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The Definition of Atmosphere
On Earth, the atmosphere defines life, making it possible for us and other respiring organisms to survive. It is the air that we breathe that envelops our planet. When looking up at the sky, it is hard to tell where the atmosphere begins and where it ends. So what exactly is the definition of the atmosphere?
The atmosphere is the layer of gases surrounding Earth.
Every planet in our Solar System (and the Sun itself) has an atmosphere. But today, we're going to be focusing on Earth's atmosphere.
Earth's atmosphere stretches from the surface of the Earth to 10,000 kilometres above the planet.
The Layers of the Atmosphere
The Earth's atmosphere is divided into five primary layers based on their temperature. From the surface upwards, they are:
The troposphere
The stratosphere
The mesosphere
The thermosphere
The exosphere
Each of the layers is bounded by a "pause". This is where the most significant changes in heat, composition, and density occur.
Let's look at the five layers in detail, starting with the lowest to us and working our way up to space.
The Troposphere
The layer closest to Earth's surface is called the troposphere. It ranges from the surface to 15 kilometres above—the temperatures in the troposphere decrease with altitude.
Most of the atmosphere's mass – up to 80% – is contained within the troposphere. The troposphere is thickest at the equator and thinnest at the Earth's poles. The majority of water vapour in the atmosphere is found here, hence why most clouds are found in this layer and weather events tend to happen here too.
Have you ever flown above the clouds in an aeroplane? Most commercial aircraft fly above the troposphere to minimise turbulence.
Clouds consist of water vapour and gases. So how exactly do clouds form? Simply, the air rises and cools; cold air cannot hold as much water vapour as warm. The water vapour becomes liquid (like condensation on your window) and clings to dust in the air, which forms the clouds we can see.
The boundary between the troposphere and the stratosphere is known as the tropopause.
The Stratosphere
Above the troposphere is the stratosphere. It reaches from 15 kilometres to 50 kilometres above the surface. The temperatures in the stratosphere increase with altitude.
Although much thinner than the troposphere, there are still gases present in the stratosphere. An important gaseous feature of the stratosphere is the ozone layer. Ozone (O3) is a toxic gas at sea level. But miles above in the atmosphere, it plays an essential role in keeping our planet safe. Ozone molecules absorb some incoming solar radiation, protecting life on Earth from harmful ultraviolet (UV) radiation.
Chemicals called chlorofluorocarbons (CFCs for short) affect the ozone layer. Chlorine (Cl) and fluorine (F) atoms react with ozone (O3) molecules, converting them into normal oxygen molecules (O2). Widespread use of CFCs led to significant ozone depletion.
Without the protective ozone layer, life on Earth would be severely impacted. Plants would die, and humans would suffer from cancer and DNA damage.
Thankfully, the Montreal Protocol, ratified in 1987, ceased the production and use of CFCs. Since then, the ozone layer has begun to recover.
Between the stratosphere and mesosphere is the stratopause.
The Mesosphere
The mesosphere, the middle layer of the atmosphere, extends between 50 and 85 kilometres above the surface. Temperatures in the mesosphere decrease with altitude. They can reach a chilly -90ºC!
This is the last layer with relatively large amounts of air.
Have you ever seen shooting stars? They are caused by meteors burning up as they travel towards Earth.
A meteor is a piece of rocky debris from space that enters Earth's atmosphere.
Meteors can easily pass through the exosphere and thermosphere since they contain very little air. In comparison, the mesosphere comprises a relatively large amount of gas, causing meteors to vaporise.
Another phenomenon unique to the mesosphere is noctilucent clouds. On a clear summer night, you may have seen these silvery or bluish-coloured clouds. Noctilucent clouds are the highest clouds in Earth's atmosphere.
Figure 2: Noctilucent clouds, high up in the mesosphere. Source: unsplash.com
The mesopause separates the mesosphere and the thermosphere.
The Thermosphere
Above the mesosphere, the thermosphere stretches from 85 to 600 kilometres above the surface. Little is known about this extremely thin layer of the atmosphere, except that temperatures increase with altitude. During the daytime, solar radiation makes the upper regions of the thermosphere get as hot as 2000ºC.
The thermopause separates the thermosphere and the exosphere.
The Exosphere
The outermost and thinnest layer of the atmosphere, the exosphere, stretches from 600 kilometres to around 10,000 kilometres above the surface. Its upper boundary isn't clearly defined. As Earth's gravity gets weaker, molecules of gas escape into space.
The Composition of the Atmosphere
Oxygen is necessary for life on Earth, but most of the atmosphere is actually made of nitrogen. A summary of the composition of the atmosphere can be found in the table below.
| Gas | Composition |
| Nitrogen (N2) | 78% |
| Oxygen (includes O2 and O3) | 21% |
| Argon (Ar) | 0.9% |
| Other Gases (including CO2 and H2O) | 0.1% |
Ozone is a rare molecule within the atmosphere. Even within the ozone layer, the concentration reaches a maximum of 15 parts per million (equivalent to 0.0015%).
