of molecules.

3

Air molecules are
everywhere.

4

Like all matter, air molecules have mass.

kg

Recall that mass and weight are not the same. Mass refers to an object’s
inertia, how hard the object is to accelerate or decelerate. If you were in space
and got hit in the head with a “weightless” hammer, it would still hurt, because
the hammer still has mass. The hammer is still hard to stop.

5

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.

Your weight is the result of gravity pulling your mass down on the bathroom
scales. Note that weight has units of a force, such as pounds.

6

Air pressure is

equal in all

directions.

Pressure = force per unit area

Because air is a fluid, force applied in one direction is distributed equally in all
directions. Thus the downward pull of gravity on air molecules produces air
pressure in all directions.

7

High altitudes = lower pressure

Low altitudes = higher pressure

Air pressure can be thought of as the column of air rising above us. As we go
up in altitude, we get closer to the top of the column. Thus there are fewer
molecules of air above us to be pulled down by gravity, so the air “weighs”
less. Therefore, pressure always decreases as one goes up.

8

As

elevation

goes up

Barometric

pressure

goes

down.

This is an inverse relationship.

9

to
measure
air
pressure.

A Barometer

is
used

10

In 1643, Evangelista Torricelli

invented the barometer

Maybe next I’ll invent

spaghetti-flavored

toothpaste!

Torricelli didn’t actually build a barometer, but he gave detailed instructions
on how to build one, so he is given credit for the invention. He was actually
trying to prove the existence of a vacuum. Many scientists in his day didn’t
believe that a vacuum could exist, hence the phrase, “nature abhors a
vacuum.”

11

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.

The abbreviation “Hg” is the chemical symbol for mercury. Some kinds of
pressure reading instruments, including some barometers, use the abbreviation
“mmHg,” meaning “millimeters of mercury.” 760 mmHg is considered the
standard “normal” atmospheric pressure at sea level. This unit is called a
“torr,” after Torricelli.

To construct a mercury barometer, fill a tube with a liquid. Invert then tube in
a dish of liquid holding your thumb over the top of the tube until the the tube is
immersed in the bowl of liquid, the atmospheric pressure will keep the liquid
in the tube from emptying such that the weight of the liquid in the tube
equalize with the atmospheric pressures. (Do not do this with mercury
because of its toxicity)

Mercury was used because it is a very heavy liquid, so the tube could be
relatively short. The tube in a mercury barometer still has to be over a meter
long. Students may want to try building a barometer using colored water.
How high would the tube need to be? Merucy is about 11 times more dense
than water. What if they used milk or some other liquid, would the height be
the same?

12

As
atmospheric
pressure
increases…

The mercury in
the tube rises.

13

The Mercury Barometer

Good:

Bad:

•Simple to construct

•Highly accurate

•Glass tube is fragile

•Mercury is very toxic!

Is there a better way to measure air pressure?

Although mercury has been used for hundreds of years, its toxic effects have
only been fully realized in the last few decades. Students should NEVER
handle mercury or broken mercury thermometers or barometers. Mercury
should also never be thrown in the trash or washed down the drain, since it
moves easily up the food chain from fish to humans. A local health
department or environmental professional can assist with disposal of old or
broken mercury instruments. GLOBE instrument specifications call for
organic-fluid (non-mercury) or digital thermometers, except for the analog
min/max thermometer, which is mounted in a shelter and is not handled by the
students.

14

The Aneroid Barometer!!

•No fragile tubes!

•No toxic chemicals!

•No batteries!

•Never needs winding!

Get yours today!!

15

An aneroid barometer
uses a cell which has
had most of the air
removed.

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.

MILLIBARS

The word “aneroid” means “no air,” and refers to the partial vacuum inside the
cell. The aneroid cell is shaped like a bellows, so that it can flex as air
pressure changes. Increasing air pressure compresses the cell, causing the
needle to register a change. Decreasing pressure allows the cell to expand,
causing the needle to move in the opposite direction.
The use of inches of mercury is a hold-over from the days of mercury
barometers. It refers to the actual height of mercury in the glass tube.
Millibars are metric system units, and as such are readily understood by
scientists around the world.

16

Since pressure changes with altitude…

…how does changing altitude affect a barometer?

As we have noted earlier, higher elevations have fewer air molecules pressing
downward, and so atmospheric pressure is lower. This means a barometer will
read lower as it is carried to a higher elevation. Airplanes use a special type of
barometer, called an altimeter, to measure altitude.

17

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.

Station pressure on a mountain top will be lower than station pressure in a
valley. Scientists need a fixed point of reference in order to compare
barometer readings in different locations. That is why barometer readings are
sometimes adjusted for elevation above sea level at the station location.

18

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

Why would this be
important?

Knowing how the air pressure is changing is as important as knowing the
actual barometric reading. The set needle allows students to compare the
current reading to the previous one. If the current reading is less than the
previous one, the barometric pressure is falling. If the current reading is more
than the previous one, the pressure is rising. If it is the same as the previous
reading, the pressure is said to be steady. Weather forecasters often use the
phrases “falling barometer,” “rising barometer,” or “steady barometer” as a
way of referring to the change in atmospheric pressure.

19

Changing Pressure

A rising barometer = increasing air pressure.

This usually means:

Rising barometer readings indicate that a high pressure system is approaching.
Higher atmospheric pressure is usually associated with fair weather and
clearing skies.

20

Changing Pressure

A falling barometer = decreasing air pressure.

This usually means:

Where’d

the hula

dancer

go?

Falling barometer readings usually indicate the approach of an area of low
pressure. Low pressure readings are usually associated with storm systems.
Tornadoes and hurricanes can produce very low barometric readings.