<p>Are you asking about the energy from the sun causing the circulation cells to circulate?  If so, that's the easy part.  I sound like I know what I'm talking about, but I'm not really a reliable source.  But here's my analysis:</p>

<p>Sun heats the earth at the equator at steep angles, thus providing a lot of heat to the air at the equator.  This air expands and grows like a mountain over the equator.  Since the air in the mountain has the same mass as before (same number of molecules), its weight on the ground would be the same.  But it is not.  The mountain of expanded air in the high atmosphere above the equator spills downhill northward because the top of the less expanded atmosphere to the north is lower.  But as this downhill falling air moves north its time spent at cool altitudes and the less direct rays of the sun at higher latitudes cause it cool and become more dense.  It becomes more dense than the air it falls over on the north edge of the Hadley Cell, so it sinks down.  Why does it move south to complete the cycle? Because the continually rising column of air encircling the equator causes a constant low pressure condition on the ground as it rises up and spills off the mountain.  If the air spills off the equatorial mountain of air, the mountain is lighter than it was before.  That the mountain got high to start with would not change its weight or the pressure at the equator.  The pressure at the equator is always low because the rising mountain of expanding air is forever spilling downhill to the north and causing fewer molecules and atoms of atmosphere to occupy that mountain that sits on the equator.</p>

<p>Note, the the Polar Cell functions just like the Hadley Cell.  But you might notice that the Ferrel Cell runs backwards against the dynamics I just described.  That's because both the Hadley Cell and the Polar Cells are "thermally direct," or run the right way.  The Ferrel cell is not run by the dynamics I just described because it is "thermally indirect," powered and supported by the boundary effects of bumping up against the Hadley and Polar Cells.  I don't have a good grasp on the thermally indirect cell being driven by its neighbor cells, but I have a feeling it has to do with atmospheric lifting at the north edge of the Ferrel Cell caused by the Polar cell ( and the converse at the border with the Hadley Cell), perhaps encouraging upper atmospheric southern downhill flow, but boy that's a reach.</p>

CC BY-SA 4.0
2020-01-27T01:53:40.280Z
2020-01-27T01:53:40.280Z
user18805
<p>I'm guessing here, but there are two things that seem to mechanically form the organized concentrated air rivers that are jet streams.  I've despaired to find no good meterologist site or link that goes beyond grand vague hand waving gestures that mostly over-explain Coriolis Forces and irrelevant complexities.  Here's how I think the actual organized streams are formed.  </p>

<p>We need to be careful about postulating that different temperatures cause the actual streams.  I don't think that's a useful perspective.  Certainly, different temperatures are required to set up organized Polar, Ferrel and Hadley cells, but the heart of the explanation for the actual stream formation is looking at only momentum and drag.  </p>

<p>If the surface wind of a Ferrel Cell (not the somewhat opposite flowing high altitude wind of the same  Ferrel Cell) blows northward, it's earth-based radius decreases.  It's becomes closer to the spin axis of the earth.  That means that its relative velocity is always more eastward than that of the air it is joining.  That's the Coriolis Force. Ok, so we get northeastern lower winds throughout the band of that Ferrel Cell, progressively becoming more eastwardly farther north.  But why the sudden discontinuous increase in velocity at the northern edge of the Ferrel Cell that produces a jet stream?  Well.  Because all throughout the transit that those low level winds negotiated on their northward trek within that Ferrel Cell, they had to gently push more eastward for every inch north they moved against the drag of the atmosphere they encountered. Their acceleration to the east was limited by atmospheric drag. </p>

