What is pressure?

Why do you sink into deep snow in boots, but not in snowshoes? Why does a drawing pin slide into a cork board so easily, when your finger cannot? And why do your ears pop on a mountain road? The answer to all three questions is pressure.

Pushing on a small or a big area

Pressure tells how hard something pushes on each bit of a surface. The same push can make a big pressure or a small one. It depends on the size of the area it pushes on.

Push a drawing pin into a cork board with your thumb. Your thumb presses on the wide, flat head. There the push is spread out, so the pressure is small and it does not hurt.

The same push travels all the way to the tiny sharp point. There, it is squeezed onto a very small spot. The pressure is huge, and the pin slides into the cork.

Snowshoes work the other way round. They spread your weight over a much bigger area than your boots do. Each bit of snow gets a smaller push, so you stay on top instead of sinking in.

Try it with modelling clay. Press it with your flat hand, then with one finger, pushing equally hard. The finger sinks in much deeper.

We live at the bottom of an ocean of air

Air may feel like nothing, but it has weight. Above you, the air reaches up for many kilometres. The Earth pulls all of it down, so it presses on everything below. We live at the bottom of an ocean of air!

This push is called air pressure, and it is surprisingly strong. Think of a patch of your skin the size of a fingernail. The air presses on it as hard as a 1-kilogram bag of sugar would!

So why don’t you get squashed? The air presses on you from all sides at once: from above, from below and from the sides. And the air and the liquids inside your body press back outwards equally hard. The pushes balance, so you don’t feel them.

You use air pressure every time you drink through a straw. When you suck, you lower the pressure inside your mouth. The air outside then presses down on the drink and pushes it up the straw.

How do we measure pressure?

Pressure is measured in pascals (Pa). They are named after Blaise Pascal, a French scientist who studied air pressure almost 400 years ago.

One pascal is a tiny pressure. A sheet of paper lying flat on a table presses on it with about one pascal.

For bigger pressures we often use the bar. One bar is 100,000 pascals. That is about the pressure of the air around us at the seaside.

Weather forecasts use hectopascals (hPa). One hectopascal is 100 pascals. On an average day at sea level, the air pressure is about 1,013 hPa.

Squeezing a gas

A gas, like air, is made of tiny particles that zoom around in all directions. They keep bumping into the walls of whatever they are in. Each bump is a tiny push. Countless bumps together make the pressure of the gas.

Now pump up a bicycle tyre. Every push of the pump squeezes more air into the tyre. More particles in the same space make more bumps on the inside of the tyre. The pressure rises, and the tyre gets hard.

Squeeze a gas and it warms up

Pump up a tyre quickly, then feel the bottom of the pump. It gets warm! When you push the handle down, you squeeze the air inside.

The pump pushes on the air particles and makes them zoom faster, like a bat hitting a ball. Faster particles mean a higher temperature. So squeezing a gas warms it up.

It works the other way round too. Let a squeezed gas spread out, and it cools down. That is one reason why a spray can gets cold when you spray it for a while.

The same happens in the sky. Air that rises up a mountain spreads out, because there is less pressure up there. As it spreads out, it cools down. That is one reason why mountain tops are cold.

This trick is very useful. Fridges, air conditioners and heat pumps squeeze a special fluid, called a refrigerant, to make it hot. Then they let it spread out to make it cold. That is an important part of how they move heat from one place to another.

Less air above you: up in the mountains

The higher you go, the less air there is above you, so the air pressure drops. Mount Everest is the highest mountain on Earth. At its top, the air pressure is only about a third of the pressure at sea level.

When a car drives up a mountain road, the pressure outside your ears drops. But behind each eardrum there is a little pocket of air that still has the old, higher pressure. It pushes your eardrum outwards until a tiny tube opens and lets some air out: pop! Swallowing or yawning helps the tube open.

A sealed bag of crisps puffs up on a mountain top or on a plane, for the same reason. The air inside the bag still has the higher pressure from below, so it pushes the bag outwards.

Lower pressure even changes the temperature at which water boils. Find out how in why water boils at different temperatures.

Go deeper: Pressure in numbers for grown-ups and the extra curious

Pressure is force divided by area:

$$p = \frac{F}{A}$$

Here $p$ is the pressure in pascals (Pa), $F$ is the force in newtons (N) and $A$ is the area in square metres (m²). One pascal is one newton on each square metre. One newton is about the weight of a small apple.

A child with a mass of 30 kg pushes down on the snow with a force of about 300 N. In boots, the soles touch about 0.03 m² of snow: $p = \frac{300}{0.03} = 10\,000$ Pa. Snowshoes touch about 0.2 m²: $p = \frac{300}{0.2} = 1\,500$ Pa. That is almost seven times less.

Air pressure at sea level is 101,325 Pa on average, which is about 1.013 bar or 1,013 hPa.

Go deeper: The gas rule in words for grown-ups and the extra curious

The pressure, the volume and the temperature of a gas are linked:

• Squeeze the same gas into half the space, at the same temperature, and its pressure doubles.
• Heat a gas in a closed container, and its pressure rises. That is why tyres get harder after a long drive on a hot day.
• Squeeze a gas quickly, and it warms up. Let it expand quickly, and it cools down.

For an “ideal” gas, all of this fits into one formula:

$$p \cdot V = n \cdot R \cdot T$$

Here $p$ is the pressure (Pa), $V$ is the volume (m³), $n$ is the amount of gas (in moles), $R \approx 8.31$ J per mole per kelvin is a fixed number, and $T$ is the temperature in kelvins.

Go deeper: Why a spray can really gets cold for grown-ups and the extra curious

Inside most spray cans, the gas is squeezed so hard that much of it has turned into a liquid. When you spray, the pressure inside drops a little, and some of the liquid boils into gas. Boiling soaks up heat, from the can and from your hand. This is the same trick a refrigerant uses in a fridge or a heat pump (see evaporation and condensation).

Go deeper: What a tyre gauge shows for grown-ups and the extra curious

A tyre gauge shows how much more pressure there is inside the tyre than outside. A completely flat tyre still has about 1 bar of air inside, the same as the air around it. A tyre pumped to 2 bar on the gauge really has about 3 bar inside.

Go deeper: Where the ocean picture breaks down for grown-ups and the extra curious

Water can hardly be squeezed, so a lake has almost the same density from top to bottom. Air is different. The air near the ground is squeezed by all the air above it, so it is denser down low and thinner higher up. Half of all the air lies below a height of about 5.5 km.

Check yourself

Why don't you sink into deep snow when you wear snowshoes?

Snowshoes make you lighter Your weight is spread over a bigger area, so the pressure is smaller Snowshoes melt the snow

Show the answer

Your weight is spread over a bigger area, so the pressure is smaller — You weigh the same, but the push is spread over a much bigger area. Each bit of snow gets a smaller push, so it can hold you up.

Check yourself

What happens to the air when you squeeze it quickly in a bicycle pump?

It gets warmer It gets colder Nothing at all

Show the answer

It gets warmer — Squeezing a gas makes its particles zoom faster. Faster particles mean a higher temperature, so the pump gets warm.