1. Air Under Pressure

2. Learning Objectives After this lesson, students should be able to:
Describe the effect of the sun on air masses in the Earth's atmosphere.
Compare and contrast high- and low-pressure air systems.
Describe how high- and low-pressure air systems are related to weather events.
Explain that engineers design instrumentation, such as a barometer, to help analyze the Earth's weather systems

3. True or False Thumbs Cold air rises and hot air sinks.
(Answer: False; hot air rises and cold air sinks.)

4. True or False Thumbs
Wind and storms are caused by the movement of air masses.
(Answer: True)

5. True or False Thumbs
The sun helps create the different air masses in our atmosphere by heating the air to different temperatures.
(Answer: True)

6. True or False Thumbs
Areas of low air pressure are often characterized by light winds and clear skies.
(Answer: False; areas of high air pressure tell us that clear skies are ahead.)

7. True or False Thumbs
Engineers design the weather instruments that are used in forecasting the weather on TV or radio.
(Answer: True; engineers design instruments to help all meteorologists make weather forecasts.)

8. True or False Thumbs
It is not possible to measure the Earth's weather from space.
(Answer: False; in additional to designing instruments to help make weather forecasts, engineers also design instruments to help us predict the weather from space as well as on Earth.)

9. Hot Air Balloons
Hot Air Balloon

10. Think about it
How do hot air balloons work? 

11. Think about it
Have you ever blown up a balloon and let go of it without sealing it with a knot?
What happens? 

12. Think about it
Both of these occurs because there is a pressure difference between the air inside the balloon and the surrounding air. 

13. Even though we cannot always see or feel the air, it actually has a mass (weight)
It also has other physical properties, such as density and pressure.

14. Air Masses
Air masses are large bodies of air in the atmosphere where all the air within that region has a similar temperature, pressure and humidity.
Air masses change and shift all the time, but on any given day, there are many different air masses in our atmosphere, all with their own unique air pressure.
Thinking back to the balloon example, we know that air pressure differences can cause air to move — similar to the balloon zipping around the room.
The movement of those air masses in our atmosphere contributes to the weather we experience every day.

15. The sun and air masses
The sun heats up our atmosphere and the Earth's surface.
The heating takes place unevenly because the sun's rays hit different areas of the Earth at different angles.
Variations on the Earth's surface provide even more room for uneven heating.

16. How does the sun relate to air masses?
The air above water is cooler than the air above land.
The air above lighter colored surfaces is cooler than the air above darker ones (dark color absorbs more heat).
The sun heats air to different temperatures and helps create the different air masses in our atmosphere.
High- and low-pressure areas are affected by the interaction of air and temperature in our atmosphere.

17. High Pressure
Areas of high air pressure often have light winds and clear skies.
With no clouds to reflect the sunlight, high- pressure regions also usually experience higher daytime temperatures.
On the other hand, rising air pressure is usually an indication that the weather is clearing up.

18. Low Pressure
Low-pressure areas are regions where the atmospheric pressure is lower than that of the surrounding area.
Low-pressure areas are associated with winds and cloudy, overcast skies.
Due to the clouds, the sun's rays have a harder time penetrating low-pressure regions, usually leading to lower temperatures.
Falling air pressure usually indicates that a storm of some sort is approaching.

19. How do air masses move?
Air masses move because the changes in air temperature also change air pressure and density.
As the temperature of air increases, the air expands.
The same amount of air occupies more space when it heats up than when it is cold.
Density of air is equal to the mass of the air divided by its volume.
When air is heated, its volume increases, but its density decreases.

20. How do air masses move?
Things with lower density float on top of things with higher density.
For example, ice cubes float in water because water is denser than the ice cubes.
The same thing happens in our atmosphere: warm air rises because it "floats" on top of colder air.
The rule of thumb is that hot air rises and cold air sinks.

22. Types of air masses

23. Types of Air Masses Continental tropical Maritime tropical
Continental polar
Maritime polar

24. Types of Air Masses Maritime Continental
Air masses that form over oceans
Humid
Continental
Air masses that form over land
Dry
Tropical air masses have low air pressure
Polar masses have high air pressure

25. Weather
Let’s observe and explore weather data!

26. Summary Remember that hot air rises and cold air sinks.
The sun heats up our atmosphere and the Earth's surface
The heating takes place unevenly because the sun's rays hit the Earth's surface at different angles.
The different angles are due to variations on the Earth's surface.

27. Summary
The moving air that circulates in our atmosphere is called an air mass.
Air masses can involve high or low air pressure.
Areas of high air pressure have light winds and clear skies.
Low-pressure areas have winds and cloudy, rainy skies.
Falling air pressure usually indicates that a storm is approaching
Rising air pressure is usually an indication that the weather is going to be fair.

28. Summary
Engineers design barometers to measure air pressure, which meteorologists use to help analyze the Earth's weather systems.