Lesson 3: A Sea of Air

We don't think about it, but we are actually living at the bottom of a large pool of air. This large pool of air is called the atmosphere. It is made up of mostly colorless, odorless gases. These include nitrogen, oxygen, and carbon dioxide.

The weight of all this air weighing down on us is what's known as air pressure. The more air that is above us, the higher the air pressure will be. At sea level, average air pressure is about fifteen pounds for every square inch. At greater heights, the air pressure is lower, because there's less air above weighing down.

Air pressure can also change with changes in temperature. Just like having hot air in a balloon to make it rise, warm bubbles of air will rise through the atmosphere. Because its rising, the air pressure under the bubble is a little lower than surrounding air pressure. On the other hand, colder air wants to sink. Because it has the tendency to sink, air pressure below the cold air is higher than its surroundings. This is where the terms "low pressure" and "high pressure" come from.

A general rule of weather forecasting is that approaching low pressure will usually bring unsettled weather while approaching high pressure will usually bring fair weather.

Activity One – Pressure's Got a Crush On You

Materials: A thoroughly rinsed one gallon can with cap (plastic or tin can, thoroughly rinsed out before use), and hot water.

Setting Up the Activity: Place about one inch of hot water into the can.. Leave the cap off for three to five minutes. Place the cap on the can and tighten. Watch what happens to the can.

Questions:

Q. What happens to the can?

A. Almost immediately, the can will start to crumple.

Q. Why does this happen?

A. As the air was heated from the water in the can, the expanding air was pushed out of the opening. When the can started cooling and the cap was put on, the air started cooling and contracting, lowering the air pressure inside the can. As the pressure on the inside continued to get lower, the air pressure on the outside stayed the same. Once the inside pressure became less than the outside pressure, the can started to crumple.

Q. The can didn't do this before, why does it crumple now?

A. Because the cap was placed back on the opening, air could not move into the can to equalize the pressure.

Activity Two – Make a Barometer

Materials: A one liter plastic bottle, rubber bands, a plastic six-inch ruler, a one-quart bowl, water.

Setting Up the Activity: Attach the ruler to the bottle using rubber bands, so that the six-inch mark of the ruler is even with the base of the bottle. Fill the bottle about 1/2 full of water. Fill the bowl about 1/2 full of water. Place the bottle upside down in the bowl, placing a finger or hand over the opening of the bottle until the bottle is standing upside down in the bowl of water. Record the water level in the bottle. Record changes in the level of water in the bottle for three days.

Questions:

Q. How did the water levels change in the bottle? What caused the changes?

A. Higher air pressure would make the water level in the bottle higher, while lower air pressure would make the water level in the bottle lower.

Taking It a Step Further: Watch your local TV weather forecast for the same period your measuring changes in your barometer.

Q. How does the movement of high and low-pressure areas on a weather map correspond to changes in your barometer? Can a barometer be used to forecast weather?

A. Lower pressure may mean stormy weather is approaching, while higher pressure should mean fair weather is coming in.