Introduction

The earth's atmosphere stretches up to 100km into space. 100km seems a long way but compared to the thickness of the earth it is like the peel on an apple.

The atmosphere has an enormous effect on the planet and is the reason why life can be sustained. The diagram on the left shows the composition of the earth's atmosphere. It is made up of several layers which have different properties. The layer of atmosphere most people are familiar with is the lowest layer called the troposphere. It is in this layer that we live and this is where the weather that we see from earth takes place. As you go up in altitude, the temperature decreases and the atmosphere becomes less dense.

At about 15km above the earth's surface the temperature begins to rise again. This is called the stratosphere. The cold air at the top of the troposphere is trapped by warmer air above and below. This makes it hard for molecules to move upwards into the stratosphere. The reverse temperature gradient of the troposphere means the gases separate out.

Above the stratosphere is the ionosphere where the suns rays heavily ionise all the molecules. Beyond that is the vacuum of space.