# AIR AND AIR PRESSURE

Air Pressure Experiments

Air pressure is the weight of the atmosphere pressing down on Earth. We measure air pressure
using a barometer. Have you ever climbed a mountain or visited an area at high altitude and
found you get out of breath more easily? This is because air pressure decreases at higher altitudes.
Have you been on a plane and felt discomfort in your ears? This is due to changes in air pressure.
Changes in air pressure allow for lots of cool science experiments.
Try and blow a ball of paper into a squash bottle and watch it fly back out, this will amaze you and hold you in hysterics and is a super simple demonstration of air pressure.
Watch an egg magically drop into a jar after heating the air inside which starts to escape past
the egg, then, as the air cools again the now higher pressure air on the outside forces the egg
into the jar.

Does Air Have Weight?

To begin these air pressure experiments wave your hand back and forth in the air. It’s easy to move
your hand around because air pressure is pressing onto your hands in all directions. Air actually
weighs 14.7 pounds per square inch at sea level.
That means that every square inch of your body is being pressed on by 14.7 pounds of pressure.
Materials Needed
Balloons
String
Scotch tape
Ruler(stick or clothes hanger may also be used
Needle or sharp pin
Directions
1. Cut three strings approximately 12 inches long.
2. Blow up two balloons so they are the same size. Tie a string to each balloon.
3. Tie one of the balloons to each end of the ruler tight enough so the string will not slip.
4. Tie a string loosely around the center part of the ruler so that you can slip the knot back
and forth until the balloons are balanced.
5. Tape the string in place so it will not move when the balloon is deflated. Prick one of the
balloons with a needle or sharp pin.
6. Watch how the ruler moves upward on the side where the balloon was deflated. If this
does not happen it might be because the center string was not tight enough and moved when the balloon was deflated.
7. Try the experiment several more times to see if the experiment works consistently. This is
the way real scientists do their work. They test their hypothesis several times to make
sure the same thing happens consistently.

Extending the experiment
Try these air pressure experiments. Balance two deflated balloons on a ruler or stick. Take one
balloon off the stick and inflate it. Return the balloon to see what happens. Try balancing several
balloons on a yard stick. When you have the yardstick in balance. Predict what you think will
happen if you deflate all the balloons, one at a time, from left to right. After writing down your
prediction, try this air pressure experiment.

Science behind the experiment
Air is a real substance and it has weight. That is why it weighs 14.7 pounds per square inch at sea
level. What scientists mean when they give this figure is that if a column of air one square inch in
size from sea level to the top of the atmosphere above Earth would weigh 14.7 pounds.
If you travel up over a mountain pass air pressure decreases as you move upward. At 18,000 feet
above the Earth the air pressure is approximately 7.35 pounds per square inch or half the
atmosphere at sea level.

Air Pressure Experiment
Emma Vanstoneon

What you need:
An empty squash bottle.,
A small rolled up ball of paper, small enough to sit inside the mouth of the bottle.
Instructions
Place the bottle on the edge of a table and put the ball of paper inside.
Try to blow the paper into the bottle.
It should shoot back out towards you.
Why does it do that?
The same principles that keep aeroplanes in the sky also apply to this neat little experiment. The
key point is that moving air is at a lower pressure than still air. This is the Bernoulli Principle. In the
case of the squash bottle the air that is blown towards the mouth is deflected around the the
sides of the bottle (very little moves past the piece of paper). This means that the air pressure in front of the ball of paper is lower than behind, and so the paper flies out.

Aeroplane wings are specially shaped so that air travels faster over the top of the wing than over the
bottom surface. Again the pressure is lower above than below and the wing is “pushed” upward by
the higher pressure air – called lift. The faster the plane moves forward the bigger the lift it
experiences.
What are the factors other than temperature that
affect atmospheric pressure?

Chris Lissner
Lake St. Louis, Missouri, USA

The main factor that affects the air pressure at a given location is the altitude (or height above sea
level) of that location. The maximum air density is at the earth’s surface; air density decreases with
height away from the surface of the earth (because the pull of the earth’s gravity is less). The fewer
number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air
pressure. The effect of altitude on atmospheric pressure is so large that meteorologists usually
adjust local air pressure measurements to what the air pressure would be if the station were actually
located at sea level, so that they can determine changes in pressure which are due to weather
patterns. Aside from altitude, temperature, and water vapor concentration are the two main factors that influence atmospheric pressure. When atmospheric air is heated (such as by radiation from the sun), the air molecules become more active, which increases the space between neighboring
molecules and reduces the air density. Lowering the air density decreases the amount of pressure
exerted by the air. Given equal volumes of air, warm air is less dense than cold air and exerts
less pressure.
Water vapor concentration affects atmospheric pressure because the molecular weight of water (18 g/mol) is less than the average molecular weight of air (about 29 g/mol). When water
evaporates and enters the atmosphere as a gas, the water vapor molecules take the place of other
gas molecules in the air. So, a volume of wet (or humid) air weighs less than an equal volume of
dry air. Therefore humid air is less dense and exerts less pressure than dry air.

Finally, atmospheric pressure can be influenced by wind patterns, which cause convergence (moving together) and divergence (moving apart) of air at the earth’s surface.

This combination of factors makes air pressure an important parameter in predicting weather. In
general weather becomes stormy when air pressures fall (generally due to warmer, humid air
and/or convergence of air masses at the surface of the earth which cause convection and rising air)
and becomes fair when air pressures rise (generally due to drier, colder air and/or
divergence of air masses). So if the television weatherperson states that the barometer (an instrument used to measure air pressure) is falling you might want to get your raincoat out!