I'm a forest ecologist and college professor who has been teaching about the Earth's convection cell circulation for many years. Explaining Hadley cells is very straightforward with differences in solar radiation and adiabatic thermodynamics.
I would like to offer my hypothesized rationale below, but I would appreciate if anyone can provide a more detailed/complete explanation (or correction!)
- proper vocabulary and explanation of physical processes a plus!
Wind travelling downward at 30o latitude from a Hadley cell will deflect in multiple directions (poleward and toward the equator).
The portion of this air moving poleward along earth's surface will be warmer than any air it's encountering as it moves closer to the pole. As a result of being warmer than the more polar air it runs into, it will begin to rise -- thus, around 60o latitude, the warmer air moves to higher atmospheric levels ("hot air rises").
As that warmer air rises high into the atmosphere at 60 degrees latitude, it's going to cool. Cool air wants to sink. But it can't sink at 60 degrees because all that warm air is pushing up. So instead, as that warm air cools, it's going to spread out both poleward and toward the equator (where it meets less resistance). Kind of like this image of water is doing (only with air):
As that cooler air continues to cool heading toward the equator, it will eventually come back down to earth around 30o -- thus completing the Ferrel cell.
However, the air that moves poleward in the atmosphere will continue cooling as it moves poleward -- the cold air sinks at the pole (90 deg). When it sinks it will hit the earth's surface and spread toward the equator. Eventually it warms enough that it will rise again around 60 deg latitude (helped from the warm air it meets from the Ferrel cell). this completes the polar cell.
Again, if you could provide a more detailed (and accurate) explanation, that would be much appreciated!