Naked Science Forum
Life Sciences => The Environment => Topic started by: Joe L. Ogan on 19/12/2011 18:35:58
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I find the explanation of how or why a Jet stream works a little confusing. I know it is caused by a high pressure area and a low pressure area coming together. There is a natural tendency for the high pressure to try to go to the low pressure. In the northern hemisphere, the high pressure area rotates clockwise and the low pressure area rotates counter clockwise. The rotation of the earth is toward the east. The rotation of the earth has something to do with the jet stream, but I can not quite understand how this works. The explanation is far from satisfactory to me. Can someone please explain just how or why this works? Thanks for comments. Joe L. Ogan
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I have thought about this a great deal and I think the reason is because the earth rotates so much faster at the equater than it does further north. Since the low pressure area travels toward the north and the high pressure travels toward the south the rotation of the earth creates a partial vacumn between the two pressure areas which bends the jet stream toward the east. The partial vacumn causes the jet stream to increase speed in its' attempt to reach the low pressure area. Thanks for comments. Joe L. Ogan
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I'm not sure Joe? Maybe you're thinking about shear forces? There are clearly wind shears (http://www.weatherquestions.com/What_is_wind_shear.htm) associated with Jet streams, but the main reason why they have so clear outlines is their momentum. Instead of jet streams think of it as drawing three footballs. One drawn just near the top of your globe (which is the pole), the second, following a straight line down, placed at the middle of the globe where the earth bulges out the most. The third, still following that straight line, placed near the bottom of your globe, near the opposite pole.
If you put one football on the top of your globe it would just spin round as you rotated that globe, right :)
But the football you put at the middle, where Earth bulges out the most from its axis, that one would start to move rather fast. It's like being on a carousel, if you're standing at the exact center of it, the axis, furthest in so to speak, you just spin around, but the closer you get to the outer edge, the rim of that carousel, the faster you will find yourself moving around and the more momentum you will have.
And that is your momentum, and those three footballs momentum too. The one near the top and the one near the bottom don't get so much momentum, but the one in the middle, the equator where earth bulges out the most, will get a lot. That football will also have the longest 'distance' to cover, and spinning the globe you can see that it does in the same amount of time as the two you placed near the top and bottom. This Noaa explanation describes it a little differently, but they mean the same thing as I, hopefully :)
The Jet Stream. (http://www.srh.noaa.gov/jetstream/global/jet.htm)
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Hot air goes up at the equator and cold air down at the poles, this gives an area where the movements are in direct conflict, couple this with the earth rotates and the air mass too it creates a band of very fast moving air.
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Hot air goes up at the equator and cold air down at the poles
I agree that this is the overall effect
- The description might be more accurate on a different-sized planet with a different speed of rotation and a different atmospheric pressure.
But on Earth, the movement of air occurs in cells, and there are 3 bands between the equator and the poles (Jupiter has a lot more bands):
- Near the equator: Hadley cells
- Mid-Latitudes: Ferrel Cells
- Near the poles: Polar Cell
- These cells are all affected by Coriolis force, producing rotation
See: https://en.wikipedia.org/wiki/Atmospheric_circulation
The exact boundaries between these cells is very fluid, as weather systems affect how strong they are.
- The jet stream flows west to east, and follows a sinuous path between the moving boundaries of the Polar and Ferrel cells
- There is a similar but less well-known jetstream between the Ferrel and Hadley cells.
See: https://en.wikipedia.org/wiki/Jet_stream