Pressure and Density of the Atmosphere CK-12

Properties of the Air

Based on the CK-12 lessons: Pressure and Density of the Air and Temperature of the Atmosphere.

Changing Pressure

Have your ears ever popped? If your ears have "popped", you have experienced a change in air pressure. Ears "pop" because the air pressure is different on the inside and the outside of your ears.

Poll

When have your ears "popped"?

• Riding in a car through the mountains

• When I had a cold

• On an airplane

• Never

Density of Air

Density is mass per unit volume. The units would be grams per cubic centimeter. Density is a measure of how closely the molecules are packed together. The closer they are, the greater the density. Since air is a gas, the molecules can pack tightly or spread out.

Multiple Choice

Which box has the highest density?

Density Varies

Density of air varies from place to place. Air density depends on several factors. One is temperature. Like other materials, warm air is less dense than cool air. Since warmer molecules have more energy, they are more active. The molecules bounce off each other and spread apart. Another factor that affects the density of air is altitude.

Multiple Select

Which two properties will affect the density of the air?

• Mass

• Temperature

• Altitude

Multiple Choice

Warm air is ___?___ dense than cold air.

• less

• more

• same

Altitude and Density

Altitude is height above sea level. The density of air decreases with height. There are two reasons: at higher altitudes, there is less air pushing down from above, and gravity is weaker farther from Earth's center. So, at higher altitudes, air molecules can spread out more and air density decreases.

Multiple Choice

Choose the correct statement.

• Air density INCREASES with height

• Air density DECREASES with height

• Air density remains CONSTANT with height

Multiple Select

Choose two correct answers:

Why does the density of air decrease with height?

• The Jet Stream blows the air molecules away

• There is less air pushing down from above

• There is less gravity higher up

Air Pressure

Because air is a gas, its molecules have a lot of energy. Air molecules move a lot and bump into things. For this reason, they exert pressure. Air pressure is defined as the weight of the air pressing against a given area.

Multiple Choice

Air pressure is defined as the __________ of the air pressing down against a given area.

• pressure

• weight

• wind

At sea level, the atmosphere presses down with a force of about 1 kilogram per square centimeter (14.76 pounds per square inch). If you are standing at sea level you have more than a ton of air pressing against you. Why doesn't the pressure crush you?

Multiple Choice

Why doesn't the air pressure crush you?

• The human body is strong enough to hold the pressure.

• Gravity pushes in the opposite direction

• Other air molecules are pushing back in all directions

Altitude and Air Pressure

Like density, the pressure of the air decreases with altitude. There is less air pressing down from above the higher up you go. Look at the bottle in the picture. It was drained by a hiker at the top of the mountain. Then the hiker screwed the cap on the bottle and carried it down to sea level. At the lower altitude, air pressure crushed it.

Multiple Choice

Why was the bottle crushed when the hiker came down from the mountain?

• The hiker squeezed all the air out of the bottle.

• The air INSIDE the bottle became more dense and used less space

• The air OUTSIDE the bottle increased in pressure and pushed the sides in.

Multiple Select

Which two decrease as you go up the mountain?

• Air density

• Air pressure

• Air humidity

Multiple Choice

The number of molecules in a given volume is called _________.

• density

• moisture

• pressure

Multiple Choice

Air density varies with __________.

• Changes in Altitude

• Changes in humidity

• Changes in temperature

• All of the Above

Multiple Choice

The force of air weighing down over a given area is know as air _____________.

• Altitude

• Force

• Density

• Pressure

Summary

• Air density and pressure decrease with increasing altitude

• Gravity pulls more air molecules towards the center of the planet

• Ears pop as air pressures inside and outside the ear equalize