The Earth is surrounded by an atmosphere. It is this that makes life possible on the planet, as the atmosphere contains the oxygen we need to breathe. It also protects the Earth from harmful radiation and particles from space.
The atmosphere of the Earth is 78% nitrogen and 21% oxygen. It also contains small amounts of argon, carbon dioxide, water vapor, ozone, methane, carbon monoxide, etc.
The atmosphere is divided into different layers. The troposphere, which extends to an altitude of 10 kilometers (up to 15 kilometers in the tropics), has many clouds. Most of the events that determine our weather happen here. The temperature drops with altitude by about one degree Celsius per 2000 meters.
Above this lies the stratosphere. Jet-propelled aircraft reach these altitudes. The stratosphere extends to an altitude of up to 50 kilometers and is separated from the troposphere by the tropopause. The temperature in this region is generally constant or rises slightly. There are almost no clouds here. The ozone layer, which protects the Earth from short-wave ultraviolet_radiation, lies at an altitude of about 25 kilometers in the stratosphere. It is currently threatened by the emission of ozone-depleting gases such as CFCs.
The next layer, from about 50 to 80 kilometers of altitude, is called the mesosphere. Here, the temperature sinks rapidly to about minus 80┬░ Celsius. This is where most meteors burn up. The transition layer between the stratosphere and the mesosphere is called the stratopause.
Beyond the mesosphere lies the thermosphere. It extends to a distance of 500 to 1000 kilometers from the surface of the planet. Here the temperature rises with altitude. The thermosphere has a region with several charged layers, the ionosphere. Temperatures here are as high as 1200┬░ Celsius (at an altitude of 400 kilometers). The transition from the mesosphere and the thermosphere is called the mesopause.
The polar_lights are a phenomenon of the thermosphere. They originate at an altitude of 80 to 120 kilometers.
They are known as northern lights or aurora borealis in the northern hemisphere and as aurora australis in the southern hemisphere. The exosphere, which lies beyond the thermosphere, has temperatures of over 2000┬░ Celsius. Although there is no distinct demarcation, it fades into outer space at an altitude of approximately 8000 kilometers. Weather satellites orbit in this layer.
The heat of the Sun is trapped on the Earth by carbon dioxide and water vapor which maintain the Earth's temperature at an average of 15┬░ Celsius. Otherwise, temperatures would rapidly drop below freezing point.
If the CO2 content in the atmosphere should increase, for instance through burning fossil fuels, the climate on Earth could change to our detriment. This is called the greenhouse_effect. Other harmful substances emitted by vehicles and factories can also cause great damage, for instance in the form of acid rain.
