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Earth's atmosphere

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Review your understanding of the composition and layers of Earth's atmosphere in this free article aligned to AP standards.

Key points

  • Earth is surrounded by a mixture of gases called the atmosphere. The composition of the atmosphere is 78, percent nitrogen and 21, percent oxygen, with the remaining 1, percent consisting of water vapor, argon, carbon dioxide, and small amounts of other gases.
  • Earth’s atmosphere consists of five distinct layers that are distinguished by temperature gradients—the layers alternate between having temperatures that increase or decrease with altitude. The five principal layers of the atmosphere are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
    • Troposphere: The troposphere is the layer closest to Earth. It is the densest layer (i.e., contains the most air particles), and is where most of Earth’s weather and cloud formation occurs. The troposphere is heated primarily by energy from the sun radiating off the Earth’s surface. This, along with the decrease in pressure that occurs with altitude, means that the troposphere has a temperature gradient that decreases with altitude.
    • Stratosphere: The stratosphere is the layer above the troposphere. It has a concentrated region of ozone gas called the ozone layer, which keeps about 95, percent of the sun's harmful UV radiation from reaching the Earth’s surface. Ozone molecules absorb UV radiation and release heat, which causes the stratosphere to have a temperature gradient that increases with altitude.
    • Mesosphere: The mesosphere is the layer above the stratosphere. The mesosphere is heated primarily by the stratosphere below, so it has a temperature gradient that decreases with altitude. The mesosphere is one of the coldest places on Earth. The average temperature is around minus 85, degreeC (minus, 120, degreeF)!
    • Thermosphere: The thermosphere is the layer above the mesosphere. It has a very low density of gas molecules. These molecules absorb highly energetic radiation from the sun, so the thermosphere has a temperature gradient that increases with altitude.
    • Exosphere: The exosphere is the highest layer of Earth’s atmosphere, where there is an extremely low density of gas molecules. These molecules often escape into space.
A graph includes an x-axis with temperature in degrees celcius, a left y-axis with altitude in kilometers, and a right y-axis with altitude in miles. A diagram of the five layers of the atmosphere is drawn on the graph. The altitude from 0 to 11km is labeled troposphere. A line at 11km is labeled tropopause. This layer includes an image of clouds and mountains. The altitude from 12-48km is labeled stratosphere. A line at at 48km is labeled stratopause. The altitude from 49km to 85km is labeled mesosphere. A line at 85km is labeled mesopause. The altitude from 85km to130km is labeled thermosphere.
Earth's atmospheric layers are distinguished by having temperatures that either increase or decrease with altitude. Image credit: Average temperature profile for the lower layers of the atmosphere, [Public domain].

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