Atmospheric Pressure and Pressure in Action

We live at the bottom of an ocean of air. The atmosphere — the layer of gases surrounding the Earth — presses down on everything at its surface, including you, with a surprisingly large force. You do not feel it because it acts equally from all sides, but it is real: it is what holds a drink up a straw, what makes a suction hook cling to a wall, and what makes your ears "pop" as you climb a mountain or take off in a plane. This lesson, part of Topic P1 (Matter) of OCR Gateway Science A, explains where atmospheric pressure comes from, why it decreases with height, and how the variation of pressure is put to use in everyday situations and instruments.

By the end of this lesson you should be able to explain how atmospheric pressure arises from the air molecules, explain why atmospheric pressure decreases with height, and describe everyday effects and applications of atmospheric pressure such as straws, suction and altitude effects.

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The Atmosphere as a Fluid

The atmosphere is the layer of gases (mostly nitrogen and oxygen) held around the Earth by gravity. Because it is a gas, it is a fluid, and like any gas it is made of particles in constant random motion. Those particles collide with every surface they meet — the ground, buildings, your skin — and each collision exerts a tiny force. Spread over the area of the surface, the countless collisions produce atmospheric pressure.

At sea level, atmospheric pressure is about $100\,000 \text{ Pa}$100000 Pa (often quoted as $101\,325 \text{ Pa}$101325 Pa, or "one atmosphere"). That is roughly the weight of a $1 \text{ kg}$1 kg mass pressing on every square centimetre — a large pressure that we do not notice only because it pushes on us equally from all directions, and because the pressure inside our bodies balances it.

Exam Tip: Atmospheric pressure is caused by air molecules colliding with a surface. An equivalent way to express it is the weight of the air above pressing down — both descriptions earn the mark.

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Why Atmospheric Pressure Decreases with Height

A crucial fact is that atmospheric pressure decreases as you go higher up. Climb a tall mountain or fly in an aircraft and the pressure outside falls. There are two complementary ways to understand this, and both are worth knowing:

• In terms of the weight of air above: the pressure at any height comes from the weight of all the air above that point. The higher you go, the less air there is above you, so its total weight is smaller and the pressure is lower. At the top of a mountain there is far less atmosphere above your head than at sea level.
• In terms of the density of the air: the atmosphere is not equally dense at all heights. Near the ground the air is squashed by the weight of all the air above it, so it is denser — the molecules are closer together, and there are more frequent collisions, giving a higher pressure. Higher up, the air is less dense (the molecules are more spread out, since there is less weight above to compress them), so there are fewer collisions per second and the pressure is lower.

So as height increases, both the amount of air above and the density of the local air decrease, and the atmospheric pressure falls. (Unlike a liquid, the air's density changes with height because a gas is compressible, which is why pressure does not fall at a perfectly steady rate, but the trend is always downward with height.)

Exam Tip: Two acceptable reasons that atmospheric pressure falls with height: (1) there is less air above you, so less weight pressing down; (2) the air is less dense higher up, so there are fewer collisions with a surface. Either, well explained, scores the mark.

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Everyday Effects of Atmospheric Pressure

Because atmospheric pressure is large and acts in all directions, it produces many familiar effects:

• Drinking through a straw. When you suck on a straw you lower the pressure inside it (above the liquid). The higher atmospheric pressure pressing down on the surface of the drink in the glass then pushes the liquid up the straw and into your mouth. It is the atmosphere, not your mouth, that does the lifting.
• Suction hooks and pads. Pressing a rubber suction pad onto a smooth surface squeezes out the air behind it, lowering the pressure there. The atmospheric pressure on the outside then holds it firmly against the surface.
• Ears "popping" with altitude. When you climb a mountain or an aircraft ascends, the outside pressure falls. The higher pressure trapped behind your eardrum then pushes the eardrum outward until the pressure equalises (often with a "pop"), which can feel uncomfortable. The reverse happens as you descend.
• Sealed packets bulging at altitude. A sealed bag of crisps taken up a mountain or onto a plane bulges or even bursts, because the air sealed inside is now at a higher pressure than the reduced atmospheric pressure outside, so it pushes the packet outward.

In every case, the effect comes from a difference in pressure between two regions — and atmospheric pressure provides one side of that difference.

Exam Tip: For "explain how a straw works", the key point is that reducing the pressure in the straw lets the higher atmospheric pressure on the drink's surface push the liquid up. Never say you "suck the liquid up" without mentioning the atmosphere doing the pushing — that is the marking point.

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How Atmospheric Pressure Changes Affect the Body and Weather

The variation of atmospheric pressure with height has real consequences: