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. |