4.4 Earth's Atmosphere

The Earth's atmosphere is mostly composed of oxygen and nitrogen. Nitrogen (N2) is released into the atmosphere through denitrification (recall the nitrogen cycle!). Oxygen (O2) is released through photosynthesis and plants.

Water vapor (H2O) can also be a major gas in the atmosphere. It is notably more concentrated around the equator than around the poles (where humidity and tropic-like conditions are common).

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. Since humans have burned an abundance of fossil fuels, greenhouse gas emissions are high and our planet is warming.

Ozone (O3) absorbs harmful UV radiation given off by the sun. This layer reduces the amount of radiation that reaches the troposphere which is beneath the ozone layer. The ozone layer prevents a lot of negative side effects from the sun, but was significantly damaged by our use of CFCs (chlorofluorocarbons).

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, about the average height of a cruising airplane. Within this layer, temperature decreases as altitude increases, as all weather occurs in this atmospheric layer.

Above the troposphere is the stratosphere that goes from 10 kilometers in altitude to 50 kilometers in altitude. The stratosphere is mainly composed of Earth's ozone layer, which is used for protection from UV rays. 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 stratosphere, whose temperature increases with altitude unlike 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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