The force of air molecules pushing on a surface
Also called air pressure or barometric pressure.
A mixture of gases containing about 78% nitrogen, 20% oxygen, and 2% other gases including carbon dioxide and water vapor.
Gas molecules are separate from each other and in constant motion.
Words to Know
air pressure atmospheric pressure
density of air
weight of air
Air pressure is a measurement of combined force of all the gas molecules in air hitting a surface.
At sea level, air is denser than is the air on top of a mountain. This means that if you spread a blanket on the beach, there are more air molecules hitting the blanket than there would be if you spread the blanket on a flat surface on a mountain.
The force of the air molecules striking a surface is generally about the same as the weight of air above the surface. Imagine a 4 in x 6 in index card laying on the beach. A column of air rising above an index card to the Earth’s atmosphere would weight about 353 pounds.
If an equal size card is laying on the ground in Denver, Colorado, the weight of the air column would be about 290 pounds. The difference in the weight is due to :
- Denver being about 1 mile above sea level. This means that the column of air is 1 mile shorter than the column of air above the beach.
- The density of the air at higher altitudes is less (fewer air molecules) than at sea level.
Discover for Yourself
With a bit of imagination, kids can better understand how altitude affects atmospheric pressure. Pretend that the floor is at sea level and the ceiling of the room is the end of the atmosphere and above the ceiling is outer space.
1. Use two 4 in x 6 in index cards. Write “Beach” on one card and lay it on the floor.
2. Write Denver, Colorado on the second card and lay it on a chair or other surface above the floor.
3. Ask These Questions:
- 1. Which is greater?
- a. The distance from the Beach (card on the floor) to the outer edge of Earth’s atmosphere (ceiling).
- b. The distance from Denver, Colorado (the card on the chair) to the outer edge of Earth’s atmosphere (ceiling).
- (Answer: a. The distance from the Beach (card on the floor) to the outer edge of Earth’s atmosphere (ceiling).)
- 2. Which region has more air above it?
- a. The beach (card on the floor)
- b. Denver, Colorado (the card on the chair)
- (Answer: a. The beach (the card on the floor).
- 3. Where would the atmospheric pressure be the least?
- a. Beach (the card on the floor).
- b. Denver, Colorado (the card on the chair).
- (Answer: b. Denver, Colorado (the card on the floor.)
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