Atmospheric pressure — AQA GCSE Physics
Test yourself on Atmospheric pressure with AQA GCSE practice questions.
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Atmospheric pressure explained
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.
Read the full explanation
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.
Air molecules colliding with a surface create atmospheric pressure.
Air pressure is the result of countless molecules moving randomly at high speed and bouncing off any surface they meet. Each collision changes the molecule's momentum, so each one exerts a tiny force on the surface; averaged over the enormous number arriving every second, the result is a steady force spread over the area, and that is what pressure means. Because the motion is random the collisions arrive from every direction, so the push acts on a surface whichever way it faces and always at right angles to it. At sea level the pressure is about 100 kPa, so the force on an outstretched palm is about 1-2 kN, the weight of an adult or two, unnoticed only because air on the other side pushes back just as hard.
The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases.
Picture a column of air standing on a surface and reaching to the top of the atmosphere. Every molecule in this column has weight, pressing down on the surface. As the surface is raised to a higher altitude, the column of air above it becomes shorter. Consequently, this shorter column contains fewer air molecules. Because there are fewer molecules above the surface, the total weight of the air pressing down decreases. For example, at the summit of Mount Snowdon, a significant portion of the atmosphere is below you, leaving fewer molecules above to exert a downward force. On the ground, you stand at the bottom of the entire column, where the number of molecules above you—and therefore the weight of the air—is at its greatest.
So as height increases there is always less air above a surface than there is at a lower height.
The key word here is 'always'. Take any two heights: everything above the upper one is also above the lower one, and the lower surface carries the extra slab of air between them. For example, a hiker at sea level has the entire atmosphere pressing down, whereas a climber on Mount Everest has far less air above them. Thus, the air above a lower surface is always greater than the air above a higher one, regardless of weather. The average density of the air above also falls as the surface rises, because the densest air near the ground is left below. This explains why the link between height and atmospheric pressure has no exceptions. Remember that this entire topic on atmospheric pressure is assessed exclusively at Higher Tier.
So atmospheric pressure decreases with an increase in height.
Fewer molecules above a surface means a smaller weight of air pressing down, and fewer molecules around it means fewer collisions each second. Therefore, atmospheric pressure falls as height rises. For example, pressure at sea level is about 100 kPa, but at an airliner's cruising altitude, it drops to near 20 kPa. This is why cabins are pressurised to around 70 kPa. Each window experiences a resultant outward force calculated by multiplying the pressure difference by its area (e.g., 4 kN for an 810 cm² window). The decrease is non-linear: pressure roughly halves for every 5.5 km climbed, resulting in a curved graph. Altimeters utilise this relationship by measuring atmospheric pressure and converting it into an altitude reading.
Students should be able to: describe a simple model of the Earth’s atmosphere and of atmospheric pressure explain why atmospheric pressure varies with height above a surface.
A complete description of the atmospheric model has four parts: air consists of particles in constant random motion; gravity holds them in a thin shell around the Earth; this shell is densest at the bottom due to compression by the air above; and pressure arises from particles colliding with surfaces. The variation in pressure follows directly from this model. For example, as a balloon rises, the column of air above it becomes shorter and less dense, reducing the weight of air pressing down. Simultaneously, the local air density decreases, meaning fewer particles strike each square metre per second. Both mechanisms explain the pressure drop. Examiners look for these logical links, so always write the full chain of reasoning.
Your focus
- Describe the Earth's atmosphere as a thin layer relative to the size of the Earth.
- Explain why air density is greatest near the ground in terms of the weight of air above.
- Compare the spacing of air particles at ground level and at high altitude.
Show all 18 objectives
- Calculate the force atmospheric pressure exerts on a stated area, such as a palm or a desk top.
- Describe why atmospheric pressure acts on a surface in any orientation.
- Relate a change in the frequency of collisions to a change in pressure.
- Describe the column of air whose weight acts on a surface at a given height.
- Explain why the number of air molecules above a surface decreases as altitude increases.
- Relate the decrease in the number of air molecules to the reduced weight of air above a surface.
- Explain why the pressure difference across an aircraft window grows as the aircraft climbs.
- Explain why the average density of the air above a surface falls as the surface is raised.
- Interpret a graph of atmospheric pressure against altitude to extract specific values.
- Explain the fall in atmospheric pressure with height using molecule number and collisions.
- Read an atmospheric pressure from an altitude graph and convert it into pascals.
- Calculate the resultant force on an aircraft window using pressure difference and area.
