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

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.

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 others.

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 blows.

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.

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.

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

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

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.