lecture note 7

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19 Jul 2010
- Two key factors determine the distribution of ecosystems on Earth more than any other:
(1) precipitation, and (2) temperature
- Tropospheric air masses are circulated by solar energy (~1% of the total average energy
input into Earth creates the global wind movements); this creates the main patterns of
- Air circulation occurs via a set of three distinct cells in the two hemispheres: the
Hadley, Ferrell, and Polar cells
- The Hadley cell (active between 0o to 30o N. and S. latitude; here we describe it for the
Northern hemisphere): the sun heats air at the equator, causing it to expand and rise; it
takes with it large quantities of moisture; as air masses rise, they cool adiabatically (the
adiabatic lapse rate: 100C/km for dry air, 60C/km for moist air) and release their
moisture content in the form of precipitation (thisfeeds the tropical rainforests); the
high-altitude air masses move away from the equator to the North, lose energy, and
descend at ~300 N. latitude (they carry almost no moisture at this point; most of the
world’s deserts are located here, in the semi-permanent high-pressure/low-precipitation
zone that results); upon descending, the air masses split to move as surface winds, with
onearm moving North (these are the Westerlies; the jet stream is a part of this) and the
other moving South, back to the equator (as the Northeast trade winds; these are the
winds that brought Columbus to America); the zone where the Hadley cells of the two
hemispheres meet is called the Intertropical Convergence Zone (ITCZ; there is a dead
zone of very little wind at the surface, known as the doldrums, right where the cells meet;
the similarly windstill zones at the 300 latitudes are known as the horse latitudes)
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