Atmospheric Layers
If we climb to the top of a tall mountain, we may notice that it becomes colder as we go higher, and that it may become more difficult to breathe. Air higher up is thinner because gravity pulls it towards the Earth's surface. It is colder because most heating takes place near the ground which is warmed by the Sun during the daytime.
The highest mountain on Earth is Mount Everest in the Himalayas. It is over 8,000 metres high, and at this altitude the temperature can fall to -40°C, way below freezing. Most of the Earth's atmosphere lies below this height, but thinner air extends upward much further. Furthermore, air temperature does not continue to fall. In fact the Earth's atmosphere is divided into a number of layers.
The lowest layer is called the troposphere and is about 10 km thick. Above the troposphere lies the stratosphere. This layer extends up to 50 km above the Earth's surface. Concorde flies within this atmospheric layer. Air temperature actually rises again in the stratosphere because ozone in the ozone layer is heated by ultraviolet rays from the Sun. This ozone layer protects life on Earth from the Sun's damaging rays. More layers lie above the stratosphere. At these altitudes however, the air becomes very thin indeed. Here meteors burn up when they strike the Earth's atmosphere, and can be seen at night as shooting stars.
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