Planet Earth

How high is the sky?

The Andromeda Galaxy (left) in the night skyThe Andromeda Galaxy (left) in the night skyThe sky is the name we give for what we can see above the horizon—an expanse of blue during the day, black at night. The sky includes the atmosphere, the blanket of air that envelopes the Earth, as well as space that lies beyond. The highest clouds are no higher than 12 kilometres (7.5 miles) above ground, so that altitude could be considered the “height of the sky”. Others might argue it is the boundary between the atmosphere and space, which scientists give as 100 kilometres (62 miles) above ground. But the upper limit of the atmosphere, beyond which there is no air at all, actually lies at least 10,000 kilometres (6000 miles) above ground—making this a third possible definition of the height of the sky. Yet another answer is the distance of the stars we can see in the night sky. In fact, the most distant object visible is a galaxy containing billions of stars: the Andromeda Galaxy. It lies 2.5 million light years or 22.5 quadrillion (15 zeros) kilometres away. 

A view of the atmosphere above the cloudsA view of the atmosphere above the clouds

Chart showing the gases that make up airChart showing the gases that make up air

Air

Air contains 78% nitrogen, 21% oxygen, along with small amounts of argon, carbon dioxide and other gases. It also contains a variable amount of water vapour—on average around 1% of its volume. Three quarters of the atmosphere’s mass is found within about 11 kilometres (6.8 miles or 36,000 feet) of the Earth’s surface.

The troposphere, the lowest layer of the atmosphere, is only one seventieth of the atmosphere’s total volume, yet it contains four fifths of all the air.

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