Lesson 4: Global wind patterns and convection

Activity 1. Follow the air globally

This lesson is composed of hands-on and kinesthetic activities during which students learn how winds move in global patterns and from high to low pressure systems. In this activity, students explore global temperature distribution and how winds move in global patterns.

Online Resource:

• Blank world map: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-038
• Simplified global temperatures and convection cells map: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-039
• Global temperature, atmospheric circulation, and trade winds map: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-040
• Coriolis Effect graphic at: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-041, part of (scroll to step 3): http://www.teachingboxes.org/catalog.jsp?id=DLESE-000-000-007-915

Advanced Preparation:

• Read through the activity
• Make two copies of the blank world map: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-038 for each student
• Either have computer w/internet connection and projector ready, or make overheads of the online resources

Time needed: Teacher preparation - 10 minutes; Class activity - 20 minutes

Procedure:

1. In your own words, start the activity with: "Let’s compare the temperatures at the extremes on Earth (the equator and the poles)." Where are the highest and the lowest temperatures? Can you predict the global wind pattern from what you know about differential heating and pressure? (Yes, let’s try!) "If you were a parcel (“piece”) of air moving from the equator to the poles or the poles to the equator, would you go straight north? Let’s find out."
   
2. "First, let’s review the main relationships between air temperature, density, pressure, and convection. What happens to air when it is heated or cooled?"

   Elicit a response that indicates an understanding of the following concepts:

   • When air is heated, it expands and rises, resulting in lower pressure over a given area.
   • When air is cooled, it contracts and descends/sinks, resulting in higher pressure over a given area.
   • Convection and convection cell circulations are the names given to air systems in which air is warmed, causing it to expand, become less dense, and rise; and in cooler nearby locations, air cools, contracts, and more dense and sinks.
   • Convective cell motions occur at all scales on Earth, wherever there is a gain or loss of heat resulting in a temperature and pressure gradient.


3. Hand out two copies of the blank world map: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-038 to each student. Describe to the students: "Let’s look at the extremes on Earth. Our objective is to examine a simplified, non-rotating Earth and determine where the high- and low-pressure systems exist because of temperature conditions at the Earth’s surface. Think about where the coolest and warmest temperatures are on Earth based on how much sun they receive annually."

   Have the students draw where the coolest and warmest temperatures are on Earth. Then have them label along the edge of the map (using arrows) where they expect warm air to rise and cool air to sink. Have them connect the rising and sinking air regions with horizontal winds creating a “convection cell.” (The students’ maps should look something like the figure below: ).

   
   source:http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-039

4. Explain: "With this approach we have two large convection cells that appear on our simplified Earth, one in the Northern Hemisphere and one in the Southern Hemisphere moving continuously between the poles and the equator.

   Unfortunately, things are not this simple. Because the Earth spins and the air changes temperature as it moves from its source region, this idealized one-cell model is really a three-cell model. Let's examine a map of the three convective cells in each hemisphere". Draw on the board or on an overhead slide the globe and the three Hadley Cells (with arrows) from the global atmospheric circulation map below.

   
   source: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-040

   In your own words explain as you draw: "Note the Hadley Cells on the global map. They are one of the three large convections cells that form in the atmosphere between the equator and 30 degrees north and south (N/S), where warm, moist rising air from the equator cools and sinks, creating descending, dry, high-pressure air masses. In addition, relatively warm air at 60 degrees north and south creates a persistent low resulting in two more convection cells between 90-60 degree N/S and 60-30 degrees N/S." Have your students draw the Hadley Cells on the second blank map you handed out to them earlier.

5. Ask your students: "How can we predict the basic global wind patterns knowing the position of these high and low systems? Do you ever get the doldrums (feel sluggish)? Who has heard of the trade winds? What are they and why they do they have this name?"

   Explain: "Air that moves vertically (for example, the areas where air is rising [low pressure at the equator] and sinking [high pressure at 30° N/S]) results in very little wind at the Earth’s surface. At the equator, this condition is known as the doldrums. It’s an area of generally low pressure where rising warm, humid air condenses and forms clouds. This area is known as the ITCZ (Intertropical Convergence Zone) because two Hadley Cells converge there.

   There’s another region with very little wind (calm conditions) at 30° latitude. This is where sailors used to get stuck for weeks with no wind to power their sails. The region was called the horse latitudes, because sailors would throw their horses overboard or eat them after running out of food and water for them.

   Remember that wind is the horizontal movement of air at the Earth’s surface between regions of high and low pressure." Have students note the general direction of the winds (bottom branch of the Hadley cell) from 30° N/S to the equator and label this: "Trade winds". Also have them draw the westerlies and polar easterlies.

6. Ask the students: "Notice that the westerlies and polar easterlies are curved on the global graphic. Why? Because the Earth is spinning, which is known as the Corliolis Force. It is sometimes called the Coriolis Effect because it is not a true force, but rather an effect caused by the Earth’s constant motion."

Use the Coriolis Effect graphic at: http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-041 (this is part of a larger page: http://www.teachingboxes.org/catalog.jsp?id=DLESE-000-000-007-915).

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Figure by Schlanger ©

Give students this analogy: Assume a plane takes off from some northern location in the U.S. (pick one from your area) and flies several hours due south (pick another location). When the plane arrives, the destination city has moved east due to the Earth’s rotation. To really reach the destination due south, the plane actually has to fly a curved path to where the city will be when the plane lands. This is what happens with the air “turning to the right.”

Explain that the trade winds are winds resulting from air moving horizontally from 30° N/S to the equator. Again, because the Earth is spinning, the winds are deflected to the right of the straight path you would expect them to take in the Northern Hemisphere and to the left in the Southern Hemisphere. This reliable belt of stronger winds helped sailors move quickly between ports of trade, hence the name ‘trade winds’ for this area of the globe’s wind patterns.

7. Explain: "The westerlies develop due to prevailing winds between 30 and 60° N/S. These winds form because air moving from 60° to 30° is deflected to the right of motion in the Northern Hemisphere and to the left of motion in the Southern Hemisphere. The resulting motion is a belt of winds that move out of the west.

Closing the activity: Discuss with your students: "What do the westerlies have to do with the general movement of storms we see in our country? (In general, weather systems move from west to east across the U.S.) Has anyone ever noticed that flights going from the east coast to the west coast take longer than the reverse? (That's because they're flying against the winds. When going from west to east, the westerlies give the plane a boost.)