The Importance of the Atmosphere
Without the atmosphere, there would be no life on Earth. The importance of the atmosphere lies in its ability to help sustain life by supplying various gases and maintaining liquid water.
Gaseous Resources
The mass of the Earth and its force of gravity retain the atmosphere above the surface. Our atmosphere provides gaseous resources. This table summarises five major gaseous resources and their uses.
| Gas | Symbol | Uses |
| Carbon dioxide | CO2 | Required for photosynthesis |
| Oxygen | O2 | Required for respiration |
| Methane | CH4 | Burned as a fuel to provide energy |
| Nitrogen | N2 | Used in industry, food production, and storage of perishable goods |
| Argon | Ar | Used as an inert shielding gas in welding |
Figure 3: Plants photosynthesise, taking in carbon dioxide and releasing oxygen. Other organisms use oxygen for respiration. Source: unsplash.com
Liquid Water
Earth's atmosphere helps maintain liquid water on the surface. Life first appeared in the oceans, and without liquid water, organisms would not be able to survive.
The Natural Greenhouse Effect
We've all heard that burning fossil fuels enhance the greenhouse effect, trapping heat in the atmosphere and contributing to global warming. But Earth has a natural greenhouse effect. Gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3) absorb incoming infrared radiation from the Sun. It is trapped in the atmosphere as heat and transported around the Earth to warm the planet as much as possible.
Without the natural greenhouse effect, Earth's average temperature would drop to -18ºC. No more liquid water, just ice!
Atmospheric Pressure
The pressure of a gas above a liquid affects its boiling point. This is called atmospheric pressure.
The greater the atmospheric pressure, the more energy is required for liquids to boil, thus the higher the boiling point.
Without the pressure of Earth's atmosphere, the boiling point of water would be much lower. No more liquid water, just steam!
Measurement of the Atmosphere
The measurement of Earth's atmosphere is taken using a barometer.
A barometer is a scientific instrument used to measure atmospheric pressure.
Barometers measure atmospheric pressure in units of measurement called atmospheres (atm).
One atmosphere is equal to the average air pressure at an altitude of 0° (sea level) and a temperature of 15ºC.
Altitude is inversely proportional to atmospheric pressure.
As altitude increases, the atmospheric pressure decreases because the density of air decreases and exerts less pressure.
As altitude decreases, the atmospheric pressure increases because the density of air increases and exerts more pressure.
Figure 4: An outdoor barometer on a frosty morning. Source: pixabay.com
Atmospheric pressure indicates weather changes. Meteorologists use barometers to predict short-term weather changes. A rapid drop in atmospheric pressure suggests that a low-pressure system is arriving. Low-pressure means there isn't enough force to push clouds away, so these systems are associated with cloudy, wet, and windy weather.
Alternatively, a rapid increase in atmospheric pressure indicates that a high-pressure system is arriving with enough force to push clouds away. These systems are associated with sunny, dry conditions.
Remember that the atmosphere has five distinct layers and plays an important role in supporting life on Earth. The atmosphere provides gaseous resources and maintains liquid water, which is essential for life.
The Atmosphere - Key takeaways
- An atmosphere is the layer of gases surrounding a planet. Earth's atmosphere extends from the surface of the planet to 10,000 kilometres above.
- Earth's atmosphere has five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.
- The atmospheric composition is 78% nitrogen, 21% oxygen, 0.9% argon, and 0.1% other gases.
- Earth's atmosphere plays a critical role in supporting life on Earth. It provides gaseous resources for photosynthesis and respiration. Furthermore, the natural greenhouse effect and atmospheric pressure maintain liquid water on the surface.
- Atmospheric pressure is measured using a scientific instrument called a barometer. The units of measurement of atmospheric pressure are atmospheres. Altitude is inversely proportional to atmospheric pressure.
1. NASA Ozone Watch, What is Ozone?, 2018
2. National Geographic Society, Atmosphere, 2022
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Frequently Asked Questions about The Atmosphere
What are the 5 layers of the atmosphere?
The 5 layers of the atmosphere are (from the surface upwards): the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.
Why is the atmosphere important?
The atmosphere provides gaseous resources for living organisms, and maintains liquid water through atmospheric pressure and the natural greenhouse effect. Without liquid water, there would be no life on Earth.
How do you measure the atmosphere?
Atmospheric pressure is measured using a barometer. The units of measurement for atmospheric pressure are atmospheres (atm).
What are the components of the atmosphere?
The atmosphere is 78% nitrogen, 21% oxygen, 0.9% argon, and the remaining 0.1% is other gases.
What is the size of the atmosphere?
The atmosphere extends from the surface of Earth to 10,000 kilometres above.
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