How Does a Wind Meter Work?

Introduction

Have you ever wondered how wind is made? Wind is caused by a difference in air pressure. Air travels from areas of higher pressure to places where there is less pressure. And just as air flows out of the high-pressure inside an inflated balloon if the opening is not tied, air in the atmosphere will move to a lower pressure area, creating wind. The speed of that wind can be measured using a tool called an anemometer.

An anemometer looks like a weather vane, but instead of measuring which direction the wind is blowing with pointers, it has four cups so that it can more accurately measure wind speed. Each cup is attached to the end of a horizontal arm, each of which is mounted on a central axis, like spokes on a wheel. When wind pushes into the cups, they rotate the axis. The faster the wind, the faster the cups spin the axis. How fast will your homemade anemometer whirl?

Background

Air is made up of tiny molecules. When molecules are heated, they move faster. Consequently, when air is heated, its molecules move faster and become spaced farther apart, which makes the air less dense (meaning that there are fewer molecules in a given volume). This also means that the air has a lower overall pressure. In comparison, cold air is made of more tightly packed molecules, and so it is denser and has relatively higher pressure.

Because air pressures are inclined to balance out, when there is an area of relatively lower air pressure, the surrounding air in higher pressure areas moves in. This movement of air from a higher pressure area to a relatively lower pressure area is what generates wind. When wind pushes the cups on the anemometer, they spin around the central axis. How fast the cups revolve can be measured in revolutions per minute (rpm), or how many times one cup returns to the position where it started in one minute. Consequently, faster wind will result in a higher rpm than will a slower air movement.

Materials

• Five three-ounce paper cups (such as Dixie Cups)
• Paper hole punch or sharpened pencil
• Ruler
• Two straws
• Pin
• Stapler
• Pencil with eraser
• Fan with different speeds (optional)
• Timer (optional)

Observations and Results

Were you able to see wind make the cups on the anemometer spin around? Did the faster winds make the anemometer cups spin faster compared with the slower winds?

Because air is made up of tiny molecules, when wind hits something it is all of these molecules that are hitting the object. This is why when we feel wind we are really feeling these molecules hitting us. A faster wind hits us harder, and also moves the cups on the anemometer more, compared with a slower wind because the faster wind is moving the molecules at a faster speed. Consequently, in the same amount of time, a faster wind hits an object with more molecules than a slower wind does.

If you have been to the beach or near a large body of water, you may have noticed it is often windy. This is because there are different air pressures where a lot of water meets the land. Water takes more time to change its temperature compared with land, so during the day the land heats up faster, and once the sun goes down the land cools faster. Consequently, during the day there is wind blowing from the ocean to the land, or from an area of higher air pressure (colder air) to lower pressure (warmer air); in the evening the wind blows from the cold land to the warmer ocean.