Learning Lesson: A Pressing Engagement

Fast Facts

We often speak of pressure in terms of "atmospheres". One atmosphere is equal to the weight of the Earth's atmosphere at sea level, about 14.7 pounds per square inch.

If you are at sea level, each square inch of your surface is subjected to a force of 14.7 pounds.

In water, the pressure increases about one atmosphere (14.7 pounds per square inch) for every 33 feet (10 meters) of water depth.

At the deepest part of all the Earth's oceans, the Marianas Trench's (east of the Philippine Islands) depth is about 35,800 feet (7 miles/11 kilometers).

The pressure of nearly seven miles of water overhead is about 1080 atmospheres or 16,000 pounds per square inch.

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.

pressure on the bottom side of the newspaper.

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.