The homosphere is the layer of an atmosphere where the bulk gases are homogeneously mixed due to turbulent mixing or eddy diffusion. The bulk composition of the air is mostly uniform so the concentrations of molecules are the same throughout the homosphere. The top of the homosphere is called the homopause, also known as the turbopause. Above the homopause is the heterosphere, where diffusion is faster than mixing, and heavy gases decrease in density with altitude more rapidly than lighter gases. Some of the processes driving this uniformity include heating convection and air flow patterns. In the troposphere, rising warm air replaces higher cooler air which mix gases vertically. Wind patterns push air across the surface mixing it horizontally. At higher altitudes, other atmospheric circulation regimes exist, such as the Brewer-Dobson circulation in the terrestrial stratosphere, which mixes the air. In Earth's mesophere, atmospheric waves become unstable and dissipate, creating turbulent mixing of this region. Earth's homosphere The Earth's homosphere starts at the Earth's surface and extends to the turbopause at about 100 km. It incorporates all of the troposphere, stratosphere, mesosphere, and the lower part of the thermosphere. Chemically the homosphere is composed of 78% nitrogen, 21% oxygen, and trace amounts of other molecules, such as argon and carbon dioxide. It contains over 99% of the mass of the Earth's atmosphere. The density of air decreases with height in the homosphere. By definition & notably, it also includes a 10 km thick band of elemental mesosphere sodium. Variations in concentration One large-scale exception to effective mixing is the ozone layer, centered at about 20 - 30 km in altitude, where the concentration of O3 is much higher than in the rest of the atmosphere. This is due to incoming ultraviolet light, which turns O2 into O3. This created ozone itself blocks most ultraviolet light from penetrating to lower layers of the atmosphere and creating similar levels of ozone there. With a half-life of about a day at room temperature, ozone breaks down before it can mix completely with the lower levels of the atmosphere. The ozone hole is a relatively stable structure caused by a combination of pollution and antarctic wind patterns in the stratosphere. Water vapor concentration (humidity) varies considerably, especially in the troposphere, and is a major component of weather. Water evaporation is driven by heat from incoming solar radiation, and temperature variations can cause water-saturated air