Exploring the Hadley Cell & Its Impact on Deserts

[physicsponderer]

If the Hadley cell, which goes all the way around Earth, causes the the Sahara, surely the desert zone should likewise go all the way around the Earth, and there should be a mirror image of it in the Southern Hemisphere, shouldn't there?

 

[TeethWhitener]

There is. In Africa, it's where you find the Kalahari and the Namib deserts. In Australia, it's where you find the Outback, and in South America, it's where you find the Atacama (though the Atacama is a bit of an anomaly because of the presence of the Andes mountains).
https://en.wikipedia.org/wiki/Desert_climate
Notice the two bands at roughly 30° north and south of the equator.

 

[physicsponderer]

There is. In Africa, it's where you find the Kalahari and the Namib deserts. In Australia, it's where you find the Outback, and in South America, it's where you find the Atacama (though the Atacama is a bit of an anomaly because of the presence of the Andes mountains).
https://en.wikipedia.org/wiki/Desert_climate
Notice the two bands at roughly 30° north and south of the equator.

So why is it not Sahara-like desert all the way across the continent, and all the way from the southern tip to about 15 degrees south? Wikipedia says the Kalahari is not even true desert, but savanna because it gets too much rain. And the Namib covers only a tiny area.

 

[DaveC426913]

Because Hadley cells are not the only thing that affects climate.
Local geography of continents, mountains, water bodies and ocean currents - as well as cyclic and non-cyclic temperature changes - all play a major factor in how moisture is brought to an area.

 

[Vanadium 50]

So why is it not Sahara-like desert all the way across the continent, and all the way from the southern tip to about 15 degrees south?

Do you think the only thing that affects weather is latitude?

 

[physicsponderer]

Because Hadley cells are not the only thing that affects climate.
Local geography of continents, mountains, water bodies and ocean currents - as well as cyclic and non-cyclic temperature changes - all play a major factor in how moisture is brought to an area.

Then it is all those things plus the Hadley cell that causes the Sahara, not simply the Hadley cell. But I read (in Wikipedia) that it was caused by the Hadley cell, full stop, which is an oversimplification, surely?

 

[physicsponderer]

Do you think the only thing that affects weather is latitude?

No, of course, not. My point is that Wikipedia should not have said simply that the Hadley cell causes the Sahara desert. It should have said something like, "The Hadley cell is thought to have played a part in the formation of the Sahara Desert."

 

[DaveC426913]

An excellent exemplar of why Wikipedia is not considered a reputable source, here on PF.

 

[Vanadium 50]

But I read (in Wikipedia) that it was caused by the Hadley cell, full stop

Which article?

 

[physicsponderer]

Which article?

As I recall, it was the one called Hadley cell.

 

[physicsponderer]

An excellent exemplar of why Wikipedia is not considered a reputable source, here on PF.

A big problem is that it easy to edit it yourself to support your point and then use it as reference.

 

[Vanadium 50]

As I recall, it was the one called Hadley cell.

"As I recall" is kind of bad. This is "I read somewhere".

If you don't tell us where you read something, we can't look at it and determine if it is wrong or if you misunderstood. It wastes our time, and you get a less good answer. Everybody loses.

That article does not have the word "Sahara" in it, not now, and not in the last half-dozen or so revisions.

 

[physicsponderer]

"As I recall" is kind of bad. This is "I read somewhere".

If you don't tell us where you read something, we can't look at it and determine if it is wrong or if you misunderstood. It wastes our time, and you get a less good answer. Everybody loses.

That article does not have the word "Sahara" in it, not now, and not in the last half-dozen or so revisions.

I read it in Wikipedia a few months ago, as I recall.

 

[physicsponderer]

'Hadley cell' in Wikipedia has in the intro: ' This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams. ' This is an oversimplification it seems. It should be explained but it isn't.

A bit later it has: 'Many of the world's deserts are located in these subtropical latitudes. However the deserts do not extend to the eastern side of the various continents because of ocean currents caused by the Trade Winds.' This is interesting, but it doesn't say how that works. Wikipedia so often is like this.

And that's all there is about deserts in the article, except for this:

https://en.wikipedia.org/wiki/File:MeanMonthlyP.gif

which I must say is very good, if it is accurate. It got an award so it probably is I suppose.

 

[lomidrevo]

So what is this thread about? Complaining about quality of information at wiki pages? Some articles are very detailed and accurate others aro not, it's not a secret. When you require more detailed explanation, why don't you search other resources? Just google can provide hundreds of relevant links, many of them with edu, org or gov top-level domains.

