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 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 atmosphere becomes thinner and thinner (meaning it contains fewer and fewer air molecules) with increasing altitude. Air suitable for breathing and for the photosynthesis of plants is found only in the troposphere, the lowest level of the atmosphere. The density of air molecules in the exosphere is extremely thin, dwindling to none at all when the outer edge is reached.

Layers of the atmosphereLayers of the atmosphere

The atmosphere

The Earth’s atmosphere is made up of several different layers: the troposphere, stratosphere, mesosphere, thermosphere and the exosphere, which merges seamlessly into space. 

The troposphere rises from the ground to about 9 kilometres (6 miles) above the poles and 16 kilometres (10 miles) above the Equator. It is only one seventieth of the atmosphere’s total volume, yet it contains four fifths of all the air. The troposphere is rich in oxygen and water vapour. This is where clouds form and all our weather occurs. Its temperature falls to –55°C (–67°F) at the boundary with the stratosphere.  

The stratosphere extends to 50 kilometres (30 miles) above the ground, between the troposphere and mesosphere. Jet planes sometimes fly in this layer to avoid storms in the troposphere. The stratosphere contains a layer of ozone, a gas that absorbs nearly all the Sun’s ultraviolet radiation—rays harmful to life on Earth. The temperature rises to 10°C (50°F) at the level where the mesosphere begins. It then plunges to a low of –75°C (–100°F) at 80 kilometres (50 miles), before rising again in the thermosphere. 

Temperatures in the thermosphere, where the Sun heats the thin air, rise to 1400°C (2500°F). The air in the thermosphere burns up most meteors (fragments of rock from space). The highest layer, the exosphere, starts at about 140 kilometres (225 miles) high. It goes all the way up to an altitude of at least 10,000 kilometres (6000 miles)—but it is impossible to give a precise measurement.

Space Shuttle and the Earth's atmosphereSpace Shuttle and the Earth's atmosphere

Kármán Line

There is no exact point where the atmosphere stops and space begins, but scientists generally agree that the Kármán Line, at 100 kilometres (62 miles), marks the “official” boundary between atmosphere and outer space. Above this altitude, the atmosphere becomes too thin to allow any aircraft to fly (they cannot get enough lift to keep them in the air). So that is, perhaps, the best answer to the question, how high is the sky. It also shows that space is not as far away as you might think. If you could drive a car vertically upwards, you could be there in an hour or so.

Other Q-&-As to explore:

What is the Moon made of?

Why is grass green?

Why is the sea salty?

Where do the stars go in the day?

What are clouds made of?

Why do we cry when we feel sad?

Where do bees go in winter?

Why do ships float?

How does an airliner fly?

What makes some drinks fizzy?

Why do our ears pop when we fly in a plane?

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See also in Earth

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.

The air in the exosphere is so thin that one particle of gas may travel hundreds of kilometres before hitting another particle.

Extending from about 80 kilometres to about 600 kilometres (roughly 50–400 miles) above the Earth's surface is the ionosphere. It is made up of a combination of the mesosphere, thermosphere and exosphere. The ionosphere can be used to "bounce" shortwave radio signals, transmitted from the ground, around the globe.

In space there is no air, so there is no friction—from air molecules—to slow a spacecraft down. This means that once a spacecraft is set in motion, no engines are needed to propel it forward. When a spacecraft re-enters the Earth’s atmosphere, however, it experiences enormous air resistance as the air molecules cause friction. This effect begins at an altitude of around 120 km (75 miles).

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