The Big Crush

By Susan M. Frack and Scott Prickett

BACKGROUND:

The surface of the earth is at the bottom of a "sea" of air called the atmosphere. The atmosphere exerts a huge 10-20 tons of pressure on everything on the earth! We don't seem to notice this "crush" because as the air pushes down, our bodies ( and everything else) push back.

The amount of pressure varies from place to place on the surface due to the temperature and moisture content of the air as well as the elevation of the place. Large masses of air with similar air pressures form as a result of these conditions. These masses of similar air pressures are divided into 2 main groups: Highs and Lows. High pressure masses generally form over a uniform land or water surface and have equal horizontal temperatures and humidities. They move in a clockwise rotation from the center out in the Northern Hemisphere and are sometimes called anticyclones. Low pressure masses form along the boundary between air masses having differing temperatures and humidities. In the Norther Hemisphere air flows toward the center of the Low and moves in a counterclockwise rotation. Lows are sometimes referred to as cyclones.

These masses of air can vary greatly in size as they form and move across the Northern Hemisphere. Many changes in weather conditions are due to the passage of differing air masses. The air pressure can be measured with a barometer and be used to predict the coming weather.

OBJECTIVES:

TLW:

Nat. Sci Obj.    
A.B  1. Define air pressure
D,E 2. Explain how air pressure is related to the density of air.
F,G 3. Describe the factors that affect air pressure.
  4. Practice using a barometer.
  5.  Describe the formation of cyclones and anticyclones.

Nebraska Frameworks Obj.:
H-1

MATERIALS:

  • current weather maps of the United States or Nebraska
  • clear plastic cups
  • index cards, 4 x 6 inch
  • 2 liter pop bottle
  • balloons
  • hot plate
  • barometer
  • straight pins
  • soup cans
  • non bending strawI-BAROM-1MOTIVATIONAL SET:

Collect US or Nebraska weather maps from the past several days. You can find these in the newspaper or download from Internet (www.weather.com). Duplicate enough copies for each student or pairs of students to have a set. Call attention to the H and L on the maps and talk about the movement, appearance, and disappearance of these letter. What do they mean? Where did they come from? Where did they go? Etc. Certainly, there will be at least 1 student who will have some knowledge about these H's and L's. If not, have students just brainstorm ideas. Then let students know that they will be studying the H's and L's to learn more about their movements and characteristics so they can be come better informed about the weather. This will help them make better choices about what to wear and what to do with their time outside of school.

TEACHING STRATEGY:

 A.

  1. Complete activity: Press On, Appendix A to demonstrate the forces of air pressure.
  2. Have students summarize: What air pressure is and its effect on objects on the earth, in a written paragraph.

 B. Explore the factors that create air pressure: temperature, humidity, and elevation.

  1. Do student activity: A Crushing Blow--Appendix B
  2. Do demonstration: Expando--Appendix C
  3. Discuss the effects of elevation of air pressure.

C. Show a mercury and an aneroid barometer and discuss how they work. Appendix D. Use the barometer to keep track of the air pressure for 2 weeks.

OR

Have students make their own barometers and keep track of the air pressure. This activity can be found in most earth science books. A good source would be: Weather in the Lab, Thomas Baker, Tab Books, 1993.

D.

  1. Watch a tornado movie to introduce the students to the power and strength of a Cyclone. A video on hurricanes will also work but tornadoes are representative of Nebraska!!
  2. Complete the reading: Highs and Lows, Appendix E.
  3. Complete the activity, The High and Low of It, Appendix F, to help explain the movement of high and low air pressure masses.

EVALUATION:

21 Question Game-- use your own game board or game show format.

APPENDIX A:

*ADAPTED FROM: METEOROLOGY, P. SEAN SMITH AND BRENT A. FORD, NSTA PUBLICATIONS. 1994.

*PRESS ON!

The air has weight but we don't feel it. The weight of the air produces air pressure. Because you have lived your whole life on the earth you are not always aware of the effects of air pressure. After these experiments you will have a better understanding of air pressure.

