Viewed from Earth’s surface, the atmosphere seems homogeneous, constantly mixed by winds and convection. In fact, the first 83 kilometers above Earth is called the homosphere because the air is kept evenly mixed. Half of the mass of the atmosphere lies less than 6 kilometers above Earth. Even this diffuse veil has differences that reflect variations in solar radiation, heating by Earth’s surface, gravity, evaporation and incoming cosmic radiation. Thus, for example, gravity holds the heavier elements closer to the ground, while lighter ones, such as helium, are found in greater relative abundance at extreme altitudes. The lowest level of the homosphere is the troposphere, where life is found and weather occurs. It averages 11 kilometers above Earth but is 8 kilometers at the poles and 16 kilometers above the equator. Above the troposphere is the stratosphere (11 to 48 kilometers above Earth), where gases become thinner; this region contains the ozone layer, between 16 and 48 kilometers above Earth. Above the stratosphere lies the mesosphere, 48 to 88 kilometers above Earth.
Earth’s air pressure is highly anomalous. A planet’s primordial atmosphere is determined by its surface gravity, its distance from its host star, and its host star’s effective temperature. 2
Diagrams of Earth’s wind belts remind us of the importance of differences in air pressure existing from one location to another. Wind belts result from the tendency of air to flow from high to low pressure. A large portion of earth’s population exists in wind belts termed prevailing westerlies or northeast/southeast trade winds. Wind belts converge or diverge in their effort to equalize pressure conditions. In terms of our healthy, dynamic weather system, these effects are necessary. Absence of differences in pressure would result in the absence of wind, the absence of precipitation-producing storm systems and the absence of a mechanism for distributing life giving water where it is needed. It is not difficult to imagine that earth life would be very different if it would exist at all. 3
1. THE SACRED BALANCE, page 71