Weather Arcade

Whenever you fly in a plane, or stick your hand out of a car window, you are demonstrating "Bernoulli's Principle." Daniel Bernoulli was a Swiss scientist who found that the speed of flow in air or fluids determines the pressure. So, when you have a curved object such as an airplane's wing, the air rushes faster over the top of the wing, which drops the air pressure. Lower pressure allows the wing to "lift" up, causing the wing and the plane to which it is attached, to rise. Here are some activities that show the effects of Bernoulli's discovery.

Paper Wing
   
Take a regular sheet of 8x11 1/2 paper and fold it in half as shown. Then bend the edges over and tape them together, being careful not to crease the rounded edge (you need that for the experiment. Now, slide a pencil through the wing, and blow across the upper edge. Wait-before you blow-what do you think will happen to the paper?

Okay, now: blow. What did happen to the paper? Why? Did it surprise you that you were able to pick the paper up by blowing on it? The next activity may surprise you even more.

Ping Pong Pressure
   
For this experiment you will need: 2 balls such as baseballs or ping pong balls, a length of string or twine approximately 1 1/2 - 2 feet long, and some scotch tape or glue. Attach a ping pong ball to each end of the string, and hang them someplace where they will not affected by wind. Make sure they are level, and about an inch apart. In a moment, we are going to blow in between the balls, but first, what do you think will happen?

Now we're ready to go; blow right in the center, between the balls (right where the arrow is in the drawing). Now try harder. Now blow as hard as you can.

Did you predict that the balls would be blown apart by the wind you created? Were they? More likely, they actually moved closer to each other. How could that happen?

(If you have a hair dryer and a couple of baseballs, you can do the same experiment, and the balls won't blow around as much.)

An Explanation
Air likes to flow from areas of high pressure to lower pressure. The flow is what we call wind. When you create an area of lower pressure (behind your hand, over the wing, or between the ping pong balls) the objects in each experiment want to rush in and fill the space. If you have enough speed ("thrust") you can lift a very large object, such as an airplane.

You might have thought that planes flew because the engines pushed them up into the air, but you would need a really powerful engine to do this-like a rocket. You will notice that a rocket doesn't need wings (except for steering).

For objects with less thrust, you need wings for the air to rush over to create that lift. If you get the chance to sit near the wing the next time you fly, watch the flaps during takeoff; the pilot uses controls in the cockpit to lengthen the wing, increasing the Bernoulli effect, and giving more lift (the fact that the air moves faster over the curved surface makes it speed up while dropping the pressure). When the plane is in the air, it doesn't need as much lift, so the pilot shortens the wing, which is more efficient for cruising.

   

The effect is also important in the weather, especially for mountainous regions, like Mount Washington, in New Hampshire. As the wind hits the mountain, it gets forced upward, which increases its speed (again, lowering the pressure). This is called "orographic" lifting, which means that the lift is caused by the mountains. The shape and location of Mount Washington make it ideal for orographic lifting, and are partly responsible for the unique and incredible winds seen on the summit. In 1934, a world record of 231 mph was recorded!

Orographic Winds