The atmosphere and oceans are constantly flowing, and this motion is critical to the climate system. What makes them flow? In general, the movement is due to pressure differences — things flow from regions of high pressure to low pressure.
The pressure changes are themselves due to density and height differences — higher density in the air or the oceans leads to higher pressures. The density differences are due to changes in composition and temperature; this works slightly differently for air and water. In air, the important compositional variable is water vapor content — more water means lower density air. When we say more water, we mean that for a given number of molecules in a volume of air, a greater percentage of them are water, and water, as shown below, is lighter than a nitrogen molecule, which is the most abundant molecule in our atmosphere.
Density of Air
Inverse relationship with Temperature:
Higher temp = lower density → rising air
Weight of H2O= 18
Lower temp = higher density → sinking air
Weight of N2 = 28
Inverse relationship with water content:
More water = lower density → rising air
Less water = higher density → sinking air
As indicated above, density differences can cause either rising or sinking of air masses. Because Earth’s gravity decreases as you move away from the surface, there is a kind of equilibrium profile of density with height above the surface as shown by the green curve below:
If we lower the density of air at the surface from A to B, then the air rises from B to C. Then, if we increase the density of air at point C, moving it to D, it will sink back down to point A near the surface.
We start with the movement of the atmosphere, which we will try to make as simple as possible by first concentrating on the flow as seen in a vertical slice from pole to pole. The story begins at the equator, where air is warmed and lots of evaporation adds water to the air, giving it a low density:
This air rises until it gets to the top of the tropopause, which is a bit like a lid on the lower atmosphere. It then diverges, with some of the air flowing north and some flowing south. As it rises and moves away from the equator, the air gets colder, water vapor condenses and rains out and the air grows drier — the cooling and drying both make the air grow denser and by the time it reaches about 30°N and 30°S latitude, it begins to sink down to the surface.
The sinking air is dense and dry, creating zones of high pressure in each hemisphere that are associated with very few clouds and rainfall — these are the desert latitudes. The sinking air hits the ground and then diverges. Some flows south and some flows north; the parts of this divergent flow that return towards the equator complete a loop or a convection cell, a Hadley Cell, named after Hadley, a famous meteorologist. Now, let’s turn our attention to the air that flows away from the equator. Moving along the surface, it warms and pick up water vapor, and so its density decreases and it eventually rises up when it gets to somewhere between 45° and 60° latitude in each hemisphere.
Once again, the rising air runs into the tropopause (which is lower at these higher latitudes) and diverges, with some of it returning toward the equator, thus completing another convection cell called the Ferrel Cell. The air that flows pole-ward sinks down at the poles, creating yet another convection cell known as the Polar Hadley Cell. These convection cells create bands of low and high pressure that roughly follow lines of latitude that exert a big influence on the climate at different latitudes. The air flowing within these convection cells does not simply move north and south as depicted above — the Coriolis effect alters the flow directions, giving us a surface pattern that is dominated by winds flowing east and west.
Note that the boundary between the Polar Hadley Cell and the Ferrel Cell (often called the Polar Front, and associated with the mid-latitude jet stream) is highly variable, with big loops in it. These loops, or waves, change over time to a much greater extent than the boundaries between the other convection cells.