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.
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?
