Layers of the Atmosphere

The Atmosphere is a blanket of gases that surround the Earth and is vital for human survival. The atmosphere consists of several gases and is a combination of 78% nitrogen, 21% oxygen, 1% water vapor and trace gases like carbon monoxide, carbon dioxide, helium, methane and argon.  The atmosphere is our life-support system, absorbing radiation from the Sun and sheltering us from the vacuum of space. The atmosphere controls the Earth’s temperature, winds and water supply.

Layers of the Atmosphere

earth atmosphere layers
Major layers of Earth’s atmosphere (not to scale).

The atmosphere is made up of four distinctive layers. Moving from the Earth’s surface upwards in altitude is the troposphere, stratosphere, mesosphere and thermosphere. The thermosphere can also be divided into different sections known as the Ionosphere and the Exosphere.  Between layers, there are three distinct boundary zones known as the tropopause, the stratopause, and the mesopause.

The Troposphere 

The Troposphere is the layer that is closest to the Earth’s surface and contains the air that we breathe. It ranges in thickness from approximately 8 km at the poles to approximately 16 km at the Equator. This layer is denser due to compression of the other layers above it weighing it down. Most of the world’s weather takes place in the troposphere. With regards to temperature, generally it will decrease with altitude. The troposphere is heated from below when sunlight warms the surface, which then radiates the heat into the air above it. This means that the Earth’s surface warms the air closest to it so the further from the surface, the cooler the air becomes. As warm air rises, air movement from the surface upwards can occur, which causes turbulence. As the surface of the Earth is warmed by the Sun’s electromagnetic rays, water vapor rises from the surface, evaporates, cools and condenses to form clouds. The tropopause is the boundary between the troposphere and the stratosphere. Stable temperatures mark this boundary along with fast moving currents of air called jet streams. Jet streams are bands of strong winds which contribute to moving weather systems around the globe.

atmosphere layers from the ISS
The troposphere (orange) is visible from the International Space Station. Source: NASA.

The Stratosphere

The Stratosphere is the second layer of the atmosphere found approximately 16 km to 50 km from the Earth’s surface. Here temperature increases with altitude due to the ozone layer, found in the lower section of the Stratosphere. The ozone layer (or ozonosphere) absorbs UV radiation, which warms the air, and as it rises upwards, it warms the upper Stratosphere. The ozone layer is extremely important to people on the surface as it absorbs harmful UV rays that could potentially damage living cells and tissues. As you move upward through this layer, the atmospheric conditions become warmer and more stable with less air turbulence. As a consequence, the stratosphere is well-suited for high-speed and long distant flights. As the warm air rises, it does not have an opportunity to cool and condense, which means clouds cannot form. The stratosphere is separated from the layer above, the mesosphere, by the stratopause.

The Mesosphere

The third level is the mesosphere. This layer extends approximately 50- 90 km in height from the Earth’s surface. Here, temperature decreases with altitude as the air is warmed by the stratosphere. As we move up through the Mesosphere, the distance from the stratosphere obviously will increases, therefore the temperature will decrease as we move through this layer. Temperatures can range from -5°C to -90°C at 90 km in height from the Earth’s surface. This can be due to the absence of any materials that could trap the incoming energy from the sun, water vapor, dust or ozone. However, these figures are approximations as this layer is hard to study. Weather balloons and planes cannot reach this layer, and satellites go beyond the mesosphere. The only data collected from this layer was done using sounding rockets, which make do not go into orbit. As a result, the data is limited. The mesosphere is also the layer where meteors burn up upon entering the Earth’s atmosphere. From Earth, we can see these as shooting stars. The mesopause is the boundary between the mesosphere and the final layer, the thermosphere.

The Thermosphere

The final layer of the atmosphere is the thermosphere. This layer extends from 90 km to approximately 1,000 km upwards. Temperatures here increase with altitude due to small amounts of oxygen in the air absorbing UV radiation. The oxygen molecules receive substantial amounts of energy from the Sun, leading temperatures to rise to approximately 1500°C. The thermosphere can be divided into different sections: the Ionosphere which is the lower region and the exosphere, the highest layer. The ionosphere is a layer approximately 90 km to 600 km, where the Sun’s energy is so strong that it breaks up molecules, leaving electrons and ions. When the Sun streams out high energy particles (as in coronal mass ejection or CME), they interact with these electrons and ions, creating light in the form of an aurora. Due to Earth’s magnetic field, the particles are attracted to the poles, where auroras can be seen more frequently. The exosphere is the highest level of the thermosphere. This level extends to 10,000 km above the Earth’s surface and much of it is commonly considered space. Hydrogen and helium are the main gases found here, along with many near-Earth satellites.

ISS thermosphere
The International Space Station (ISS) orbits in the thermosphere.

The Importance of the Atmosphere

The Atmosphere is vital for life on planet Earth. The atmosphere is critical to regulate Earth’s climate and distribute heat around the globe. However, human actions are beginning to significantly change the composition of the atmosphere through burning fossil fuels, deforestation and pollution. The level of greenhouse gases in the troposphere is increasing, causing the Earth’s temperature to rise by trapping more of the Sun’s incoming short-wave radiation. Humans are also responsible for contributing several new air pollutants to the atmosphere, such as nitric oxide from the exhaust of cars and airplanes, and CFC’s (chlorofluorocarbons), which are used as refrigerants. These gases are man-made and are believed to contribute to climate change and depletion of the ozone layer.

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