 

[physicsponderer]

So what is this thread about? Complaining about quality of information at wiki pages? Some articles are very detailed and accurate others aro not, it's not a secret. When you require more detailed explanation, why don't you search other resources? Just google can provide hundreds of relevant links, many of them with edu, org or gov top-level domains.

I guess I wanted to see a convincing argument that the Sahara is caused by the Hadley cells, or find out that there isn't such an argument. It seems widely accepted that the Sahara is caused that way, but Wikipedia didn't explain why the Sahara was so much bigger than the Namib. I was also wondering just how idealized the idea of the Hadley cell is.
I certainly was not complaining about Wikipedia in the OP.
I had hoped for some discussion.

 

[anorlunda]

If you are able to find literature on historical climates, you may get closer. From what I've heard, the Sahara is a recent development. In early human history, it was green. Try searching for the historical climate origin of the Sahara.

 

[snorkack]

Note that Hadley cell creates discontinuous rings of deserts on both hemispheres:
In North Old World, Sahara is continued east through Arabia, Iran and Pakistan to West India, but then India and eastwards to China are wet;
in North America, deserts are in Baja California, Arizona and western Mexico, but do not reach Gulf of Mexico;
in South America, deserts are in Atacama and continue to interior of Argentina, but Uruguay is wet;
in South Africa, Namib and Kalahari are deserts but Mozambique is not;
in Australia, interior and west coast are deserts but east coast is wet.
Trade winds are one mechanism that causes the deserts not to extend across east coasts. Another mechanism is monsoons. And monsoons was probably the underlying cause for Green Sahara.

 

[physicsponderer]

Note that Hadley cell creates discontinuous rings of deserts on both hemispheres:

Thanks for the informative post.
How does the Hadley cell differ from the idealized representation one normally sees. E.g. the diagram in Wikipedia:

https://en.wikipedia.org/wiki/Hadley_cell#/media/File:Earth_Global_Circulation_-_en.svg

This chart seems to suggest it differs a lot:

https://en.wikipedia.org/wiki/Hadley_cell#/media/File:Omega-500-july-era40-1979.png

And why is there no sign at all, or almost no sign of, the Ferrel cells and Polar cells? Are the latter even more idealized? Kudos to Wikipedia for the latter chart if it is accurate, because it gives some indication, I think, of how much of an idealization the former chart is. A case of too many cooks spoiling the illusion, perhaps.

 

[DaveC426913]

Thanks for the informative post.
How does the Hadley cell differ from the idealized representation one normally sees. E.g. the diagram in Wikipedia:

https://en.wikipedia.org/wiki/Hadley_cell#/media/File:Earth_Global_Circulation_-_en.svg

This chart seems to suggest it differs a lot:

https://en.wikipedia.org/wiki/Hadley_cell#/media/File:Omega-500-july-era40-1979.png

That second chart is measuring velocity of vertical movement of air. While that has a correlation with Hadley cell movement, no perfunctory glance at such a map could reasonably result in an assessment of how it differs from ideal.

 

[physicsponderer]

That second chart is measuring velocity of vertical movement of air. While that has a correlation with Hadley cell movement, no perfunctory glance at such a map could reasonably result in an assessment of how it differs from ideal.

Agreed. And that chart was probably chosen because it illustrates the Hadley cell theory relatively well. Other charts are likely even more doubt-producing.

 

[snorkack]

This chart seems to suggest it differs a lot:

https://en.wikipedia.org/wiki/Hadley_cell#/media/File:Omega-500-july-era40-1979.png

And why is there no sign at all, or almost no sign of, the Ferrel cells and Polar cells? Are the latter even more idealized? Kudos to Wikipedia for the latter chart if it is accurate, because it gives some indication, I think, of how much of an idealization the former chart is.

Polar cell is actually there, if you notice.
Look at the lower edge. The bright yellow spot over Antarctic.
And then look at the contrast... very little polar cell in North. A small pink patch... on Greenland, not a coincidence.
It is July - Northern summer, Southern winter. The polar cells are seasonal. Northern polar cell nearly shuts down in polar summer, with a small remnant over Greenland ice sheet, but the Southern polar cell is in full power in polar winter.

 

[physicsponderer]

Polar cell is actually there, if you notice.
Look at the lower edge. The bright yellow spot over Antarctic.
And then look at the contrast... very little polar cell in North. A small pink patch... on Greenland, not a coincidence.
It is July - Northern summer, Southern winter. The polar cells are seasonal. Northern polar cell nearly shuts down in polar summer, with a small remnant over Greenland ice sheet, but the Southern polar cell is in full power in polar winter.

