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Name: Gary Lawrence, Jonathan Wray
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Question:
Why do weather systems move from west to east? We know that water currents and wind currents cause this, but why do they move in that direction?



Replies:
This one is a bit tricky. I found the answer in "It's Raining Frogs and Fishes," by Jerry Dennis, Harper Perennial, 1992.

Here is the scoop: Wind is the result of heating by the sun and the fact that hot air rises. The sun burns brightest near the equator, heating up the air and causing the air to rise. The rising air at the equator is replaced by air coming from the north and the south. This leads to a rotation of the air, with the higher altitude winds traveling away from the equator and the lower altitude winds traveling toward the equator.

Now, if it were not for the rotation of the earth, that is where the situation would end. But the rotation of the earth complicates the process through the introduction of Coriolis effects. The speed of the earth due to rotation is most rapid at the equator, slowing as you approach the poles. This is analogous to a record player (remember them?). The speed of the record is most rapid at the outer edge of the record and slows as you approach the center.

The higher speed at the equator is important because the high altitude winds that rise from the equator maintain the speed of rotation at the equator. So, as these winds travel away from the equator, they move eastward relative to the ground beneath them - since the winds have a greater rotational speed than the ground.

This explains why high altitude winds blow from west to east. And it is these high altitude winds that, to a large extent, control the weather.

If this is not clear, try finding the above mentioned book, where the answer is explained in more detail.

Grant L



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