The Atmosphere

Breathing comes naturally to humans. If it didn't, we'd forget to breathe and suffocate! We breathe to take in oxygen, a gas used in respiration to produce energy. Oxygen is found in the atmosphere – the bundle of gases surrounding Earth like a protective blanket. Thanks to this blanket, we don't need to worry about breathing. If you're interested in the other cool things that the atmosphere does to support us, keep reading!

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?

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:

1. The troposphere

2. The stratosphere

3. The mesosphere

4. The thermosphere

5. 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.

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.

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!

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 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 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.

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.

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.

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.

Measurement of the Atmosphere

The measurement of Earth's atmosphere is taken using a barometer.

Barometers measure atmospheric pressure in units of measurement called atmospheres (atm).

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

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.

^{1. NASA Ozone Watch, What is Ozone?, 2018}

^{2. National Geographic Society, Atmosphere, 2022}