# Breaking under pressure

What’s happening?

To think about air pressure, we have to look up into the atmosphere. Gravity pulls air (and us) to the Earth, and the part of the atmosphere that has the most air is right here, close to the Earth at sea level. As you go up there is less and less air until you reach a point where there is no air at all. The lower layer of the atmosphere that has 75% of all the air in the atmosphere is called the troposphere and it is around 10 km thick above Australia (this layer gets thicker over the equator and thinner over the poles).
Air has weight and air pressure is the force of air pushing on and around everything on the Earth. At sea level, every one centimetre by one centimetre square has one kilogram of air pushing upon it. Phwoa – that’s a lot of pressure.
Why does the ruler snap? Air pressure pushes from all directions, it pushes down, up and from the sides. Your ruler will break only when there is air pushing down on the paper. This is why it is important to carefully make sure that there is no air between the paper and the table as this air will push up, cancelling out the air pushing down.

Applications

Air pressure and the movement of air is an important part of the weather. Knowing the air pressure can let us know what type of weather to expect. The weather report lets you know where there are high and low pressure systems in the atmosphere.
A high pressure system is when the air moves downwards, or sinks. As the air sinks, it warms. High pressure systems usually mean dry, stable weather conditions.
A low pressure system is when the air is heated by the sun and moves upwards, or rises. Low pressure systems usually mean that there will be clouds forming that may bring showers and unstable weather conditions.
Joke: Q. How come it never rains inside a barn?
A. It’s a stable atmosphere.

Guest writer Beth Askham shows you how to break a ruler with one hand using only a piece of newspaper and air pressure.

What you need

• A wooden ruler
• A piece of newspaper