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.

Air

“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, 0.041% 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.

Layers of the atmosphere

The atmosphere has several layers which rise and fall in temperature as the air gets thinner.

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. 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 rays—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.

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

Ionosphere

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

Why the sky is blue

When sunlight passes through the atmosphere, some of its waves are scattered (sent in all directions) by the gas molecules in the atmosphere. Sunlight is not colourless: it is made up of all the colours in the spectrum. Different colours of light have different wavelengths. Blue waves of light travel in shorter, “choppier” waves than other colours. Shorter wavelengths of light are scattered by gas molecules more than other colours—and this is why the sky usually looks blue.