The Earth's atmosphere shields life from deadly ultraviolet radiation from the sun and provides the planet with stable temperatures. It contains a number of layers, the most well known of which are the troposphere, stratosphere, mesosphere and thermosphere. The vast majority of the weather occurs in the troposphere, but some clouds can appear higher up in the stratosphere and mesosphere.
Life forms on the planet inhabit the troposphere, the lowest level of the atmosphere, which extends from the surface to between 7 and 20 kilometers (4 to 12 miles) above it. It creates almost all the known weather phenomena, and clouds that reside there generate rain, hail and snow. Stratus clouds are the lowest type found in the troposphere; they are often found at ground level as fog or mist. Displaying a dull gray appearance, they rarely produce any precipitation.
The stratosphere, where jetliners fly, can be found in the zone between 20 and 50 kilometers (12 to 31 miles) from the surface. Water vapor can only be found at a very low concentration in the stratosphere, making the presence of clouds very rare. Volcanic eruptions, however, can eject vast amounts of dust into the stratosphere, and this sometimes combines with ice particles to produce nacreous clouds that often have a colorful appearance.
The mesosphere can be found between 50 and 85 kilometers (31 to 53 miles) from the surface. Its position makes it very difficult for scientists to study, since it is too high for balloons or planes to fly in and yet too low for orbital spacecraft. This makes the mesosphere one of the most poorly understood regions of the atmosphere. Noctilucent clouds were found to exist within the mesosphere in the late 1800s. These special clouds only form when water vapor is released by methane in a chemical reaction. The increase in methane within the Earth's atmosphere has led to an increase in the observation of noctilucent clouds.
The thermosphere extends from 90 kilometers (56 miles) to between 500 and 1,000 kilometers (310 and 620 miles) above the Earth's surface. Although it is considered a part of the Earth's atmosphere, the air density is so low that it can be considered space. The International Space Station orbits within the thermosphere at an altitude of approximately 370 kilometers. No clouds are found within the thermosphere.
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