• Wind Cave National Park - Two Worlds

    Wind Cave

    National Park South Dakota

What Causes the Wind of Wind Cave

Natural Entrance to Wind Cave

Natural Entrance to Wind Cave

NPS Photo

Objectives:
Students will be able to:

  • · define barometric pressure and explain how changes in surface pressure can cause the winds of Wind Cave.
  • · explain how to predict volume of space using a balloon

Materials:
Balloons
Video of the
Wind at the Entrance of Wind Cave - See how the winds at the entrance of the cave can be blowing in or out of the cave depending upon changes in the barometric pressure.

Background Information:
Barometric pressure, also called air or atmospheric pressure, is the pressure exerted by the weight of air over a given area of Earth's surface. While it is uncommon for us to think of gas as weighing anything, as matter it does have mass. That mass is related to how many molecules of air there are in a specific area; how fast they are moving; and how often they collide. Because the air has mass, gravity has an effect on it causing the air above us and around us to weigh down on us and on the surface of the earth. This force is called barometric, atmospheric or air pressure. This pressure is measured by an instrument called a barometer.

At sea level (the level of the ocean's surface) gravity is strongest and air pressure is greatest. Because gravity weakens as you go up, air pressure is lower at higher altitudes. However, the barometric pressure at any given location on the earth is constantly changing. On the surface, weather is driven by the sun, which heats some areas of the earth more than others. Temperature differences lead to pressure differences.Those changes produce winds, bring in clouds, or clear the way for sunny skies. Understanding barometric pressure readings help forecast the weather. A rising barometric pressure often suggests clearing skies and fair weather where falling pressure indicates that wet or stormy weather may be on the way. Areas of very low pressure are associated with severe storms, such as tornadoes.

Because Wind Cave is so large and has a lot of space, it also has an air pressure system. That air pressure system is always working to be equal to the air pressure system on the surface. So if a high pressure system is on the surface air will be forced into the cave to create a high pressure system in the cave. When there is a low pressure system on the surface the high pressure in the cave forces air out so the cave will have a low pressure system also. This is referred to as cave breathing.

Barometric airflow through the natural entrance of Wind Cave not only gave the cave its name, but also provides an opportunity for determining the approximate volume or size of the cave passages. Monitoring and recording the barometric airflow through the cave natural entrances help us understand the volume of air in the cave and that can be used to calculate the total volume of cave passage. By using the amount of air that comes from the cave we can determine the volume of space in the cave. At this time we have determined that we have found less than 10% of the cave. However, that does not mean we can determine the mileage of the cave. We could have many very small passages or one great huge one. But it does give us an idea of how big the cave could be.

Procedures:

  1. Give each student a balloon and ask them what will make it big. They should respond that blowing air into it will make it big.
  2. Have them blow the balloon up, but do not tie the end. Now have them let the balloon go. What happened? What caused the balloon to lose its air? The difference in air pressure from the outside of the balloon to the inside causes the balloon to expel its air. The air in the balloon was trying to equalize the pressure system and because of this it released all of the air the student had forced into it.
  3. Have the students draw air into their lungs to show them what a high pressure system is like. Have them blow the air out to show a low pressure system.

Volume of Space:

Procedures:

  1. To demonstrate volume of space, ask the smallest and the largest (or you) student in the class to come forward with their balloons.
  2. Have the class predict which person will be able to blow the most air into the balloon with just one breath. (The largest person will have more lung capacity and should be able to put more air into the balloon.)
  3. Have the 2 people blow up their balloons.
  4. Whose balloon was bigger? Were the predictions correct? By measuring the amount of air that comes out of the cave during a change in barometric pressure we can determine the amount of space in the cave. However, that does not mean we can determine the mileage of the cave. We could have many very small passages or one great huge one. But it does give us an idea of how big the cave could be.

Winds at the Cave Entrance:
Many caves are big enough to have barometric winds. However the wind at Wind Cave is very noticeable because of its very small natural entrance.

Procedures:

  1. To demonstrate this have the students open their mouths very wide and blow onto their hands.
  2. Then have the students close their mouth to a very small circle and blow onto their hands. They should easily feel the difference.
  3. Tell the students that because of the small entrance to Wind Cave, we have clocked the winds at the walk-in entrance of the cave at over 70 miles per hour!

Did You Know?