- Describe a particle model of the atmosphere including gravity and density variation.
- Explain the fall of atmospheric pressure with height as a linked sequence of steps.
- Evaluate an incomplete explanation of pressure variation and identify the missing logical links.
Atmospheric pressure exam tips
Marking 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.
- one mark for air molecules colliding with the surface
- one mark for each collision exerting a force on the surface
- one mark for pressure as that total force spread over the area of the surface
- one mark for the collisions arriving from all directions because the molecular motion is random
- State that a column of air above a surface has weight due to the mass of the air molecules.
- Identify that as the height of a surface above ground level increases, the column of air above it becomes shorter.
- Explain that a shorter column means there are fewer air molecules above the surface.
- Conclude that fewer molecules result in a decreased total weight of air acting on the surface.
- State that the amount of air above a surface always decreases as height increases.
- Explain that a lower surface carries an extra layer of air compared to a higher one.
- Identify that the average density of the air decreases as altitude increases.
- Conclude that atmospheric pressure always decreases with height due to the reduced weight of air above.
- State that there are fewer air molecules above a surface at a greater height.
- Explain that this results in a smaller weight of air acting on the surface.
- Identify that fewer molecules lead to fewer collisions with the surface each second.
- Conclude that atmospheric pressure is therefore lower at greater altitudes.
- Describe the atmosphere as a thin layer of particles in random motion held by gravity.
- State that the atmospheric layer is denser near the ground due to compression.
- Explain that atmospheric pressure is produced by particle collisions with a surface.
- Link greater height to fewer molecules above and fewer collisions, resulting in lower pressure.
Examiner 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.
- 💡Three words earn the marks here: collisions, force, area.
- 💡If a change in pressure is asked for, say whether the collisions become more frequent, harder, or both.
- 💡Use the phrase 'fewer molecules' rather than 'less air' when explaining the decrease in the weight of air with height.
- 💡Always explicitly link the decrease in the number of air molecules above a surface directly to the reduced weight of air acting on that surface.
- 💡Answer comparison questions by explicitly naming the extra layer of air that the lower surface carries.
- 💡Keep in mind that questions on atmospheric pressure and height are exclusively for Higher Tier candidates.
- 💡Extract the pressure value from the graph and write it down before starting any calculation.
- 💡Always check units; expect to convert kilopascals to pascals and square centimetres to square metres.
- 💡Structure your answer in ordered steps: particles, gravity, density, and collisions.
- 💡Use linking words like 'so' or 'therefore' to show the logical progression of your explanation.
Common Mistakes
- Drawing or describing the atmosphere as a thick shell comparable with the radius of the Earth; correction: it is a very thin layer.
- Saying the atmosphere has a definite upper surface; correction: it gradually thins out into space.
- Confusing lower density with lower temperature as the reason the air is thin at altitude; correction: density decreases due to less weight of air above.
- Claiming the air is less dense high up because gravity does not act there; correction: gravity acts at all altitudes but the compressive weight of air above is less.
- describing the air as pressing down only, so that a vertical surface would feel nothing
- saying air pressure is caused by the weight of molecules resting on the surface rather than by collisions
- confusing the number of collisions with the speed of the molecules when explaining a change
- leaving out the word area, which turns a pressure answer into a force answer
- Reversing cause and effect by stating that lower weight causes fewer molecules; correction: the reduced number of molecules at altitude is the cause of the decreased weight of air.
- Assuming the column above a high surface is identical to the one above a low surface, only shorter; correction: the air is also less dense at higher altitudes, further reducing the number of molecules.
- Confusing the weight of the air above with the weight of the object on the surface; correction: atmospheric pressure depends only on the weight of the air column above the surface.
- Arguing that weather or temperature could reverse the relationship; correction: the decrease in the amount of air with height is absolute and always holds true.
- Treating the air between the two heights as though it sits above both surfaces; correction: the extra slab of air only sits above the lower surface.
- Confusing the density of the air at a specific height with the average density of all the air above it; correction: average density of the column must be considered.
- Stating only that there is 'less air', without linking this to weight or collision frequency.
- Failing to convert kilopascals into pascals before using pressure values in calculations.
- Assuming the graph of pressure against altitude is a straight line, rather than a curve.
- Describing the particle model but failing to use it to explain the variation in pressure.
- Stating 'because it is higher up' as an explanation, without providing the physical mechanism.
- Claiming there is less gravity at altitude, rather than explaining the reduced weight of air.