MATERIALS:

  • clear plastic cup
  • 4 x 6 inch index card
  • straight pin
  • water
  • sink or dishpan

WHAT TO DO:

  1. Fill the cup to the top with water. Suggest what will happen if you turn the cup upside down over the sink.
    Explain why you believe this will happen: Now turn the cup over and tell what actually happened:
  2. Refill the cup with water and cover it with the index card. The index card must extend over all of the edges of the cup so there are now "leaks". Again, predict what will happen when you turn the cup upside down over the sink. Explain why you believe this will happen. Now, hold the card in place and turn the cup upside down. Slowly remove your hand. Tell what happened:
  3. Refill the cup and use a dry card to cover the top. Predict what will happen if you turn the cup and card sideways over the sink. Explain your prediction. Now, turn the cup sideways while holding the card. Slowly remove your hand from the card. Tell what happened:
  4. Refill the cup and place a dry card over the top. You are going to turn the cup and card upside down and use a pin to poke a hole in the bottom of the cup. Predict what you think will happen after you pole the hole in the bottom of the cup. Now, do it! You may need a partner to help you with this one!! Tell what happened?
  5. Refill the cup, holding your finger over the hole in the bottom. Cover with a dry card and turn upside down. Remove your finger. Describe what happens:

CHECKING FOR UNDERSTANDING:

  1. In step 1, what caused the water to fall out of the cup?
  2. In step 2 and 3, what held the index card in place? What prevented the water from falling out of the cup?
  3. Explain why the water and the index card fell in step 4:
  4. Based on your trials and observations of the water and index cards. In which direction is air pressure being exerted? Draw a picture to help explain this idea and give a written explanation below the picture.
  5. Try to explain why you do not usually feel air pressure. Then explain a situation where you would feel air pressure.

APPENDIX B:  *ADAPTED FROM WEATHER IN THE LAB, THOMAS RICHARD BAKER, TAB BOOKS, 1993.

*A CRUSHING BLOW

OBJECTIVE: To demonstrate the effect of different temperatures on air pressure.

 MATERIALS:

  • 2 liter pop bottle with lid
  • hot plate
  • liter beaker

WHAT TO DO:

  1. Heat 1 liter of water until almost boiling. Fill a sink or dishpan with cool water.
  2. Using oven mitts, pour the water into the 2 liter bottle and put on the top for 1 minute.
  3. Unscrew the cap and quickly pour out the water. Quickly replace the top.
  4. Put the bottle into the sink or pan of cool water.
  5. Describe what happens:
  6. Explain what happened.
  7. Describe a way that you "re-inflate" the bottle:
  8. Compare the air pressure inside the bottle to the air pressure outside the bottle. Which is greater? Why?
  9. What effect would different temperatures of water in the pan have on the bottle? Ice water? Warm water? No water?

APPENDIX C: 

EXPANDO

OBJECTIVE: To show that warm air is less dense but takes up more space than cool air.

 MATERIALS:

  • 2 glass bottles (Snapple bottles are great!)
  • balloons that will have a tight fit over the bottles' opening
  • hot plate
  • balance
  • small sauce pan--must fit on the hot plate
  • tape measure
WHAT TO DO:

 1. Have the bottles and balloons at room temperature for 24 hours. Fit 1 balloon over the opening of each bottle. Label 1 bottle A and 1 bottle B.

2. Find the mass of each bottle/balloon;
Mass of A=____________________ 
Mass of B=____________________ 
3. Find the volume of each bottle/balloon: length x width x height (This is not the most accurate volume but it will work to show your point.)
Volume of A=______________ 
Volume of B=______________ 
Bottle and balloon 

4. Put about 1 inch of water in the pan and bottle A. Heat gently on the hot plate. As the water warms and the bottle warms, the balloon should expand. Bottle B should be kept away from the heat. Have students compare the 2 bottles as A heats. Be
careful not to let the water boil.

5. After the balloon has expanded, find its mass and volume again.

Mass of A after heating=
Volume of A after heating=
6. Have the students calculate the densities of Bottle A before and after heating. They should find the density less in the heated bottle. Discuss why. Explain how the molecules of air move apart, etc, therefore leaving room for humidity, etc. in the air. Students should be able to see that the warm air takes up more space and there would be more room in that balloon for water vapor than there would be in the cool (B) balloon.

 

APPENDIX D: 

MERCURY BAROMETER:

A mercury barometer is made of a glass tube closed on one end and filled with mercury. The open end is put into an open container of mercury. The air pressure pushing down on the mercury in the container forces the mercury in the tube to rise. As the air pressure decreases, there is less push on the mercury in the container and the mercury lowers in the tube.