<p>But suddenly these northward winds are stopped from going further north when they encounter the Polar cell and are forced to deflect directly eastward. They are no longer dragged down by the slower rotating atmosphere north of them because they are moving only eastward.  This means that the atmosphere that would normally drag down these northeastward tending winds is mostly made of their neighbor Ferrel Cell winds, that had been northeastwardly parallel to them for a while, that are similarly suddenly deflected due east by encountering the obstruction of the Polar Cell.  Now, at the northern border of the Ferrel Cell, these winds all line up moving east as they begin to rise up towards the top of the Ferrel Cell and turn around back to the south to continue their cycle in the Ferrel circulation Cell, their eastward momentums now all lined up together fully aligned with each other along the lines of latitude (parallel to the equator) and pushing forth with their excess momentum in line concentrated in a much smaller area (than the circumferal distance of the Polar Cell and Ferrel Cell border).  The eastward change in momentum has been concentrated at this border by having the Ferrel Cell winds line up trying to accelerate each other because they are forced to suddenly turn east.  A visual picture of a similar idea is five people thrashing through a forest line abreast - like the low level northeasterly Ferrel Cell winds.  Then, suddenly they turn 90º and have a lot less thrashing to do to get through the forest because of their alignment - like those Ferrel Cell winds when they turn to line up behind each other at the circulation cell boundary.  The atmosphere these winds encounter that would normally limit their eastward acceleration is now made up of similar Ferrel Cell winds trying to accelerate eastward during the big turn back to the south.  The result is a reduction in atmospheric drag in relatively short distance.  Boom:  Jet stream.  </p>

<p>You may wonder why it would be that at the northern border of, say, the Ferrel Cell where there is no longer a northward component of velocity would still provide an eastward force to prople the jet stream since the eastward component of the Coriolis Force disappears when the northward velocity goes to zero.  It's because of lag.  The eastward pressures are not fully equalized instantly when the air stops moving northeastward and is deflected due east.  </p>

<p>If all this is the case, then why does the subtropical jet also move east?  Only air that is approaching a cell border can produce a jet; not air that is retreating from such a border because of the mechanism described above.  So only the upper air of the Hadley cell can contribute to a jet stream  at the border between the Ferrel and Hadley cells.  </p>

<p>Finally, why doesn't the southern flowing air in the upper reaches of the Ferrel Cell create a westward moving jet?  If you examine the profile view of the circulation cells, notice the altitude where the reversal from north/south occurs.  In the both the Polar and Hadley cells, due to the odd sloped frontal border of the various cells, the reversals that should spawn westward blowing jet streams happen at low altitude where too many disturbances prevent organized flow.  Remember even the low altitude portion of the Ferrel Cell can't form a jet stream until it has shifted up into the high region of the cell at the turn reversal. Furthermore, the Ferrel Cell runs backwards.  That is, it is thermally indirect, so not driven by disparities in surface heating like the Polar and Hadley Cells, hence it is weaker, driven by its neighboring cells, which are thermally direct.  Occaionally, a west flowing subtropical jet forms from these normally turbulent regions if conditions are just right, but not normally.</p>

<p>It should be remembered that on a planetary scale these jets are rather weak, broken, discontinuous, and easily disrupted.  For example, if global warming reduces the sharpness of the temperature variation between the Polar Cell and the Ferrel Cell, that border becomes less defined and can undulate down into the central US in what the news calls a Polar Vortex.</p>

<p>Alas, I can't back any of this up because I've not seen it anywhere.  This could all be wrong, but it seems straight forward enough to be plausible.</p>

CC BY-SA 4.0
2020-01-26T18:33:44.733Z
2020-01-26T18:38:45.710Z
2020-01-26T18:38:45.710Z
user18805
<p>Because the tropical easterly jet is formed as a result of warming over the Tibetan plateau leading to rising motion and finally the Easterly maxima due to corriolis effect</p>

CC BY-SA 3.0
2018-02-13T10:28:19.583Z
Yes, this hints at a useful answer to the question.  It'd be great if you could explain how this warming produces such wind, especially in a fairly approachable way for a variety of people.  If you're up to it, a question like this could greatly use such an answer, as there are kids and people throughout society who likely have this question :-)  But either way, I'd think you could give a more thorough answer of the thought process and any references/graphics/etc to support your answer.  Thanks :-D
JeopardyTempest
CC BY-SA 3.0
2018-02-13T07:00:11.123Z
2018-02-13T07:56:34.487Z
2018-02-13T07:56:34.487Z
Ayana
<p>If most of jet streams are westerly, that is to say, they blow from west to east, why does the tropical/equatorial jet stream blow from east to west?</p>

<meteorology><climatology><jet-stream>
Easterly Jet Stream
James
# Easterly Jet Stream

(Asked by: username_0 on 2016-12-03)

If most of jet streams are westerly, that is to say, they blow from west to east, why does the tropical/equatorial jet stream blow from east to west?

