#### Transcript Air Pressure and Weather Tools

```Air Pressure and Weather
Tools
Air Pressure
Air pressure is the force
applied by the weight of all
the air above an area.
Think of a tall column of air
straight above an area.
The weight of that column
pushes down. We don’t
think of the weight because
air pressure is pushing us
from every direction.
Factors that Change Air Pressure
 Volume (how much space an object takes up)
If
volume increases and amount of air matter stays the
same, then air pressure decreases.
Factors that Change Air Pressure
 Temperature:
When air is heated, molecules are
farther apart. When it is cooled, molecules are closer
together. Therefore, cool air has greater air pressure
than warm air.
Factors that Change Air Pressure
 Height above Earth’s surface:
The column of
air above a mountain is shorter than a column of air
above sea level. So lower altitudes (height in
comparison to sea level) have lower air pressure.
Last Factor that Affects Air
Pressure
 Humidity (the amount of water vapor in air)
 Which do you think weighs less, dry air or moist air?
 Moist air weighs less!
 So which do you think exerts more air pressure, dry or moist
air?
 If you said “dry air” then you are right!
 When water vapor is added to air, the air becomes lighter,
and it exerts less pressure than dry air.
Weather Tools
 Thermometer
 Barometer
 Hygrometer
How can we measure humidity?
 A hygrometer is a tool used to measure the relative
humidity in the air.
 Relative humidity measures the amount of water vapor
in the air compared to the total amount the air could
hold at that temperature. A relative humidity of 50%
means that the air contains half the water vapor it could
possibly hold at that particular temperature.
Hygrometer – measures humidity
What do you notice about these hygrometers?
• digital hygrometer tells relative humidity as a percent.
• analog hygrometer’s top measurement - 100.
• Since relative humidity is a comparison between how much water the
air could hold and how much water it does hold, it is always given as a
percent. 100% humidity would
mean that the air was holding all the
vapor it could.
Thermometer – measures temperature
Compare and contrast these thermometers.
• Both measure temperature. Both measure
positive and negative temperatures, and
Celsius (metric) and Fahrenheit (standard)
temperatures.
• One uses a dial, and the other uses a
substance that expands as the temperature
increases.
Barometers- measure air pressure
Mercury thermometer
How it works: As air
pressure increases,
mercury is pushed up.
Aneroid Barometer
How it works: It measuring changes in
the size of an air-tight container. As the
air pressure increases, the container
shrinks. As the air pressure decreases,
the container swells.
Video
 Think about . . .
 How does air pressure affect weather?
 How can I see changes in air pressure?
 How can I build a barometer?
 Is this home-made barometer more like an aneroid or a
mercury barometer?
Atmospheric Pressure
Review
 Complete each statement with “increases” or “decreases.”
 Volume: If volume increases while air mass remains the
same, air pressure ____________________.
 Temperature: When air temperature rises, air pressure
______________________ .
 Altitude: As height above sea level increased, air pressure
_____________________.
 Humidity: As humidity increases, air pressure
________________________.
Thank you for your time and
attention.
 Next lesson, we’ll be learning about wind.
 To better understand and study the information in this
power point, refer to pages 284-291 in your science
book.
```