• identify the layers of the Earth’s atmosphere
  • describe the contents of each layer of the Earth’s atmosphere
  • describe the importance of each of the layers of the Earth’s atmosphere

The Earth is surrounded by layers of gases which we call the atmosphere. The atmosphere of our planet is a very thin layer, relatively. The Earth’s diameter is about 12,800 km across at the equator, and yet 75% percent of the atmosphere is within about 12km of the surface. This can be compared to the outermost skin of an onion around the rest of the onion!

Most people can name two important gases in the atmosphere: oxygen (O2) and carbon dioxide (CO2). However, the most prominent gas by far is, in fact, nitrogen (N2)! Nitrogen makes up 78% of our atmosphere, while the next most common is oxygen at 21%. After that, the most common is either Argon or water vapour at 1%. Water vapour is the gaseous form of water, as seen in steam, fog, mist and clouds; its concentration in the atmosphere varies between 0.5-4%. Carbon dioxide makes up only about 0.03%, and the remaining gases are a combination of Neon, Helium, Methane, Krypton, Hydrogen, Ozone, Xenon and Nitrous Oxide.
Percentages of Gases that make up the Earth’s Atmosphere
Did you know?

We are losing small amounts of our atmosphere everyday! Gravity holds most of the atmosphere down on the Earth, but the vacuum of space is continually trying to suck the atmosphere away from us. Although we lose small amounts each day, we are actually gaining small amounts; also, meteors and cosmic dust arrive from space.

Our atmosphere is generally divided into 5 horizontal layers. These are, in order from nearest to the Earth to the farthest: the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere.

The layers of the Atmosphere. Note that in this image the relative size
of the layers is not accurate. The troposphere is actually incredibly
thin compared to the thermosphere.
The layer closest to the Earth is the troposphere. It starts right at ground level and continues up to an altitude of about 12km above the Earth. This is a very short distance when compared to the size of the Earth which gives us the ‘onion skin’ analogy. Our lives are conducted totally within this layer. Mount Everest does not even reach above this layer, as it has an altitude of about 8.85km.

The word “troposphere” comes from the Greek word “tropos” which means “turning” or “mixing.” The light from the Sun adds heat to the gases within this layer of the atmosphere causing them to mix and turn. We experience this movement as wind and weather. Airplanes fly at about 30,000 ft or 9.1km, near the top of the atmosphere to avoid the turbulence of the gases.

Heat is one of the important characteristics that create divisions between layers of the atmosphere. Near the surface of the Earth the air is warm, averaging about 20?C, but as you move to higher altitudes it begins to cool. We see this with the ice and snow that is common on mountaintops. At the top of the troposphere the atmosphere has cooled to about -50?C. The boundary that divides the troposphere from the next layer is called the tropopause.
Above the tropopause, the stratosphere, from the Latin word “stratum” meaning “layer”, begins. The stratosphere goes from about 12km above the Earth to about 50km in altitude. Very little human activity occurs within this layer, but weather balloons float into these altitudes to study weather from above.
Beginning at the tropopause and moving upwards, the temperature begins to increase again. This is because of the large amount of ozone within the stratosphere. Ozone (O3) is a molecule that is excellent at absorbing the energy of skin-damaging ultra-violet light from the Sun. In absorbing this light, the ozone molecules become heated causing increased temperature within this layer.

Ultra-violet light has very negative effects on humans as it can cause mutations in DNA. Alterations in our cells’ DNA is one of the causes of cancer, most commonly skin cancer. With the depletion of the ozone layer in the past 40 years, cancer rates have increased dramatically.

The depletion of ozone within the stratosphere has been primarily caused by chlorofluorocarbons (CFCs). These chemicals were used largely in refrigerators, solvents and propellants such as in the original aerosol cans. CFCs were banned in 1980s by most countries and this has lead to a decrease in the release of CFCs to the atmosphere.


Canadian Contributions

In September 1987, an international congress was held in Montreal where an agreement was signed to eliminate the use of CFCs, and other chemicals, that were damaging the ozone layer. The “Montreal Protocol” has since been signed by 191 countries around the world. Thanks to this agreement, the level of CFCs in the atmosphere peaked in about the year 2000 and is now decreasing. It is expected that by the year 2050 the Ozone Layer will have returned to nearly its original state. Due to its wide acceptance and implementation, the Montreal Protocol has been called one of the most successful international agreements ever.


Did you know?

There is more atmosphere above the equator than there is at the poles. Just as the Earth is slightly thicker at the equator than at the poles (see Unit 1, Lesson 1), the atmosphere is also thicker at the equator. The troposphere is as thin as 8km at the poles and as thick as 16km over the equator. This is also caused by the increased temperature at the equator. As a gas is heated, it increases its pressure and wants to expand. So the air at the equator is expanding and forcing the gases higher within the atmosphere.


Starting at about 50km and going to about 80km is the mesosphere. There is not much remarkable about the ‘meso’sphere or “middle” layer. This layer contains essentially the same gases as the previous layers, except it does not contain water vapour and ozone. The temperature decreases again through this layer reaching a minimum of about -100?C.


The next layer of the atmosphere is more exciting. The thermosphere extends from about 80km to about 500km, making it the thickest layer of our atmosphere. Its physical size is misleading though, as it contains very little gas. In fact the density of particles is so low that even though it’s over 400km thick, it contains far less gas than the troposphere, which is only about 12km thick.

The International Space Station in orbit, with its newly added solar panels visible in the
bottom right. Space Shuttle Atlantis delivered these panels in September 2006.

This region is very similar to outer space. The International Space Station orbits here at an altitude of about 350km, through the middle of the thermosphere. There is still enough gravity at this height to keep the gases present above the Earth. However, there is very little oxygen, and mostly lighter gases such as hydrogen. This layer is still important though, as it protects us from two main dangers: meteors and x-rays


The “shooting stars” you see in the night sky are meteors, small particles of dust, usually no bigger than a grain of sand, that burn up quickly from atmospheric friction as they enter the thermosphere. Some larger particles do make it through and burn up in the lower levels of the atmosphere, appearing as large and bright shooting stars. Some are even large enough not to burn up entirely. They strike the Earth as meteorites. These are few and far between, as most meteors burn up in the thermosphere.


The second protective function of the thermosphere is to block x-rays from shining down on us. Remember that x-rays are a high-powered form of light, constantly being produced by the Sun. They shine down on Earth everyday, just as UV light and visible light do. However, the small amount of oxygen in the thermosphere is very good at absorbing the x-rays, and these harmful rays do not reach the surface of the Earth. The energy that is absorbed is transformed into heat energy, giving the thermosphere its name of “thermo” or “heat”. This layer of the atmosphere actually has the highest temperature! During periods of high sunspot activity where lots of x-rays are produced the temperature in the thermosphere can rise to as much as 2000?C. However, since there is so little gas, a normal thermometer would still read well below zero.

The thermosphere also contains most of what is known as the Northern (and Southern) Lights, but that is a topic for a later module (see Unit 5, Lesson 2).

The last layer of the atmosphere is called the exosphere. "Exo" means "outer" as this layer has so little gas in it, that it is hardly differentiable from outer space. The exosphere begins at about 500 km and extends to about 1000-10,000 km. This number is highly variable as our atmosphere just blends off into outer space. At this level there are only the lightest elements present such as hydrogen and helium, and it is here that there is any noticeable loss of atmosphere to the vacuum of space. The exosphere is still important to mention though, because it is where many satellites are in orbit. It is important for satellites to maintain their orbit for long periods of time, so they must experience very much drag from the particles of the atmosphere. In the exosphere they are able to obit the Earth with almost nothing to slow them down.

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