Overview
We typically can not feel atmospheric air pressure. Why? Since air surrounds everything, including our bodies, the air pressure is applied equally on all sides.
For example, if someone holds an 8½x11" sheet of paper by their hand at arm's length, the weight of the air directly above the sheet is over 1,300 pounds. However, that same pressure (14.7 pounds per square inch) is also pressing up on the bottom side of the paper. The equal pressure on all sides cancels out, so all that's left is the weight of the material that comprises the paper.
This activity will show students the effect of air pressure on two sheets of paper in a way that can be seen.
| TOTAL TIME | 2 minutes |
|---|---|
| SUPPLIES | Ruler; a sheet of printer paper; newspaper |
| PRINTED/AV MATERIAL | None |
| TEACHER PREPARATION | None |
| SAFETY FOCUS | Thunderstorm safety |
Procedure
- Lay a ruler on a table with about 3" (8 cm) hanging over the edge.
- Lay a sheet of printer paper on the part of the ruler in direct contact with the table.
- Press the paper against the table until it is flat as possible.
- Press down on part of the ruler hanging over the edge.
- Repeat the above steps, but replace the printer paper with a large sheet of opened newspaper in the second step.
Discussion
The student will discover the newspaper was much harder to lift than the printer paper. As the ruler lifted the printer paper, air rushed in under the rising paper, thereby quickly allowing the air pressure to equalize on all sides. Therefore, the weight of the air above the paper had little effect on the difficulty in lifting the paper.
However, as the ruler lifted the newspaper, the edges of the newspaper remained in contact with the desk. Very little air was allowed to rush in and equalize the pressure on the bottom side of the newspaper. Since there is less air below the paper, the pressure is less as well, and the weight of all the air above the paper now becomes evident.
Building a Weather-Ready Nation
The weight of air molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. The National Weather Service defines a severe thunderstorm as one containing wind speeds of 58 mph (50 kt / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater.
At those wind speeds, the weight of the air molecules creates the force that knocks down phone and power lines, trees, vehicles, and people. When the National Weather Service issues a Severe Thunderstorm Warning, it means a thunderstorm with those hazardous conditions is occurring or about to occur near you.
Discuss severe thunderstorm safety with your family. Know where your safe rooms are. Know what to do in case all family members are not together. Preparing for a disaster ahead of time helps reduce fear and lets everyone know how to respond during a severe thunderstorm.
Take an American Red Cross first aid and CPR course to learn how to treat burns and administer CPR. You need to know how to respond in an emergency, because severe weather can strike almost anywhere in the country.