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Name: Paul
Status: student
Grade: 6-8
Location: WI
Country: USA
Date: Fall 2009

Question:
Why does the atmosphere rotate with Earth? Why does it not stay more or less in place as Earth rotates underneath it?



Replies:
The atmosphere moves for a variety of reasons, but I think the most important concept is "viscosity". Viscosity is a term that describes attraction between fluid molecules. It means that when one molecule in a fluid moves, it attracts/pulls its neighbors with it. But this is a molecular-level definition -- what does this mean in real life? At people-scale, viscosity is a little like "thickness" -- pancake syrup is thicker (more viscous) than water, and molasses is thicker (more viscous) than syrup. When fluid molecules are strongly attracted to each other, the fluid seems "thicker" and flows more slowly.

How does this relate to the atmosphere? Air is a very low-viscosity fluid, but it still has viscosity. On large time scales like the age of the earth, that viscosity is very important. The molecules of air next to the earth get dragged along with the earth's surface, and they progressively drag the rest of the atmosphere with them. This has been going on for a very long time, so the 'top' of the atmosphere has had more than enough time to "catch up" with the bottom, so the result is that the atmosphere moves with the earth.

(technically there is a lot more going on than just viscosity, including conservation of momentum and auto-convection -- feel free to respond back if you want to know more about these things)

Hope this helps,

Burr Zimmerman


The atmosphere rotates with the Earth for the same reason that water inside a bucket will eventually rotate with the bucket if the bucket is rotated. Friction between the solid surface and the contacting layer of fluid sets the fluid into motion; the internal viscosity of the fluid then ensures that adjacent layers of fluid will eventually move together.

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming



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