1. AIR PRESSURE

2. Earths Atmosphere
The earth’s atmosphere is not made of pure air. It contains tiny solid and liquid particles of dust, smoke, and other particles.
Gases that make up the earths dry air 78% Nitrogen, 21% Oxygen, 1% Other
The earths air is not dry though. It contains water vapor

3. Water Vapor Water vapor is water in the form of gas.
The amount of water vapor in the air varies greatly depending on the place and time.
Water vapor plays an important role in the earth’s weather. It creates clouds and turns into precipitation as heat is removed.

4. Properties of air
We often don’t think that air is made of anything; however, air consists of atoms and molecules.
These molecules have mass so air must have mass.
Because air has mass it also has other properties like density and pressure.
The more molecules of gas in an area the greater the density and the fewer the molecules of gas in an area the lower the density.

5. Pressure Pressure is the force pushing on an area or surface.
Dense air creates more pressure than less dense air.
Example: think about carrying a back pack. The more stuff you put in the back pack the harder it gets to carry. The weight presses the straps into your shoulders (pressure). When you take off the backpack it feels like the pressure has been removed.

6. Air Pressure
Air pressure is the result of the weight of a column of air pushing down on an area.
The weight of the column of air above your desk right now is about the weight of a school bus. So why isn’t the desk crushed?

7. Answer
The desk is not crushed because the air pushes in all directions up, down, and sideways. So the air pushing down on the air is balanced equally by the air pushing up on the bottom of the desk.

8. Air pressure can be increased by density of the air as discussed earlier or by altitude.
Air pressure decreases as altitude increases.

9. Examples
Think of a stack of ten books. Which book has more weight on it a book at the top or at the bottom? The book at the bottom has the weight of all the other books on top of it.
Likewise a man standing on top of a mountain has less air on top of him than a man standing on the beach. Air at sea level is like the bottom book it has the weight of all the atmosphere on top of it.
At sea level, the air pressure is about 14.7 pounds per square inch. As your altitude increases (for example, if you climb a mountain), the air pressure decreases. At an altitude of 10,000 feet, the air pressure is 10 pound per square inch.

10. Answer on Paper How is atmosphere important for living things?
What are the two most common gases in dry air?
What percentage of gas is nitrogen?
What percentage of dry gas is oxygen?
What percentage of dry gas is not nitrogen or oxygen?
Name two things other than dry gas found in air.
What three properties does air have? What gives air these properties?
Why is air hard to breath on the top of a mountain?
What changes in air pressure would you expect as you walked down a mine shaft? Explain.