Global Weather Machine
by Mark Hoover
We live in an ocean of air, seething and flowing around us,
changing-sometimes violently-every day. In the heart of this swirling
machinery of rain clouds and jetstreams, hot desert winds and frozen arctic
storms, there is one constant: change. A trillion and a half days have
passed since the Earth was born in a spinning disk of stardust, and no two
of those days have ever had the same weather.
Driven by the heat of the sun, weather is an interlocking system of cycles.
Water evaporates, rises, cools, and falls as rain, only to evaporate once
again. The sun rises and sets every day, with the air warming and cooling
in response, and the cycle endlessly repeating. Low pressure systems suck
high pressure systems into their vacuum, creating spinning masses of wind
and clouds bigger than Texas; these cyclones are swept across the skies by
persistent high-speed winds miles up in the atmosphere, rivers of air in a
relentless race around the globe. Weather, in all its cycles and clashes,
arises from a simple fact: the sun heats some parts of the Earth more than
The major surface wind bands of Earth. Each hemisphere is divided into
three belts. The path of a storm greatly depends upon the wind belt in
which it is located. The easterly (west-blowing) trade winds of both
hemispheres collide near the equator. This Intertropical Convergence
Zone (ICTZ) can be seen as a narrow band of clouds and thunderstorms
wrapped around the globe. This zone is a prolific contributor of
storms and clouds to the world's weather.
Because the Earth is a globe, and not a flat board, the sun shines almost
straight down on the tropics, baking them every day of the year. But at the
poles, the angle is small and the sun's rays are weak, and the poles are
therefore cold. Nature "abhors" this imbalance, and tries to fix it. As
quickly as solar heat flows in to the tropics, it begins flowing out toward
the poles, seeking to equalize the difference. The unrelenting march of
this energy-on-the-move, from high concentration to low concentration, is
the piston in the engine that propels weather.
When warm air leaves the tropics and heads toward the poles, cold air from
near the poles is sucked back toward the tropics. This exchange sets up
two-lane highways for air rushing to and from the tropics. These highways
of air are called convection cells, and they are the reason wind
Air flowing back and forth in these great cells is pushed sideways by the
Earth's rotation, dragged by friction with the land and the sea, and
squeezed by gravity. All of these distortions cause turbulent mixing of
the winds, and soon lead to the organization of storm centers due to
unevenness between warm and cold. In particular, the sideways push given
the winds by the spinning of the planet-called the Coriolis Effect-causes
the constant convective flows to organize in bands, where the flow
direction varies according to latitude. These bands are responsible for
prevailing winds on the surface, and jetstreams high in the atmosphere.
The ITCZ on this satellite image is the band of bright clouds located
just north of the equator. While El Niño conditions prevail, the ITCZ
is disrupted due to the unusually warm sea surface.
This rare shot of the south pole, a composite of photos taken by the Galileo probe as it
swung by Earth on its way to Jupiter, clearly shows the band of cyclones that predominates
in higher latitudes. A similar band surrounds the north pole, and its cyclones sweep across
the US in a never-ending parade of changing weather.
We can see these bands of wind clearly in Jupiter's atmosphere, because
Jupiter rotates at a furious pace, once every ten hours. We can also see
them clearly on Earth when we take a picture from far out in space. El Niño exploits this organization
of winds into bands when it causes major weather changes around the world.
Specifically, El Niño can affect the path of flow in these bands, and the
cyclones that are ushered across the surface by them are now delivered to
different areas than normal. Think of the wind bands—both at the surface
and high in the sky—as a tram, a streetcar on which storm systems hitch a
ride as they travel around the Earth. El Niño moves the tracks—the
stormtracks—of this tram. The answer to the puzzle of how this happens is
literally blowing in the wind.
This animation of Jupiter, photographed by the Voyager probe, shows the wind banding effects created when convection and rotation forces interact.
See the animation (4MB): QuickTime | AVI.
See a similar supercomputer-model animation of water vapor circulation on Earth (4MB): QuickTime | AVI |
Get the QuickTime software
Continue: How does El Niño take over such a large system?
Photos/Illustrations: (1) NASA; (2) University of Illinois WW2010 Project;
(3) NASA/JPL; (4) NASA; (5) National Center for Atmospheric Research/Scientific Computing
Division: Sponsored by the NSF.