Excerpts from Jim Conrad's
from the February 16, 2007 Newsletter issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
CONTEMPLATING THE HADLEY CELL
When I'm with visitors and we have a good view of a broad valley of scrubby, semidesert vegetation with lots of cacti, agaves and such, I can't keep myself from talking about the Hadley Cell. The Hadley Cell is named after George Hadley, who lived from 1685 to 1768. Let me tell you about it.
Visualize the Earth suspended in space. The Sun's energy falls most directly on equatorial regions, so those regions heat up much more vigorously than areas closer to the poles. Hot air rises, and rising air cools. Since cool air holds less moisture than hot air, rising air at the Equator loses much of its moisture as rain, thus accounting for the stormy, rainy tropics.
When the recently-dried-out equatorial air ascends as high as it can it branches into two vast rivers of air -- one river flowing northward, the other southward.
Now, if you visualize these Earth-size rivers of air flowing toward their respective Poles, you should see that the amount of air rushing toward the Poles remains the same, but, below the air, the width of the Earth at that latitude diminishes drastically. At the Poles themselves, which are infinitely small, theoretical entities, there's no land at all.
Therefore, at some point the vast rivers of sky-high air flowing toward their respective poles have to do something. Since the air can't go higher, or east or west because of all the rest of the air crowding at the sides, it can only descend. In fact, both rivers descend to the Earth, one at about 30° North, the other at about 30° South.
Remembering that this air was dried out when it rose at the Equator, it shouldn't surprise us to find arid lands where it descends. In fact, at about 30° North, behold the Sahara, the Arabian Desert, western India's Thar desert and our own Chihuahuan Desert in northern Mexico. At about 30° South, behold southern Africa's Kalahri, Australia's various deserts, and Chile's Atacama. Of course here and there local exceptions to this rule are found: Some deserts are caused by other than dried-out air descending from the sky, and some places at 30° latitude have lots of rain because of ocean currents and the like. Still, isn't it something how all those major deserts occur exactly where theory says they ought to?
Much of the descending dry air flows back toward the Equator, to take the place of heated air rising there. The circulating pattern consisting of that air, the rising equatorial air, the sky-air flowing away from the Equator, and the air descending at 30° North & South Latitudes -- that's the Hadley Cell. It explains so many things.
The Earth rotating beneath the descending air is responsible for our weather fronts arriving from generally eastern or western directions, not directly from the south or north.
Visualizing it all as a dynamic system consisting of the Hadley Cell, the rotating Earth deflecting descending air, air gaining and losing moisture, and more, requires effort. But it's a majestic concept worth working toward.
You can read more about the Hadley Cell and see a good diagram of it halfway down a fine NASA page at http://mynasadata.larc.nasa.gov/Cloud_Compare.html.
Another important climatological feature associated with the Hadley Cell is the Intertropical Convergence Zone. This, the Hadley Cell and other phenomena influencing the General Circulation of the Atmosphere (GCA) are discussed and illustrated at a University of Florida page at http://ess.geology.ufl.edu/usra_esse/ENSO_Atmosphere.html.
Why didn't they tell me about these things back when I was taking Climatology 101 in college? I think these concepts were so large and revolutionary that the conservative professorship at my school simply felt awkward dealing with them. Certainly that was the case with Continental Drift. I acquired a B.Sc. in Geology just as Continental Drift was being introduced to the general public in Scientific American magazine, but during several years of taking geology classes only one of my professors in only one class spoke of it for only about ten minutes.