ANEROID BAROMETER:

Another, more common type of barometer is the aneroid barometer. It is made of an airtight metal box that has had most of the air removed. A needle on a spring is attached to the box. A rise in air pressure presses the box and spring and causes the needle to go up. A fall in air pressure relaxes the spring and the needle falls.

APPENDIX E: 

HIGHS AND LOWS

The weather of the United States is dominated by large weather systems called Highs and Lows. Highs are usually very large, fair weather systems. Lows, on the other hand, are smaller and show changeable and stormy conditions. Either system can last for several days or even as long as a week. However, Highs generally persist for longer periods of time than do Lows.

CHARACTERISTICS OF A HIGH:

  • forms when a mass of air stays over a uniform body of land or water for several weeks. It gradually takes on the temperature and humidity of this surface.
  • dry, cold air masses form over Central Canada; warm, humid masses form over the Gulf of Mexico, and cool, humid masses form over the North Pacific or North Atlantic oceans.
  • air flows from the center, near the surface, to the outer edges. This horizontal movement makes for relatively calm conditions with little wind. The earth's rotation causes this air to move in an upward spiral as it nears the outer edges. This spir al movement is clockwise.
  • more common name is an anticyclone.
  • the movement of highs moves colder air from the higher latitudes to the lower latitudes in an easterly direction.
  • the weather conditions are generally clear and calm and favor night time cooling with the formation of dew, frost, or fog possible.

CHARACTERISTICS OF A LOW:

  • a variety of weather that can include cloudy and stormy conditions, warm and cold sections separated by frontal boundaries, and rapidly changing conditions over short periods of time.
  • air flow is toward the center with a counter-clockwise rotation commonly referred to as a cyclone.
  • the spiraling air rises and expands creating clouds and precipitation.
  • the rapid converging and lifting of air masses create strong winds.
  • lows tend to form over the Pacific Ocean, the Gulf of Mexico, on the Great Plains east of the Rocky Mountains, and off the mid-Atlantic coast.
  • winter lows are more intense than summer lows because of the greater contrasts in temperatures.
  • over the Great Plains, Lows often produce severe thunderstorms and/or strong winds.

APPENDIX G: 

20 QUESTIONS

  1. Moving toward the centr of a High, the surface pressure (increases, decreases). When moving toward the center of a Low, the surface pressure (increases, decreases).
  2. The surface wind movement in a High is:
    1. Counterclockwise and spiraling outward
    2. Counterclockwise and spiraling inward
    3. Clockwise and spiraling outward
    4. Clockwise and spiraling inward
  3. The surface wind movement in a Low is:
    1. Counterclockwise and spiraling outward
    2. Counterclockwise and spiraling inward
    3. Clockwise and spiraling outward
    4. Clockwise and spiraling inward
  4. Vertical air movements in a High are (upward, downward).
  5. Vertical air movements in a Low are (upward, downward).
  6. In a High pressure system the air flows:
    1. Downward and outward in a clockwise spiral
    2. Downward and inward in counterclockwise spiral
    3. Upward and outward in a clockwise spiral
    4. Upward and inward in a counterclockwise spiral
  7. In a Low pressure system, the air flows:
    1. Downward and outward in clockwise spiral
    2. Downward and inward in a counter clockwise spiral
    3. Upward and outward in clockwise spiral
    4. Upward and inward in acounter clockwise spiral
  8. Air expands and cools when it is (ascending, descending).
  9. Air is compressed and warms when (ascending, descending).
  10. The rising air in a Low shows (increasing, decreasing) air pressure. The descending air in a High shows (increasing, decreasing) air pressure.
  11. The upward movement of the air in a (High, Low) leads to cloud formation and precipitation.
  12. The upward movement of the air in a (High, Low) leads to clear skies.
  13. Descending air in a High leads to (fair, stormy) weather. The rising air in a Low tens to make the weather (fair, stormy).
  14. In a High, air to the east of the High is (colder, warmer) than the air to the west of the High.
  15. In a Low, air to the east of the Low is (colder, warmer) than the air tot he west of the Low.
  16. A High is approaching your area. You can expect the weather to become__________________.
  17. A Low pressure system is passing through your area. You can expect the winds to (increase, decrease).
  18. The weather has been warm and sunny for 5 days in your area. A (High, Low) pressure system is most likely responsible for this weather.
  19. Another name for a High is (cyclone, anticyclone).
  20. Another name for a Low is (cyclone, anticyclone).