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Planet Earth


Sunlight passing through the atmosphereSunlight passing through the atmosphere The atmosphere is an “ocean of air” that surrounds our planet. A mixture of gases, mainly nitrogen (four-fifths) and oxygen (one-fifth), it gets thinner or less dense with height and fades away completely about 800 kilometres (500 miles) above the ground. Here the atmosphere ends and the nothingness of space begins. Besides providing oxygen for us to breathe, the atmosphere traps enough heat to avoid extremes of hot or cold. It also shields us from harmful radiation from the Sun and from bombardment by objects such as asteroids or meteors, from space.

The composition of Earth’s atmosphereThe composition of Earth’s atmosphere


“Air” is the name we give to the atmospheric gases that surround the Earth. Dry air contains around 78% nitrogen, 21% oxygen, 0.93% argon, 0.039% carbon dioxide and small amounts of other gases. Air also contains water vapour, making up on average around 1% of its volume. Air suitable for breathing and for the photosynthesis of plants is found only in the troposphere, the lowest level of the atmosphere.

The Kármán line, located within the thermosphere at an altitude of 100 km (62 miles), is sometimes used to define the boundary between the Earth's atmosphere and space.

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