Why Does The Wind Blow?
Wind is simply moving air. If there wasn’t any wind, weather reports wouldn’t be very interesting, as there would be little change in day-today weather. Wind is produced by pressure differences.
In this activity, students will produce a region of high pressure and then see how this high pressure causes air to flow to a region of low pressure—they will make the wind blow!
Above the earth, an ocean of air surrounds us. The air pressure from the air above us produces large forces on all objects. The force of normal atmospheric pressure in Colorado on one side of a typical office door is about 15 tons! But there is an equal force on both sides, meaning the net force, or total force, is zero. Now, if the pressure is larger on one side than the other, there will be a force. In the atmosphere, the pressure isn’t constant. This is primarily because the sun heats the earth’s surface unevenly. As heat is transferred to the air, we get regions of warm and cool air which can turn into regions of low and high pressure. This difference in pressure makes a force that causes the wind to blow. On a large scale, the rotation of the earth and other factors can make the source of the pressure differences that drive the wind hard to determine, but on small scales the sources are easier to determine. If you live near the ocean, you have noticed that, in the summer, the land will be warmer than the ocean during the day.
Here is a laboratory experiment you can also make from the Little Shop of Physics at Colorado State University.
This activity can be performed as a demonstration, but is
much more effective if students can see, feel, and hear the
experiment while working in small groups.
• 1 clean and dry 1 liter bottle
• Styrofoam packing peanuts
• 1 Fizz Keeper pump cap
• 1 small piece of masking or duct tape
Prepare your experiment by punching a hole in the side of the 1 liter bottle toward the bottom. Cover
the hole with the piece of tape. Fill the bottle with styrofoam packing peanuts and then put the special Fizz Keeper pump cap on the bottle. If you don’t have styrofoam packing peanuts, minimarshmallows can be used. The styrofoam peanuts can compress to half their size and help students see the effect of high pressure in the bottle.
Pressure air over the ocean will cause a wind to blow toward the shore—a welcome sea breeze. At night, the ocean stays warm longer than the land, so we get the reverse—a land breeze. On the front range of Colorado, we see a similar effect. In the morning, the eastward-facing foothills
warm first; the air here warms and rises, and the higher-pressure region on the plains causes the wind to blow toward the foothills. At night, the eastward-facing foothills lose the light first, and so cool down first. The process is reversed, and the wind blows from the mountains. You may have noticed this before; if not, pay attention on your morning and evening commute! It’s not always true that the wind blows west in the mornings and east in the evenings, but it’s true more often than not. Doing the Experiment Hold a brainstorming session with your class to elicit their ideas about the wind and what causes it to blow. Ask them to tell you if they have noticed any trends. What direction is the wind blowing when they walk to school? When they walk home? Follow this with a brief explanation or review of the differential heating of the earth that leads to pressure
differences in the atmosphere, the proceed as follows:
• Tell your students that in this experiment, they will make a high-pressure system that will then flow to
an area of low pressure, causing wind to blow. This experiment will also help them see, feel, and hear
the effects of air pressure.
• Show students the supplies they have for the experiment and ask them to identify the two main ingredients
of the styrofoam peanuts, a plastic foam concoction of plastic and air. (If you are using
marshmallows, the main ingredients are sugar and air.)
• Explain that the Fizz Keeper is a special cap that can put more air molecules into the bottle. Ask
them not to pump it yet. Have them squeeze the bottle and note how it feels. Then listen as they
shake the bottle, and note what they hear. Ask them what they think will happen if they pump a lot
of air molecules into the bottle.
• Have one student hold his/her thumb over the taped hole, while another student pumps the cap as
much as he/she can. Squeeze the bottle. How does it feel? Has the temperature changed at all?
What’s happening to the styrofoam peanuts? Now carefully shake the bottle, keeping the hole covered.
Does it sound any different then before?
• Have students predict what will happen when they take the tape off the hole.
• Before removing the tape, tip the bottle horizontally and shake the peanuts evenly over the surface.
• Remove the tape and have them discuss and explain what they observed. (When you add more air
molecules to the bottle, the air pressure increases, compressing the air in the styrofoam peanuts. The
bottle feels solid, and the peanuts may sound noisy as you shake the bottle. When you release the
tape over the hole, the high pressure moves horizontally to an area of lower pressure, creating a
wind. The air pressure in the bottle equalizes, and the packing peanuts return to their original size.)
This is a nice demonstration of how wind is created as air moves horizontally from regions of high pressure
to low pressure.
For More Information
the Center for Multi-Scale Modeling of Atmospheric Processes: cmmap.colostate.edu
Little Shop of Physics: littleshop.physics.colostate.edu