**Tags:** meteorology, climatology, jet-stream


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## 3 answers

### Answer by username_1 on 2020-01-27. Score: 0

Are you asking about the energy from the sun causing the circulation cells to circulate? If so, that's the easy part. I sound like I know what I'm talking about, but I'm not really a reliable source. But here's my analysis:

Sun heats the earth at the equator at steep angles, thus providing a lot of heat to the air at the equator. This air expands and grows like a mountain over the equator. Since the air in the mountain has the same mass as before (same number of molecules), its weight on the ground would be the same. But it is not. The mountain of expanded air in the high atmosphere above the equator spills downhill northward because the top of the less expanded atmosphere to the north is lower. But as this downhill falling air moves north its time spent at cool altitudes and the less direct rays of the sun at higher latitudes cause it cool and become more dense. It becomes more dense than the air it falls over on the north edge of the Hadley Cell, so it sinks down. Why does it move south to complete the cycle? Because the continually rising column of air encircling the equator causes a constant low pressure condition on the ground as it rises up and spills off the mountain. If the air spills off the equatorial mountain of air, the mountain is lighter than it was before. That the mountain got high to start with would not change its weight or the pressure at the equator. The pressure at the equator is always low because the rising mountain of expanding air is forever spilling downhill to the north and causing fewer molecules and atoms of atmosphere to occupy that mountain that sits on the equator.

Note, the the Polar Cell functions just like the Hadley Cell. But you might notice that the Ferrel Cell runs backwards against the dynamics I just described. That's because both the Hadley Cell and the Polar Cells are "thermally direct," or run the right way. The Ferrel cell is not run by the dynamics I just described because it is "thermally indirect," powered and supported by the boundary effects of bumping up against the Hadley and Polar Cells. I don't have a good grasp on the thermally indirect cell being driven by its neighbor cells, but I have a feeling it has to do with atmospheric lifting at the north edge of the Ferrel Cell caused by the Polar cell ( and the converse at the border with the Hadley Cell), perhaps encouraging upper atmospheric southern downhill flow, but boy that's a reach.


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### Answer by username_1 on 2020-01-26. Score: 0

I'm guessing here, but there are two things that seem to mechanically form the organized concentrated air rivers that are jet streams. I've despaired to find no good meterologist site or link that goes beyond grand vague hand waving gestures that mostly over-explain Coriolis Forces and irrelevant complexities. Here's how I think the actual organized streams are formed.

We need to be careful about postulating that different temperatures cause the actual streams. I don't think that's a useful perspective. Certainly, different temperatures are required to set up organized Polar, Ferrel and Hadley cells, but the heart of the explanation for the actual stream formation is looking at only momentum and drag.

If the surface wind of a Ferrel Cell (not the somewhat opposite flowing high altitude wind of the same Ferrel Cell) blows northward, it's earth-based radius decreases. It's becomes closer to the spin axis of the earth. That means that its relative velocity is always more eastward than that of the air it is joining. That's the Coriolis Force. Ok, so we get northeastern lower winds throughout the band of that Ferrel Cell, progressively becoming more eastwardly farther north. But why the sudden discontinuous increase in velocity at the northern edge of the Ferrel Cell that produces a jet stream? Well. Because all throughout the transit that those low level winds negotiated on their northward trek within that Ferrel Cell, they had to gently push more eastward for every inch north they moved against the drag of the atmosphere they encountered. Their acceleration to the east was limited by atmospheric drag.

