Atmosphere of Earth

The atmosphere is a thick gaseous envelope that wraps around the planet on all sides like a blanket. Earth's gravity holds it in place, and it carries the life giving gases we depend on, such as oxygen and carbon dioxide. Without it there would be no weather, no breathable air and no shield against the harsh radiation of space.

The atmosphere is made up mostly of gases, along with water vapour and tiny solid particles. These are standardised notes you can revisit any time in our study library at /resources, and this topic sits with the rest of our earth science material under Geography.

What the atmosphere is made of

Gases in Earth's atmosphere

The dry air of the atmosphere is a mixture of several gases. By volume, the main ones are:

• Nitrogen: 78.08 per cent
• Oxygen: 20.95 per cent
• Argon: 0.93 per cent
• Carbon dioxide: 0.04 per cent
• Small traces of neon, helium, ozone, hydrogen and other gases

Ozone is present in very small amounts at a height of about 20 to 25 kilometres. It filters incoming solar energy and stops most of the harmful ultraviolet rays from reaching the ground.

Carbon dioxide is a greenhouse gas. It traps some of the heat leaving the Earth and helps keep the average surface temperature at a liveable level of around 15 degrees Celsius. Too much of it, however, traps extra heat and warms the planet.

Water vapour in the atmosphere

Water vapour is water in its gas form. Depending on the region, it makes up anywhere from 0 per cent to about 4 per cent of the air by volume. It decreases from the equator towards the poles as the temperature falls. The warm tropical zone holds the most (about 4 per cent), while polar and desert regions hold the least (about 1 per cent).

The amount of water vapour also drops as you rise higher into the sky. All condensation and precipitation events, such as clouds, fog, dew, rainfall, frost and snowfall, are caused by the moisture in the atmosphere.

Particulate matter

These are tiny solid particles floating in the air, including dust, salt, pollen, smoke, soot and volcanic ash. Most of them are found in the troposphere, the lowest part of the atmosphere.

These particles scatter sunlight. The scattering of solar radiation by them gives us the beautiful colours seen at sunrise and sunset, and the scattering of light by tiny particles and gas molecules is also why the daytime sky looks blue.

Structure of the atmosphere

Although the exact outer boundary of the atmosphere is not sharp, it is usually taken to reach roughly 1600 kilometres above the surface. Based on how pressure and air temperature change with height, the atmosphere is divided into five layers:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

Troposphere

This is the first and lowest layer, and it holds around 75 per cent of the mass of the whole atmosphere. The name comes from two Greek words, tropos meaning turning or mixing and sphaira meaning sphere, which describes the constant churning of gases in this layer.

It reaches about 8 kilometres high at the poles and about 18 kilometres at the equator. It is thicker at the equator because warm air there rises to greater heights. This is where nearly all weather happens, so it is often called the weather zone. Clouds, cyclones, storms, rainfall, fog and hailstorms are almost all confined to this layer.

As you climb through the troposphere the temperature falls at a rate of about 6.5 degrees Celsius per kilometre. This rate of cooling is called the lapse rate. The temperature reaches around minus 45 degrees Celsius above the poles and around minus 80 degrees Celsius above the equator.

What is the tropopause? The boundary between the troposphere and the stratosphere is called the tropopause. It is the topmost part of the troposphere and acts as a transition zone in which the temperature stays roughly constant.

Stratosphere

The stratosphere is the next layer above the troposphere and reaches up to about 50 kilometres above the surface. Its temperature ranges from about minus 51 degrees Celsius at the bottom to about 0 degrees Celsius near the top. The temperature stays steady for a short distance and then rises with height.

This layer is mostly free of clouds and the air is calm, which makes it ideal for jet aircraft to fly in. Thin cirrus clouds are sometimes seen at its lower levels.

What is the ozonosphere? Sitting between about 30 and 60 kilometres up, spanning the stratosphere and lower mesosphere, is a region rich in ozone molecules. This ozone layer absorbs most of the Sun's harmful ultraviolet radiation, which is why life on the surface is protected. Because of the chemical activity here it is also called the ozonosphere or chemosphere, and the temperature rises by about 5 degrees Celsius per kilometre through it.

