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AP Enviro



Unit 4

4.4 Earth's Atmosphere

2 min readmay 19, 2020

Cody Williams

Major Gases

The major gases of the atmosphere include oxygen and nitrogen. These two gases make up most of the atmosphere. Nitrogen (N2) is released into the atmosphere through denitrification. Oxygen (O2) is released through photosynthesis.

Water vapor (H2O) can also be a major gas in the atmosphere. In areas such as the equator, over oceans, and forests, the percentage of water vapor in the atmosphere can be up to 4%. However, in polar regions and deserts, it is closer to 0%. 

There are also many other gases that play important roles in the atmosphere including methane (CH4), carbon dioxide (CO2), nitric oxide (N2O), and ozone (O3). Carbon dioxide, methane, and nitric oxide are all greenhouse gases that trap heat in Earth’s atmosphere. All of these gases are released when fossil fuels are burned.

Ozone (O3) is a big component of the atmosphere. This layer of ozone absorbs UV radiation given off by the sun. This layer reduces the amount of UV radiation that reaches the troposphere which is beneath the ozone layer. 


Image Courtesy of Wikimedia

Layers of the Atmosphere


The atmosphere is composed of the troposphere, stratosphere, mesosphere, and thermosphere. 

The first layer of the atmosphere (closest to the earth) is the troposphere. The troposphere starts at ground level and goes up to around 10 kilometers in altitude. Within this layer, our weather occurs and most of the living organisms live here. As you go higher in this layer, the temperature decreases. Temperature gradients in the atmospheric layers are important because they determine where the layers are.

Above the troposphere is the stratosphere that goes from 10 kilometers in altitude to 50 kilometers in altitude. The important thing about this layer is that it houses the ozone layer. As mentioned above, the ozone layer absorbs UV rays from the sun. Due to this, the temperature actually increases as you go up in this layer. Thanks to the ozone layer, the troposphere doesn’t receive 100% of the UV rays given off by the sun. This plays a big role in the temperature in the troposphere

Next is the mesosphere that goes from 50 kilometers to 80 kilometers in altitude. In this layer, the temperature decreases as you increase in altitude. This layer is very cold, and temperatures in the mesosphere can reach below -80 °C (-115 °F).

Finally, the thermosphere goes from about 80 kilometers to 100 kilometers in altitude. This layer is the thinnest layer of the atmosphere. Another name for the thermosphere is the ionosphere because this layer often traps protons, electrons, and other ions given off by the sun. As you increase in altitude in this layer, the temperature increases because this layer receives a lot of UV radiation and energy from the sun.

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Unit 1: The Living World: Ecosystems

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