1. Chapter 11 Section 6 How is air pressure measured?
Objective:
Explain how a barometer measures air pressure.

2. Key Terms
barometer: instrument used to measure air pressure

3. Mercury Barometer
Air pressure is measured with an instrument called a barometer.
A mercury barometer is a glass tube filled with mercury.
It is open at one end.
The space at the closed end of the tube forms a vacuum.
The open end of the tube sits in a container of mercury.

4. Mercury Barometer
Air pressure pushes down on the surface of the mercury in the container.
The mercury is pushed up the vacuum.
At sea level air pressure can raise a column of mercy to a height of 760mm.
As the air pressure changes, the level of mercury in the tube rises or falls.

5. Mercury Barometer

6. DEFINE:
What does a barometer do?

7. Aneroid Barometer
Another kind of barometer is called an aneroid barometer.
The word aneroid means “without liquid.”
An aneroid barometer is made of an airtight metal container.
The sides of the container are very thin.
They can bend in or out.

8. Aneroid Barometer
When the air pressure increases, the sides of the metal container bend in.
When the air pressure decreases, the sides of the container bend out.
A pointer is connected to the container.
As the container changes shape, the pointer moves along a scale.
The scale shows air pressure in millimeters of mercury.
Some aneroid barometers, called barographs, keep a continuous record of air pressure.

9. Aneroid Barometer

10. 2. DESCRIBE:
How does an aneroid barometer work?

11. Measuring Air Pressure
Standard air pressure at sea level measures 760mm of mercury.
This is sometimes called one atmosphere.
Air pressure is also measured in millibars (mb).
Standard air pressure is equal to 1,013.20mb.

12. 3. ANALYZE:
How many millimeters of mercury equal 1,013.20mb?

13. Measuring Altitude An altimeter is a device used to measure altitude.
Pilots, scientists, surveyors, and mountain climbers all use altimeters.
At sea level, air pressure will raise a column of mercury 760 mm.
As you go higher, air pressure decreases.
The mercury column drops.

14. Measuring Altitude Mount Everest is about 8,900 m high.
It is the highest point on Earth.
The mercury column here is only about 250 mm high.
Skydivers and parachuters wear devices that contain altimeters.
These let them see how high they are as they descend.
This type of altimeter is actually an aneroid barometer.

15. 4. INFER:
Why do parachuters wear altimeters?

16. 11-6 Classwork Air pressure is measured with a ___.
Two kinds of barometers are mercury barometers and ___ barometers.
The mercury column in a mercury barometer rises or falls with changes in ___.
Air pressure is measured in ___.
CALCULATE: Mercury is 13.6 times more dense than water. If a container of water can hold 20 g of water, how much more massive would an equal volume of mercury be?