Mesosphere

This middle layer extends up to about 80 kilometres above the surface. The temperature falls steadily to around minus 100 degrees Celsius at the top, a limit known as the mesopause. Most meteorites and other objects from space burn up as they pass through this layer, which is why we see shooting stars.

Thermosphere

The thermosphere lies just above the mesopause and stretches from about 80 to 400 kilometres. Here the temperature rises very quickly with height. This layer also contains the ionosphere.

What is the ionosphere? This is an electrically charged region within the thermosphere, spanning roughly 80 to 400 kilometres. Its gas atoms are ionised by solar radiation. Because of this electric charge, radio waves sent up from the ground are reflected back to Earth, which is how long distance radio communication works.

The International Space Station and many satellites orbit within this layer. The air here is extremely thin, with gas molecules separated by large distances. The glowing light displays of the Aurora Borealis at the North Pole and the Aurora Australis at the South Pole also occur in this layer.

Exosphere

This is the topmost layer of the atmosphere. It reaches from the top of the thermosphere out to great heights before fading into space. The air is so thin that no real heat can be felt even though temperatures are high. From here, light gases like helium and hydrogen slowly escape into outer space.

Why it still matters

The atmosphere is not a fixed background. Its make up is changing right now, and two of the gases in these notes are at the centre of the biggest environmental stories of our time.

Carbon dioxide is at a record high. In its Greenhouse Gas Bulletin published on 15 October 2025, the World Meteorological Organization reported that the global average concentration of carbon dioxide reached 423.9 parts per million in 2024. The jump of 3.5 parts per million from 2023 to 2024 was the largest single year rise since measurements began in 1957. The WMO said this was driven by continued human emissions, a rise in wildfires, and weaker absorption of carbon dioxide by forests and oceans. That tiny 0.04 per cent slice of the air is exactly the part that is growing, and it is what traps extra heat and warms the planet.

The ozone layer is slowly healing. There is good news too. The WMO reported that the 2025 Antarctic ozone hole was the fifth smallest since 1992 and closed on 1 December, one of the earliest closures in years. Scientists say the ozone layer is on track to recover to its 1980 levels by about 2066 over the Antarctic and by about 2040 over most of the rest of the world. This recovery is credited to the Montreal Protocol, the 1987 global treaty that phased out the chemicals that were destroying ozone. It is one of the clearest examples of countries acting together to fix a shared problem.

Put together, these two stories show why understanding the atmosphere is not just an exam topic. You can follow how these gases link to climate, ecosystems and everyday weather across the wider Learnacy Hub, and revise the full set of earth science notes any time under Geography.

Sources

1. World Meteorological Organization, Greenhouse Gas Bulletin, 15 October 2025: https://wmo.int/sites/default/files/2025-10/GHG-21_en.pdf
2. UN News, "Record rise in carbon dioxide levels during 2024", 15 October 2025: https://news.un.org/en/story/2025/10/1166110
3. World Meteorological Organization, "Small and short-lived 2025 ozone hole confirms long-term recovery trend": https://wmo.int/media/news/small-and-short-lived-2025-ozone-hole-confirms-long-term-recovery-trend
4. NOAA, "2025 ozone hole is 5th smallest since 1992": https://www.noaa.gov/news-release/noaa-nasa-2025-ozone-hole-is-5th-smallest-since-1992

Key takeaways

• Earth's atmosphere is a protective gaseous envelope held by gravity that provides breathable air, creates weather, and shields the planet from harsh space radiation.
• The dry air is primarily composed of nitrogen (78.08%) and oxygen (20.95%), with trace amounts of other gases like argon and carbon dioxide.
• Water vapor and tiny solid particles in the air are responsible for all precipitation, condensation, and the scattering of sunlight that gives the sky its color.
• The atmosphere is structured into five distinct layers based on changes in air pressure and temperature: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
• The lowest layer, the troposphere, contains about 75% of the atmosphere's mass and is where nearly all weather occurs, while the calm stratosphere above it houses the protective ozone layer.

Test yourself

What is the most abundant gas in Earth's atmosphere?

Nitrogen, which makes up 78.08 percent of the dry air by volume.

Why do jet aircraft typically fly in the stratosphere?

The air in the stratosphere is calm and mostly free of clouds.

What is the lapse rate?

The lapse rate is the rate at which temperature decreases as altitude increases in the troposphere, dropping about 6.5 degrees Celsius per kilometer.