Revision notes for AQA GCSE Physics Atmospheric pressure. Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
Atmospheric pressure
What you'll learn
- What the atmosphere is, and why it is “thin” compared with the Earth.
- How air molecules colliding with surfaces create atmospheric pressure.
- Why air density decreases as altitude increases.
- How to explain why atmospheric pressure is lower higher up.
The atmosphere: Earth’s layer of air
The atmosphere is the layer of air around the Earth. It contains gases such as nitrogen, oxygen, carbon dioxide and water vapour.
Compared with the size of the Earth, the atmosphere is very thin. Earth’s radius is about 6400 km, but most of the atmosphere’s mass is much closer to the surface than that.
Definition
Atmosphere
The atmosphere is the thin layer of air surrounding the Earth.
A useful simple model is to imagine the Earth wrapped in a thin “blanket” of gas. The blanket is not equally packed everywhere: it is densest near the ground and becomes less dense as you go higher.
Analogy
Thin compared with Earth
If the Earth were a large ball, the part of the atmosphere where most weather happens would be like a very thin skin on the outside. This helps you remember that the atmosphere is small compared with the whole planet.
Air is made of moving molecules
Air may look like “nothing”, but it is made of tiny particles called molecules. These molecules are moving in random directions all the time.
When air molecules hit a surface, they push on it for a tiny moment. Billions and billions of collisions happen every second, so the overall effect is a steady pressure.
Definition
Atmospheric pressure
Atmospheric pressure is the pressure caused by air molecules colliding with surfaces. Pressure means force per unit area, and it is measured in pascals, Pa.
The pressure is not caused by one giant push. It is the combined effect of many tiny particle collisions.
Key Idea
What causes atmospheric pressure?
Atmospheric pressure is caused by air molecules constantly colliding with surfaces.
Example
Explaining pressure on a window
A window has air on both sides. Explain why the air produces pressure on the glass.
- Air molecules are moving randomly, so molecules strike the glass from many different directions.
- Each collision changes the motion of a molecule, so the molecule exerts a tiny force on the glass.
- Because a very large number of molecules collide with every square metre of the glass each second, the total effect is a measurable pressure.
Common Mistake
Pressure is not only downward
Atmospheric pressure acts on surfaces in all directions because air molecules move randomly. Gravity matters for explaining why pressure changes with height, but the particle collisions themselves come from all directions.
Density: how packed the air is
Density tells you how much mass there is in a certain volume. For air, higher density means more air molecules in each cubic metre.
Near the Earth’s surface, the air is more dense because there is more air above it. The weight of the air above compresses the air below, so the molecules are packed more closely together.
Definition
Density
Density is mass per unit volume. For air, a higher density means more air molecules in the same volume.
As you go higher up, the atmosphere becomes less dense. This means that in the same size box of air, there are fewer air molecules.
Key Idea
Density changes with altitude
The atmosphere gets less dense as altitude increases, so there are fewer air molecules per cubic metre higher up.
Altitude: height above the Earth’s surface
Altitude means height above sea level or height above the Earth’s surface, depending on the context. In this topic, you mainly use it to compare low places, like sea level, with high places, like mountains.
Definition
Altitude
Altitude is height above a reference level, usually sea level or the Earth’s surface.
At a low altitude, there is a large amount of air above you. At a high altitude, there is less air above you.
That matters because air has mass, so air has weight. The more air there is above a surface, the greater the weight of air pushing down through the atmosphere.
Why atmospheric pressure decreases with height
Now combine the ideas:
- Air molecules collide with surfaces, causing atmospheric pressure.
- Air has weight because it has mass and is pulled by gravity.
- Lower down, there is more air above a surface.
- Higher up, there is less air above a surface.
So, as height increases, atmospheric pressure decreases.
Key Idea
The main explanation
Atmospheric pressure decreases with increasing altitude because there is less air above you, so there is less weight of air pressing down and fewer air molecules nearby to collide with surfaces.
Example
Comparing pressure at sea level and on a mountain
A student stands at sea level. Another student stands near the top of a high mountain. Explain who experiences the greater atmospheric pressure.
- The student at sea level has a larger height of atmosphere above them than the student on the mountain.
- This means there is more air above the sea-level student, so the weight of air above them is greater.
- The air is also more dense at sea level, so there are more air molecules per cubic metre colliding with surfaces.
- Therefore, the atmospheric pressure is greater at sea level than on the mountain.
Tip
Use a cause-and-effect chain
For full explanations, link the ideas in order: higher altitude → less air above → lower weight of air above → fewer/less frequent molecular collisions → lower atmospheric pressure.
A simple particle model of the atmosphere
For GCSE Physics, you should be able to describe a simple model of the Earth’s atmosphere.
In this model:
- The atmosphere is a thin layer of gas around the Earth.
- The gas is made of moving air molecules.
- The molecules collide with surfaces and create pressure.
- There are more molecules per cubic metre near the ground.
- The number of molecules per cubic metre decreases with height.
This model is not perfect, because the real atmosphere has different layers and changing weather conditions. But it is very useful for explaining the main pattern: pressure decreases with altitude.
Common Mistake
Model, not exact map
You do not need to memorise exact atmospheric layers for this section. Focus on the simple model: particles, collisions, density and height.
Why you do not usually feel atmospheric pressure
Atmospheric pressure is large, but you normally do not notice it because it acts on you from all directions. The pressure inside your body and lungs also helps balance the pressure outside.
You tend to notice changes in atmospheric pressure more than the pressure itself. For example, your ears may “pop” when going up or down a steep hill, in a lift, or in an aircraft. This happens because the pressure outside your ear changes.
Example
Explaining why ears pop at different heights
Explain why your ears may pop when you travel up a mountain road.
- As the car travels upward, its altitude increases.
- At the higher altitude, there is less air above the car, so atmospheric pressure outside your ear is lower.
- The pressure inside your ear may not change immediately, so there is a pressure difference across the eardrum.
- Your ears pop when the pressure becomes more balanced again.
Language examiners like
When answering this topic, try to use precise scientific words:
- Atmosphere: the thin layer of air around Earth.
- Molecules: tiny particles that make up the air.
- Collisions: impacts between air molecules and surfaces.
- Density: how much mass of air there is in a certain volume.
- Altitude: height above the Earth’s surface or sea level.
- Pressure decreases: becomes smaller with increasing height.
Common Mistake
Saying there is no air higher up
There is still air at high altitude, just less of it per cubic metre. Say “less dense” or “fewer air molecules”, not “no air”, unless you are talking about space far beyond the atmosphere.
Exam technique
In the exam
- If asked what causes atmospheric pressure, start with air molecules colliding with a surface.
- If asked why pressure decreases with height, include less air above and less weight of air above.
- For a comparison question, clearly state which place has the greater pressure and then give the reason using altitude and density.
Self review
Check yourself
- Why do air molecules colliding with a surface create pressure?
- Why is atmospheric pressure greater at sea level than at the top of a mountain?
- What does it mean to say that the atmosphere becomes less dense with altitude?
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