Earth's atmosphere

Key points

• Earth is surrounded by a mixture of gases called the atmosphere. The composition of the atmosphere is $78\mathrm{\%}$‍ nitrogen and $21\mathrm{\%}$‍ oxygen, with the remaining $1\mathrm{\%}$‍ 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 $95\mathrm{\%}$‍ 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 $85\mathrm{°}$‍C ($-120\mathrm{°}$‍F)!
  • 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.