The Earth’s atmosphere
Definition
Atmosphere
The atmosphere is the thin layer of air surrounding the Earth, thin compared with the size of the Earth.
- The atmosphere is made of air molecules that are constantly moving and spread throughout it.
- Its density is not the same at every altitude: near the surface the molecules are closer together, and higher up they are more spread out, so the atmosphere is less dense.
Key Idea
- The density of the atmosphere decreases as altitude increases; there is still air high up, but fewer molecules in a given volume.
Atmospheric pressure
Definition
Atmospheric pressure
Atmospheric pressure is the pressure exerted on a surface by air molecules colliding with that surface.
- Air molecules move in different directions; when they collide with a surface they exert a force on it.
- The combined effect of a very large number of these collisions produces atmospheric pressure on any surface exposed to the air.
- Individual molecules are extremely small, but there are so many collisions that their total effect is significant.
Why atmospheric pressure changes with height
- Atmospheric pressure decreases as height above the surface increases.
- At a lower height there is a larger amount of air above the surface, with more molecules and so a greater total weight.
- At a greater height there is less air above, with a smaller total weight, so the pressure is lower.
- The lower density at height reinforces this: with molecules more spread out, fewer collide with the surface, so the pressure is lower.
Example
A student measures atmospheric pressure at the bottom and at the top of a mountain. Explain why the atmospheric pressure is lower at the top.
- The top is at a greater altitude, so there is less air above it.
- The air above therefore has a smaller total weight.
- The atmosphere is also less dense there, so fewer air molecules collide with a surface.
- Therefore the atmospheric pressure is lower at the top of the mountain.
Exam technique
- Do not say pressure is lower on a mountain because it is “closer to space” or “there is no air”; air is still present, just less dense with less air above.
- Do not give a weaker gravitational field as the main reason; the required explanation is fewer molecules and a smaller weight of air above the surface.
- Give the full chain: greater height, less air above, smaller weight of air above, lower atmospheric pressure.
- Strengthen it by adding that the atmosphere is less dense higher up, so fewer molecules collide with the surface, and always finish with the effect on pressure.
Self review
- What is the atmosphere?
- What creates atmospheric pressure?
- How does atmospheric density change with altitude?
- How does atmospheric pressure change with height?
- Why is pressure lower at greater height?
