National Weather Service United States Department of Commerce

Learning Lesson: Heavy Air


To show that air has weight, the air is removed from one of two balanced balloons throwing the balance off.

TOTAL TIME 10 minutes.
SUPPLIES Yard/meter stick; two large balloons; string; transparent adhesive or masking tape
You can have someone hold the string (attached to a year/meter stick) or you may want to have a piece of string hanging from the ceiling before class.
SAFETY FOCUS Severe thunderstorm safety


  1. Inflate two balloons so they are the same size.
  2. Tape one balloon to each end of the yard/meter stick.
  3. Tie a string to the center of the stick and adjust it so the stick balances when held by the string. Tape the string in place to prevent it from slipping.
  4. Ask the students, "If one end were heavier, would the heavier end move up or down?"
  5. Carefully deflate the other balloon. Try poking the balloon with a pin in its neck to prevent the balloon from tearing apart as it pops.
  6. Let both balloons hang freely on the yard/meter stick. Ask the students to explain what happens to the balance.


Air is all around us. This air is composed of atoms and molecules. Despite their small size, the quantity of atoms and molecules exert weight on us known as pressure. Since our bodies are designed to live in this environment, we do not notice the pressure.

Since the inflated balloon now weighs more than the deflated one (due to the air inside of the balloon) it will sink creating an imbalance. Now, imagine the weight of air if that balloon were now 15 miles (24 km) tall.

That is actually what is occurring at this moment in your classroom. When we measure air pressure with a barometer, we are measuring the weight of a column of air 15 miles (24 km) high directly over us.

Building a Weather-Ready Nation

The weight of molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kt / 93 km/h) or greater.

The weight of all of the molecules in wind of 58 mph (50 kt / 93 km/h) is the force that can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorm with wind gusts to at least 58 mph (50 kt / 93 km/h) and/or hail size of 1" (2.5 cm) or greater 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.

Fast Facts

Baseballs travel farther in moist air than in dry air. For any given volume of air, moist air (at the same temperature and pressure) has exactly the same number of molecules as dry air.

Dry air is composed of mostly of heavy oxygen (O2) and nitrogen (N2) molecules. However, in moist air, some molecules are the lighter weight water molecules (H2O), rather than heavier O2 or N2 molecules.

Therefore, the air is less dense in moist air and this decrease in density equates to less resistance to the ball's motion through air. So, for two baseballs hit with equal force, the one hit in the moist air would travel farther than the one hit in dry air.