Earth is surrounded by its atmosphere, which is the body of air or gases that protects the planet and enables life. Most of our atmosphere is located close to Earth's surface, where it is most dense. It has five distinct layers. Let's look at each, from closest to farthest from the Earth.
The layer of the atmosphere closest to the Earth is the troposphere. It begins at the surface of the Earth and extends out to about 4 to 12 miles (6 to 20 km). This layer is known as the lower atmosphere. It's where weather happens and contains the air humans breathe. The air of our planet is 79 percent nitrogen and just under 21 percent oxygen; the small amount remaining is composed of carbon dioxide and other gases. The temperature of the troposphere decreases with height.
Above the troposphere is the stratosphere, which extends to about 31 miles (50 km) above the Earth's surface. This layer is where the ozone layer exists and scientists send weather balloons. Jets fly in the lower stratosphere to avoid turbulence in the troposphere. Temperature rises within the stratosphere but still remains well below freezing.
From about 31 to 53 miles (50 to 85 km) above the surface of the Earth lies the mesosphere, where the air is especially thin and molecules are great distances apart. Temperatures in the mesosphere reach a low of -130 degrees Fahrenheit (-90 C). This layer is difficult to study directly; weather balloons can't reach it, and weather satellites orbit above it. The stratosphere and the mesosphere are known as the middle atmospheres.
The thermosphere rises several hundred miles above the Earth's surface, from 56 miles (90 km) up to between 311 and 621 miles (500-1,000 km). Temperature is very much affected by the sun here; it can be 360 degrees Fahrenheit hotter (500 C) during the day than at night. Temperature increases with height and can rise to as high as 3,600 degrees Fahrenheit (2000 C). Nonetheless, the air would feel cold because the hot molecules are so far apart. This layer is known as the upper atmosphere, and it is where the auroras occur (northern and southern lights).
Extending from the top of the thermosphere to 6,200 miles (10,000 km) above Earth is the exosphere, where weather satellites are. This layer has very few atmospheric molecules, which can escape into space. Some scientists disagree that the exosphere is a part of the atmosphere and instead classify it actually as a part of outer space. There is no clear upper boundary, as in other layers.
Between each layer of the atmosphere is a boundary. Above the troposphere is the tropopause, above the stratosphere is the stratopause, above the mesosphere is the mesopause, and above the thermosphere is the thermopause. At these "pauses," maximum change between the "spheres" occur.
The ionosphere isn't actually a layer of the atmosphere but regions in the layers where there are ionized particles (electrically charged ions and free electrons), especially located in the mesosphere and thermosphere. The altitude of the ionosphere's layers changes during the day and from one season to another.