1. AIR PRESSURE

2. Keeping an Atmosphere Atmosphere is kept by the world’s gravity
Low mass (small) worlds = low gravity
=almost no atm.
High mass (large) worlds = high gravity
= thick atm.
Gravity and pressure
Air pressure depends on how much gas there is
i.e. The atmospheric thickness.

3. Gravity and Atmospheric Pressure
The stronger the gravity, the more gas is held by the world and the greater the weight of atm. on a point

4. Earth’s Atmosphere About 10 km thick
Consists mostly of molecular nitrogen (N2) and oxygen (O2)

5. The air is made up of molecules.

6. Gravity pulls the air molecules toward the earth, giving them weight
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.

7. High altitudes = lower 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.
Low altitudes = higher pressure

8. Atmospheric Pressure
Gas pressure depends on both density and temperature.
Adding air molecules increases the pressure in a balloon.
Heating the air also increases the pressure.

9. Air pressure is equal in all directions.
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.
Pressure = force per unit area

10. Barometric pressure goes down.
As elevation goes up
Barometric pressure goes down.
This is an inverse relationship.

11. A Barometer
is used
to measure air pressure.

12. 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.

13. 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.
Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

14. Changing Pressure A falling barometer = decreasing air pressure.
This usually means:
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.
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.