Editing Atmospheric circulation (section)

=== Hadley cell ===
{{Main|Hadley cell}}
[[File:IntertropicalConvergenceZone-EO.jpg|thumb|right|The [[ITCZ]]'s band of clouds over the [[Eastern Pacific]] and the [[Americas]] as seen from space]]
The atmospheric circulation pattern that [[George Hadley]] described was an attempt to explain the [[trade wind]]s. The Hadley cell is a closed circulation loop which begins at the equator. There, moist air is warmed by the Earth's surface, decreases in density and rises. A similar air mass rising on the other side of the equator forces those rising air masses to move poleward. The rising air creates a low pressure zone near the equator. As the air moves poleward, it cools, becomes denser, and descends at about the [[30th parallel (disambiguation)|30th parallel]], creating a [[high-pressure area]]. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop of the Hadley cell.<ref>{{Cite journal |last=Baines |first=Peter G. |date=2006 |title=The zonal structure of the Hadley circulation |journal=Advances in Atmospheric Sciences |language=en |volume=23 |issue=6 |pages=869–883 |doi=10.1007/s00376-006-0869-5 |bibcode=2006AdAtS..23..869B |s2cid=122789933 |issn=0256-1530}}</ref> The poleward movement of the air in the upper part of the troposphere deviates toward the east, caused by the [[coriolis acceleration]]. At the ground level, however, the movement of the air toward the equator in the lower troposphere deviates toward the west, producing a wind from the east. The winds that flow to the west (from the east, easterly wind) at the ground level in the Hadley cell are called the trade winds.

Though the Hadley cell is described as located at the equator, it shifts northerly (to higher latitudes) in June and July and southerly (toward lower latitudes) in December and January, as a result of the Sun's heating of the surface. The zone where the greatest heating takes place is called the "[[thermal equator]]". As the southern hemisphere's summer is in December to March, the movement of the thermal equator to higher southern latitudes takes place then.

The Hadley system provides an example of a thermally direct circulation. The power of the Hadley system, considered as a heat engine, is estimated at 200 tera[[watt]]s.<ref name="RERLHadley">{{cite journal |author=Junling Huang and Michael B. McElroy|title=Contributions of the Hadley and Ferrel Circulations to the Energetics of the Atmosphere over the Past 32 Years|journal=Journal of Climate|issue=7| volume=27 |pages=2656–2666 |year=2014 |doi=10.1175/jcli-d-13-00538.1|bibcode = 2014JCli...27.2656H |s2cid=131132431 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:11857805|doi-access=free}}</ref>