By Shapley H.
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Extra resources for A Contribution to the Study of Galactic Dimensions
For both atmospheres and oceans in a state of rest, the pressure, p, must support the weight of the fluid above it. This is called a state of hydrostatic equilibrium. With increasing height in the atmosphere, the density decreases as the pressure decreases (Boyle's Law). With increasing depth in the ocean, this also holds true but the density change is slight. Consider the atmospheric case. In differential form, the weight of the air (mass times the acceleration of gravity g) in a small volume element dV is gdM, where dM is the mass of the air inside the volume.
An important property of an atmosphere or ocean is its tendency to arrange itself into a vertically stratified and horizontally homogeneous medium. Quasi-horizontal motions tend to homogenize properties along constant-pressure (more correctly constant-entropy) surfaces. Horizontal variations do occur (after all, this is the origin of weather), particularly in temperature, but usually on spatial scales much greater than a scale height. However, all over the Earth, the pressure and density at sea level are nearly the same on a horizontal plane.
Again, as for the solar spectrum, the deviations are attributed to the nonisothermal character of the Earth's atmosphere. The spectral regions of minimum emission arise from the upper cold regions of the Earth's troposphere where the opacity of the overlying regions is ~ 1 . Those of highest emission originate from the warm surface in transparent spectral regions ("windows"), with the exception of the Antarctic spectrum, where the surface is actually colder than the overlying atmosphere (see Fig.