1. Air Pressure and Weather Tools

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. Weather Tools Thermometer Barometer Hygrometer

8. 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.

9. 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.

10. 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.

11. Barometers- measure air pressure 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. Mercury thermometer How it works: As air pressure increases, mercury is pushed up.

12. 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?

13. Atmospheric Pressure

14. 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 ________________________.

15. 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.