1. How could a can be crushed by air?
Understanding Air Pressure
How could a can be crushed by air?

2. Air is heavy
Air is constantly pushing down on our bodies and everything around us with a force.
Similar to how the weights push down on the weightlifters body.
Air weighs about 14 pounds for every square inch it presses down on.
We saw this when we tried to pull the plastic bag out of the jar.
The force from air pushed down on the bag so it was difficult to lift up.
Air Pressure is the force from air that pushes on objects.
(A Force is a push or pull)
This means hundreds of pounds of force push against your body every second.
We are not crushed because we have air in our body that presses back against the outside air.
We can see this idea when discovering why air can crush a soda can.

3. An Empty soda Can is Not Really Empty
An empty soda can has air inside of it. Air enters from the hole in the top.
All air has pressure and exerts force. When air can flow freely into an object
The air pressure in the object will be the same as that outside of it.
So the air in the pop can has the same pressure or pushes just as hard as the air outside of it.
The soda can keeps it’s shape because the two forces cancel each other out.
When two forces push against each to cancel each other out it is called Balanced Force.
Two football players push
Against each other with balanced
Force and they go nowhere.
Air pushes against the
outside walls of the can.
Air pushes against inside
the walls of the can.

4. What would happen if you could remove the air from the inside of the can
and cover up the hole that lets the air inside of it?
When the air is removed from the inside of the can it creates high and low pressure
High pressure is when the air is dense with matter or mass.
Low pressure is when the air has lower density.
The higher the density of the air, the greater the pressure and force it pushes with.
Areas of high pressure air naturally try to go to areas of low pressure.
So the air outside of the can pushes to get to the inside of it. With the hole being covered on top it has no easy way in.
The can no longer has balanced forces working and 14 pounds per square inch of pressure pushing on the outside
Two football players collide
With an unbalanced force. L H
Air pushes against
The outside of the
Can with 14 pounds
Per square inch of pressure.
There is no air
Inside the can to push
Back against the outside
Air pressure. H
Guess what happens next L

5. L H H Air Pressure Crushes the Can
The can is crushed because the high pressure air pushes to get to the low pressure air on the inside.
Normally the high pressure air would have traveled through the hole in the top, but tried to go
through the sides of the can instead since that was the only way.
Air pressure can be a very powerful force H
High pressure always moves
toward low pressure. H L

6. Watch a video and learn more
Barometer
The Barometer Measures Air Pressure.
Scientists use this tool to predict when storms are coming. This works because air pressure becomes
lower when a storm is moving in.
Air pressure pushes down on the liquid inside the
Barometer and forces it up inside the tube.
The higher the air pressure the more liquid is inside
The tube.
Watch a video and learn more

7. What Naturally Changes Air Pressure
By boiling water in a can and flipping it into ice water we change the air pressure inside of the can.
But what types of things cause air pressure to change in nature?
Click on the pictures below to learn more
Altitude
Temperature
Water Vapor
After viewing the above pictures, click on the picture to the left to learn more.

8. Altitude The higher the altitude the lower the air pressure.
Altitude is the height above sea level . People who live next to the ocean live at sea level.
People that live in the Blue Ridge mountains are thousands of feet above it.
Air pressure depends on the weight of molecules pressing down on an area from above.
The higher the altitude a place is located the less the air pressure because there is less air pushing down above you.
The density of the air and pressure from it is much higher at low altitudes.
The higher the altitude the lower the air pressure.
Marnie has more air pushing down
On her so the air pressure is much
Greater where she lives
Purple = High density air
Light Pink = Low Density air

9. Temperature Air pressure depends on temperature.
When air is heated it spreads out and becomes less dense.
The lower density causes lower air pressure.
When air is cold it becomes more dense.
Denser are creates higher pressure.
Warm air
Less dense
Low pressure
Cold Air
More Dense
High Pressure
The higher the temperature the lower the air pressure.

10. Water Vapor
Air is made up of Nitrogen, Oxygen and other gasses including water vapor.
Water Vapor weighs less than nitrogen or Oxygen.
The more evaporated water in the air, the less that air will weigh creating lower pressure.
The more water vapor in the air the less the air pressure will be.
Rainforests have lower pressure because of all the water vapor in the air.
Deserts have higher pressure do to less water vapor in the air.

11. Pick Higher or Lower Pressure
Read the conditions of where the air below is located.
Based on what you have just learned pick if the air will have higher or lower pressure.
Click the H or L below to make your pick
Air at sea level
Air on a warm sunny day
Air above a lake
Air on a very cold winter day
Air on the top of a mountain
Air at night in the desert
H or L
H or L
H or L
H or L
H or L
H or L

12. Hmmm, lets rethink that.

13. Right!!!! Air at Sea Level has a lot of air above pushing down creating higher pressure

14. Right!!! Air is heated by the sun, expands and rises.
This creates low pressure on the surface.

15. This creates lower pressure.
Right!!!! Air above a lake would have more water vapor in it making the air lighter.
This creates lower pressure.

16. Right!!! Air on a cold winter day would be more dense,
making it heavier and creating higher pressure.

17. Right!!! Air on the top of a mountain is at a high altitude.
This means less air above pushing down. This creates lower pressure.

18. Right!!! The air in the desert would be very dry.
Less water in the air makes it heavier and creates higher pressure.

19. Watch a video and learn more
What causes wind to blow?
Remember the pop can? High pressure air likes to push its way to areas of low pressure.
Remember the factors that cause different air pressure? (Temperature, Altitude, Water Vapor)
Air at different areas on the Earth can have different pressures. When high pressure air moves
Close to low pressure air it pushes towards it to fill up the less dense space.
The pushing of high pressure air is the wind that we feel blowing.
Watch a video and learn more L H
Wind blows across Charlotte
From an area of high pressure
To an area of low pressure.

