Atmospheric and ocean physics

Both the atmosphere and the ocean are fluids sitting in a gravitational field, so both obey hydrostatic balance: at any depth or altitude, the pressure supports the weight of everything above it. Because air is compressible and obeys the ideal gas law, this means pressure and density fall off roughly exponentially with altitude — roughly halving every 5.5 km — rather than dropping off linearly. Water is nearly incompressible, so the ocean's pressure instead grows almost linearly with depth, by about one atmosphere for every 10 metres.

The atmosphere's vertical temperature structure comes from a different piece of the same physics. When an air parcel rises, it moves into a region of lower pressure and expands; that expansion does work on its surroundings, and with essentially no heat exchanged on the timescale of the rise, the parcel's internal energy — and therefore its temperature — falls. This adiabatic cooling, not distance from the warm ground, is why the troposphere gets colder with altitude, and it is the same physics (the first law of thermodynamics applied to an expanding gas) that governs any adiabatic process.

The ocean has no real equivalent of pressure-driven adiabatic cooling, since water barely compresses, but it develops its own layered structure for a related reason: density. Sunlight warms only the surface layer, and evaporation and precipitation change surface salinity, so temperature and salinity together set up a stable density gradient — warm, often less salty water near the surface sitting on cold, denser water below, with a sharp transition layer (the thermocline) in between. Like the atmosphere's layering, this structure does not mix away on its own; it takes energy — wind, tides, or a density difference strong enough to overturn — to break it up.