HiSET Science: Structure of the Atmosphere

Layers of the Atmosphere
The atmosphere consists of 78% nitrogen, 21% oxygen, and 1% argon. It also includes traces of water vapor, carbon dioxide and other gases, dust particles, and chemicals from Earth. The atmosphere becomes thinner the farther it is from the Earth's surface. It becomes difficult to breathe at about 3 km above sea level. The atmosphere gradually fades into space.
The main layers of the Earth's atmosphere (from lowest to highest) are:
Troposphere (lowest layer): where life exists and most weather occurs; elevation 0–15 km
Stratosphere: has the ozone layer, which absorbs UV radiation from the sun; hottest layer; where most satellites orbit; elevation 15–50 km
Mesosphere: coldest layer; where meteors will burn up; elevation 50–80 km
Thermosphere: where the international space station orbits; elevation 80–700 km
Exosphere (outermost layer): consists mainly of hydrogen and helium; extends to ~10,000 km

Tropospheric Circulation
Most weather takes place in the troposphere. Air circulates in the atmosphere by convection and in various types of 'cells.' Air near the equator is warmed by the Sun and rises. Cool air rushes under it, and the higher, warmer air flows toward Earth's poles. At the poles, it cools and descends to the surface. It is now under the hot air, and flows back to the equator. Air currents coupled with ocean currents move heat around the planet, creating winds, weather, and climate. Winds can change direction with the seasons. For example, in Southeast Asia and India, summer monsoons are caused by air being heated by the Sun. This air rises, draws moisture from the ocean, and causes daily rains. In winter, the air cools, sinks, pushes the moist air away, and creates dry weather.