20. Watch a video and learn more
What is Wind?
Wind is simply air moving across the surface of the earth.
The direction that wind moves in determines its name.
Watch a video and learn more
Wind is air that moves horizontally across the earth
Wind
Updraft
Downdraft
Air that moves to higher altitudes is an updraft.
Air that moves to lower altitudes is a downdraft.
Updrafts and downdrafts are not called wind.

21. Anemometer Weather Vane An Anemometer is used to measure wind speed.
Weather Vanes are used to measure the direction of the wind.
Watch a video and learn more
Anemometer
An Anemometer is used to measure wind speed.

22. Convection cell
Wind is more complicated than just simply blowing from areas of high pressure
To areas of low pressure. Wind blows in a continuous cycle. (A series of repeated events.)
A convection cell is the rising and falling of air that creates the wind.
Different parts off the Earth are heated differently by the sun. Some places are warmer than others.
This unequal of air creates a pattern of air rising and sinking. This cycle is called a convection cell.
The surface is heated by the sun causing it to rise and expand creating updrafts and an area of low pressure.
The air blows horizontally as wind at higher altitudes and cools.
Cold air becomes more dense and forms a area of high pressure.
The dense air falls blowing as a downdraft.
The high pressure air at the surface blows as wind back to the area of low pressure.
The air is warmed and repeats the cycle. 2. 3.
Wind
Updraft
Downdraft 4. H 5.
Wind L 1. 6.

23. Types of Common Convection Cells in Nature.
There are examples of convection cells that occur in nature.
Click on the picture to learn about each convection cycle.
Sea Breeze
Land Breeze
Valley Breeze
Mountain Breeze

24. Sea Breeze
If you have ever felt the wind blowing on you from the ocean you have experienced a sea breeze.
Sea Breezes happen because the land and water warm up unevenly.
A sea breeze is wind from the ocean created because cool water creates high pressure. Sea breezes
Happen during the day when the sun heats up the land.
The land warms faster than water so air warms, rises as a updraft and a low pressure area forms over the land.
The air blows as wind at higher altitudes out toward the sea, cools and becomes more dense.
The dense air falls as a downdraft creating a high pressure area over the water.
The high pressure air blows toward the beach as wind. (The wind blowing toward the beach creates waves)
The air is heated on the land and the cycle is repeated.

25. Land Breeze
A land breeze is wind blowing from the land out to the ocean because cool air on the land creates high pressure. Land breezes happen at night when the land cools quicker than water.
The ocean is warmer than the land so air warms, rises as a updraft and a low pressure area forms over the water.
The air blows as wind at higher altitudes out toward the land, cools and becomes more dense.
The dense air falls as a downdraft creating a high pressure area over the land.
The high pressure air blows toward the water as wind.
The air is warmed over the water and the cycle is repeated.

26. Valley Breeze
A Valley Breeze is when cool air blows from a valley up the side of a mountain.
Valley Breezes happen during the day.
This happens because as sun shines on the slope of a mountain it heats up
Much faster than the valleys below creating areas of low pressure.
The areas of higher pressure in the valleys blow up to the areas of low pressure.
Where it is heated rises, cools and falls back toward the valley repeating the cycle. H H

27. Mountain Breeze
A Mountain Breeze happens when cool air blows down from mountains into the valley.
Mountain breezes happen at night.
This happens because mountains cool down much quicker than the valleys below.
This creates high pressure that blows towards the lower pressure in the valleys below.
The areas of lower pressure rise, cool and continue the cycle . L L

28. Important Questions to Check Your Understanding
Click on NY (not yet) if you need to review that concept
Are you able to describe the differences between the ideas of wind, updraft, downdraft? NY
Do you know the main reason why wind blows? NY
Would you be able to identify each step in a convection cell and describe why it happens? NY
Are you able to tell why Sea, Land, Mountain and Valley Breezes happen? NY

29. Coriolis Effect
Once air has been set in motion it undergoes an apparent deflection from its path, as seen by an observer on the earth. This apparent deflection is called the "Coriolis Effect" and is a result of the earth's rotation.
As air moves from high to low pressure in the northern hemisphere, it
is deflected to the right by the Coriolis effect. In the southern
hemisphere, air moving from high to low pressure is deflected to the
left by the Coriolis effect.
The amount of deflection the air makes is directly related to both the
speed at which the air is moving and its latitude. Therefore, slowly
blowing winds will be deflected only a small amount, while stronger
winds will be deflected more. Likewise, winds blowing closer
to the poles will be deflected more than winds at the same speed
closer to the equator. The Coriolis effect is zero right at the equator.
Check this out!

30. Wind Patterns
Don’t travel North and South because of the Earth rotating on its axis.
4 Types of Global Winds
Doldrums-At the equator, surface winds are calm and weak.
Trade Winds-30 degrees N & S of Equator
Calm winds, few clouds, little rain fall
Warm air rising from Equator cools and sinks
Also known as Horse Latitudes
Prevailing Winds-Strong winds
• Located in the belt from degrees
latitude in both hemispheres.
• Has an impact on the US weather
Polar Easterlies-Cold, but weak winds
• Near the north and south poles
• US weather is influenced by these
• Cooling takes place between the degree
latitude as it approaches the poles

31. Isobaric Maps
A line connecting points of equal pressure is called an isobar. That means, that at every point along a given isobar, the values of pressure are the same.
Wind is a direct consequence of air pressure differences. The greater the pressure contrast over an area, the shorter the distance between isobars on a weather map depicting the area. Wind blows from areas of high to low pressure. The greater the contrast in pressure difference between two areas, the faster the wind will blow, so closer isobars on a weather map predict higher velocity winds.