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# On the Move: What Causes Wind?

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Source:
Author: Janice VanCleave

### Problem

What Causes Wind?

### Materials

• Ruler
• Scissors
• Tissue paper
• Paper hole-punch
• Tape
• New, unsharpened pencil

### Procedure

1. Make a wind detector by following these steps:
• Measure and cut a 1-by-3-inch (1.25- by-7.5-cm) strip of tissue paper.
• Use the hole-punch to make a hole in one end of the paper strip.
• Cut a 2-inch (5-cm) piece of thread.
• Tie one end of the thread through the hole in the paper strip.
• Use a small piece of tape to attach the free end of the thread to the pencil about 2 inches (5 cm) from the pencil's end.
2. Open an outside door about 2 inches (5 cm). Hold the wind detector at the bottom of the opening in the door.
3. Observe the direction of any movement in the hanging paper piece.

### Results

Diagram A shows the paper strip blowing in toward the room. This is the correct result during the winter. Diagram B shows the reverse movement of the paper that would occur during the summer if the room were air-conditioned.

### Why?

Warm air molecules have more energy and move around faster than do less energetic cold-air molecules. The speedy warm-air molecules tend to move away from each other. So warm air, with its molecules spaced farther apart, is lighter than cold air, with its sluggish molecules huddled closer together. This causes warmer, lighter air to rise, and colder, heavier air to sink. This up and down movement of air due to differences in temperature is called convection currents.

Vertical movements of air, or any fluid, are called currents. The movements of air in general horizontal directions are called winds. Wind depends on differences in atmospheric pressure. High pressure is associated with cold air, and low pressure with warm air. As warmer, lighter air in low pressure areas rises, the cooler, heavier surface air moves in to take the place of the rising currents of warm air. Thus, winds move from high pressure areas to low pressure areas.

When the door is opened during the winter, cold air rushes into the bottom of the room and replaces the rising warm air. In the summer, however, the air-conditioned room is colder than the warm air outside. When the door is opened, the cold air from the bottom of the room rushes out. The paper on the wind detector used in this experiment is pushed in the direction in which the wind is blowing.

### Let's Explore

1.
1. Would holding the wind detector at different heights affect the results? Repeat the experiment holding the detector at the top and then at the middle of the opening in the door.
2. You do not have to wait until next winter or summer to see what the results would be in that particular season. To test the results during the opposite season, repeat the original and previous experiments while standing on the outside of the door.

### Show Time!

1.
1. A refrigerator can be used to test the movement of warm and cold air. Open the refrigerator door about 8 inches (20 cm). Hold the wind detector at the bottom of the refrigerator just inside the door opening.
2. Test the flow of air at the top of the open refrigerator door. Close the door and then open it again. Hold the tester inside the door at the top of the refrigerator.
2.

1. Air currents move around the earth as the warm air near the equator rises and flows up toward the poles. The cold polar air sinks and flows down toward the equator. Simulate the movement of air currents due to convection by filling a small jar with warm water mixed with food coloring. Cover the mouth of the jar with aluminum foil and secure it with a rubber band. Stand the small jar inside a quart Oiter) jar filled with cold clear water. Use a pencil point to make two small holes in the foil. Use photographs of the results along with diagrams of the movement of winds around the earth found in earth science texts to prepare a poster showing wind movement due to differences in pressure.
2. A convection cell is a pattern of air circulation caused by the unequal heating of the earth's surface. Use an earth science text to find out more about convection cells. Prepare and display a diagram showing zones of rising and sinking air, updraft and downdraft regions, and the direction of air between the areas of different pressure.

### Check it Out!

1. A cyclone is a rotating wind system with low pressure at the center. Find out more about cyclones. What causes the winds in a cyclone to rotate? How do anticyclones and cyclones differ? In what direction do the winds in a cyclone spin in the Northern Hemisphere? Southern Hemisphere?
2. The major wind systems around the earth are called prevailing winds. Find out more about prevailing winds. What factors determine the direction of prevailing winds? Where are the northeast and southeast trade winds? Why are they called trade winds? Where are the prevailing westerlies? What is the Coriolis effect? A diagram of the earth showing the prevailing winds can be displayed.