The atmosphere is the layer of air surrounding Earth. The atmosphere consists of four layers: the troposphere, stratosphere, mesosphere, and thermosphere. Figure 6.1 shows the different layers and how the temperature changes with height from the ground to the very thin top of the atmosphere. The troposphere, the lowest layer of the atmosphere, is where we live and where weather occurs. Temperatures generally decrease with height. The boundary between the troposphere and the stratosphere is called the tropopause, which is higher in the tropics and lower at mid to high latitudes. The temperature in the stratosphere generally increases with height because it encompasses the ozone layer which absorbs ultraviolet (UV) rays from the Sun. In the mesosphere the temperature again decreases with height and it is well mixed with a similar composition to the troposphere, except there is little water vapor. The thermosphere is the uppermost layer of the atmosphere where the temperature increases with height because it is directly heated by the Sun. Above about the mesopause, which divides the mesosphere from the thermosphere, the molecules of the atmosphere become ionized by the active very shortwave radiation from the Sun (X-rays, UV) or cosmic rays (which may come from elsewhere) that do not make it to the lower levels, and their abundance is greatly affected by solar activity. The ionosphere, which overlaps with the other layers defined by temperature, plays a major role in transmission of radio waves over great distances around the world. At these very high levels of the atmosphere, above about 80 km altitude, the composition of the atmosphere is very different than at lower levels.