But suddenly these northward winds are stopped from going further north when they encounter the Polar cell and are forced to deflect directly eastward. They are no longer dragged down by the slower rotating atmosphere north of them because they are moving only eastward. This means that the atmosphere that would normally drag down these northeastward tending winds is mostly made of their neighbor Ferrel Cell winds, that had been northeastwardly parallel to them for a while, that are similarly suddenly deflected due east by encountering the obstruction of the Polar Cell. Now, at the northern border of the Ferrel Cell, these winds all line up moving east as they begin to rise up towards the top of the Ferrel Cell and turn around back to the south to continue their cycle in the Ferrel circulation Cell, their eastward momentums now all lined up together fully aligned with each other along the lines of latitude (parallel to the equator) and pushing forth with their excess momentum in line concentrated in a much smaller area (than the circumferal distance of the Polar Cell and Ferrel Cell border). The eastward change in momentum has been concentrated at this border by having the Ferrel Cell winds line up trying to accelerate each other because they are forced to suddenly turn east. A visual picture of a similar idea is five people thrashing through a forest line abreast - like the low level northeasterly Ferrel Cell winds. Then, suddenly they turn 90º and have a lot less thrashing to do to get through the forest because of their alignment - like those Ferrel Cell winds when they turn to line up behind each other at the circulation cell boundary. The atmosphere these winds encounter that would normally limit their eastward acceleration is now made up of similar Ferrel Cell winds trying to accelerate eastward during the big turn back to the south. The result is a reduction in atmospheric drag in relatively short distance. Boom: Jet stream.

You may wonder why it would be that at the northern border of, say, the Ferrel Cell where there is no longer a northward component of velocity would still provide an eastward force to prople the jet stream since the eastward component of the Coriolis Force disappears when the northward velocity goes to zero. It's because of lag. The eastward pressures are not fully equalized instantly when the air stops moving northeastward and is deflected due east.

If all this is the case, then why does the subtropical jet also move east? Only air that is approaching a cell border can produce a jet; not air that is retreating from such a border because of the mechanism described above. So only the upper air of the Hadley cell can contribute to a jet stream at the border between the Ferrel and Hadley cells.

Finally, why doesn't the southern flowing air in the upper reaches of the Ferrel Cell create a westward moving jet? If you examine the profile view of the circulation cells, notice the altitude where the reversal from north/south occurs. In the both the Polar and Hadley cells, due to the odd sloped frontal border of the various cells, the reversals that should spawn westward blowing jet streams happen at low altitude where too many disturbances prevent organized flow. Remember even the low altitude portion of the Ferrel Cell can't form a jet stream until it has shifted up into the high region of the cell at the turn reversal. Furthermore, the Ferrel Cell runs backwards. That is, it is thermally indirect, so not driven by disparities in surface heating like the Polar and Hadley Cells, hence it is weaker, driven by its neighboring cells, which are thermally direct. Occaionally, a west flowing subtropical jet forms from these normally turbulent regions if conditions are just right, but not normally.

It should be remembered that on a planetary scale these jets are rather weak, broken, discontinuous, and easily disrupted. For example, if global warming reduces the sharpness of the temperature variation between the Polar Cell and the Ferrel Cell, that border becomes less defined and can undulate down into the central US in what the news calls a Polar Vortex.

Alas, I can't back any of this up because I've not seen it anywhere. This could all be wrong, but it seems straight forward enough to be plausible.


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### Answer by username_2 on 2018-02-13. Score: 0

Because the tropical easterly jet is formed as a result of warming over the Tibetan plateau leading to rising motion and finally the Easterly maxima due to corriolis effect

> **Comments:**
> **username_3**: Yes, this hints at a useful answer to the question.  It'd be great if you could explain how this warming produces such wind, especially in a fairly approachable way for a variety of people.  If you're up to it, a question like this could greatly use such an answer, as there are kids and people throughout society who likely have this question :-)  But either way, I'd think you could give a more thorough answer of the thought process and any references/graphics/etc to support your answer.  Thanks :-D  

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