The density of the gas molecules per unit of air volume varies throughout the atmosphere because gravity pulls its gas molecules toward the earth’s surface. About 75-80% of the earth’s air mass is found in the atmospheric layer closest to Earth’s surface – the troposphere. This layer extends about 11 miles above sea level at the equator and 4 miles above sea level over the poles. Most of the weather we experience occurs in this layer. Air currents, winds, and concentrations of CO₂ and other greenhouse gases in the troposphere play a major role in the planet’s weather and climate.

The layers of the atmosphere are based on differences in temperature as altitude increases. Because of radiation from Earth and the objects on it, the troposphere is warmer at sea level (0 km altitude) and cools as altitude increases.

Moving away from Earth, the next layer of the atmosphere is the stratosphere. The stratosphere contains a layer of air with a high concentration of ozone molecules (O₃), called the ozone layer. Stratospheric ozone is formed when oxygen molecules (O₂) in this layer interact with ultraviolet (UV) radiation from the sun. The ozone layer keeps about 95% of the sun’s harmful UV radiation from reaching the Earth’s surface. This UV filtering effect allows life to exist on Earth and protects us from sunburn, skin and eye cancers, cataracts, and damage to our immune systems. This absorption of UV also makes the stratosphere warm as altitude increases, a trend opposite to that of the troposphere.

Above the stratosphere is the mesosphere and then the thermosphere. Without any heat-absorbing materials, the mesosphere is the coldest layer of the atmosphere.

The thermosphere is sometimes broken down further into the ionosphere (an area of highly charged particles where auroras occur) and the exosphere (where many satellites orbit). Despite this distinction, the entire thermosphere warms as altitude increases.

Although there is no distinct beginning or end to any of the layers, the transition from one to another is known as a “pause”. For example, the transition from troposphere to stratosphere is called the tropopause. The stratopause divides the stratosphere and mesosphere and the mesopause divides the mesosphere and thermosphere. There is no pause after the thermosphere because it blends into space as the concentration of atmospheric molecules gets lower and lower.