AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The atmosphere is a thin layer (relative to the size of the Earth) of air…
Full specification wording
Atmospheric pressure: The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.
Held on by gravity, the atmosphere is a shell of gas roughly a hundred kilometres deep around an Earth whose radius is over six thousand kilometres.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Held on by gravity, the atmosphere is a shell of gas roughly a hundred kilometres deep around an Earth whose radius is over six thousand kilometres. Proportionally, it is thinner than the skin of an apple, so diagrams drawing it as a thick band are misleading. It has no sharp top; it simply thins out with increasing altitude. Air is a gas and can be compressed, so the weight of all the air above squeezes the air at the bottom into a smaller volume. That makes the air near the ground the densest, with its particles closest together, and the density falls steadily as you rise. Half of the whole atmosphere by mass lies below about 5.5 km. This thinning is why aircraft cabins are pressurised and why climbers at altitude breathe harder.
What earns the marks
- Describe the atmosphere as a thin layer relative to the size of the Earth.
- State that the density of the atmosphere decreases as altitude increases.
- Explain that air particles are spaced further apart at higher altitudes.
- Link the greater density of air near the ground to the weight of the air above compressing it.
Common mistakes and how to put them right
Drawing or describing the atmosphere as a thick shell comparable with the radius of the Earth.
Put it right: It is a very thin layer.
Saying the atmosphere has a definite upper surface.
Put it right: It gradually thins out into space.
Confusing lower density with lower temperature as the reason the air is thin at altitude.
Put it right: Density decreases due to less weight of air above.
Claiming the air is less dense high up because gravity does not act there.
Put it right: Gravity acts at all altitudes but the compressive weight of air above is less.
Exam tips
- Use the phrase 'thin compared with the size of the Earth' when describing the atmosphere's extent.
- Remember that density relates to particle spacing; explicitly state that particles are closer together near the surface due to the weight of air above.
Try the lesson
The start of the Atmospheric pressure: The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes how the density of the atmosphere changes as altitude increases?
Listen
Held on by gravity, the atmosphere is a shell of gas roughly a hundred kilometres deep around an Earth whose radius is over six thousand kilometres. Proportionally, it is thinner than the skin of an apple, so diagrams drawing it as a thick band are misleading. It has no sharp top; it simply thins out with increasing altitude. Air is a gas and can be compressed, so the weight of all the air above squeezes the air at the bottom into a smaller volume. That makes the air near the ground the densest, with its particles closest together, and the density falls steadily as you rise. Half of the whole atmosphere by mass lies below about 5.5 km. This thinning is why aircraft cabins are pressurised and why climbers at altitude breathe harder. A strong answer earns marks for these points. Describe the atmosphere as a thin layer relative to the size of the Earth. State that the density of the atmosphere decreases as altitude increases. Explain that air particles are spaced further apart at higher altitudes. Link the greater density of air near the ground to the weight of the air above compressing it. A common slip: Drawing or describing the atmosphere as a thick shell comparable with the radius of the Earth. Instead, it is a very thin layer. Exam tip: Use the phrase 'thin compared with the size of the Earth' when describing the atmosphere's extent.
Key words
Say what each word means before you turn the card.
structure of the atmosphere
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density with altitude
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compression by the air above
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particle spacing
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See every definition
- structure of the atmosphere
- A thin layer of air round the Earth, thin relative to the size of the Earth.
- density with altitude
- The atmosphere gets less dense with increasing altitude, so the density falls steadily as you rise.
- compression by the air above
- Air is a gas and can be compressed, so the weight of all the air above squeezes the air at the bottom into a smaller volume.
- particle spacing
- The air near the ground is the densest, with its particles closest together, and at higher altitudes the particles are spaced further apart.
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