It didn't occur to me that those small patches, 15 degrees away (which seems rather far) from the pole in each case could be all that is seen of the polar cells. I was looking for a more or less complete ring around each pole. Sobering.
But what about the corresponding rising air in the polar cells?

 

[physicsponderer]

In this diagram:

https://en.wikipedia.org/wiki/File:Earth_Global_Circulation_-_en.svg

the flow in the cells is shown as north, down, south, up, but really it is northeast, down, southwest, up. Am I right? And even then, only in one small part at anyone time, meaning that the 'cell' is in fact merely a zone where a small ring or airflow is allowed, a bit like how a probability wave in quantum mechanics says where you might detect a subatomic particle. Right? The cell isn't a continuous vortex that extends all the way around the planet, is it?

 

[physicsponderer]

This photograph:
https://en.wikipedia.org/wiki/File:The_Earth_seen_from_Apollo_17.jpg
is captioned :'Cloud formations in a famous image of Earth from Apollo 17 makes atmospheric circulation directly visible' but it is not directly visible or even visible at all to me. Can anyone see the atmospheric circulation here?

CORRECTION (edit by physicsponderer): The link shows the photo without the caption. To see the photo and caption click instead on https://en.wikipedia.org/wiki/Hadley_cell and scroll down to the fourth image.

 

[DaveC426913]

This photograph:
https://en.wikipedia.org/wiki/File:The_Earth_seen_from_Apollo_17.jpg
is captioned :'Cloud formations in a famous image of Earth from Apollo 17 makes atmospheric circulation directly visible'

I see no such caption associated with any images on that page.

I see this:

"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.

 

[physicsponderer]

I see no such caption associated with any images on that page.

I see this:

"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.

You are correct. My apologies. You can see the photo with the caption if you scroll down on this page:
https://en.wikipedia.org/wiki/Hadley_cell . I have edited the post accordingly. Thanks for pointing that out.

 

[physicsponderer]

I wonder what that vertical motion chart https://en.wikipedia.org/wiki/Hadley_cell#/media/File:Omega-500-july-era40-1979.png would look like if it were not ' July average ' but for a particular instant.

'500 hPa vertical velocity (Pa/S) in July from ERA-40 reanalysis, 1979-2001 average.

Negative (blue) values represent rising air; positive (red) values indicate sinking air. This map is an excellent illustration of the Hadley cell.' says Wikipedia.

Also, my suspicion that the chart was chosen for it's close correspondence to the Hadley cell theory seems to have been confirmed by the last bit.

Edit by physicsponderer: It would be interesting to see what the chart looks like for a random month average, annual average, decade average, and some randomly selected instants.

 

[DaveC426913]

You are correct. My apologies. You can see the photo with the caption if you scroll down on this page:
https://en.wikipedia.org/wiki/Hadley_cell . I have edited the post accordingly. Thanks for pointing that out.

OK. Atmospheric circulation is pretty directly evident in that pic. Do you not think so?

 

[physicsponderer]

OK. Atmospheric circulation is pretty directly evident in that pic. Do you not think so?

It's not evident to me. Would you care to explain it?

 

[DaveC426913]

It's not evident to me. Would you care to explain it?

Those very long contiguous streamers of cloud banks show the circular interaction of air masses on the scale of thousands of kilometres, notably between temperate and polar latitudes.

 

[physicsponderer]

Those very long contiguous streamers of cloud banks show the circular interaction of air masses on the scale of thousands of kilometres, notably between temperate and polar latitudes.

Would you care to expand on that?

 

[phinds]

I had hoped for some discussion.

You got it. You used a crappy, and ill-specified source for your statement and you got a discussion of your crappy and ill-specified source. As said earlier in the thread, that's why PF insists on acceptable and well-specified sources.

 

[physicsponderer]

You got it. You used a crappy, and ill-specified source for your statement and you got a discussion of your crappy and ill-specified source. As said earlier in the thread, that's why PF insists on acceptable and well-specified sources.

My post was a question. I thought it was clear that I am a beginner at meteorology. The source was simply to provide some context. I am indeed grateful for the interesting discussion.

 

[DaveC426913]

Would you care to expand on that?

At this point, I think it would make more sense to pass the conch to you, and ask you explain why - when you are directed to "very long contiguous streamers of cloud banks showing the circular interaction of air masses on the scale of thousands of kilometres, notably between temperate and polar latitudes" - you don't see that as "direct evidence of atmospheric circulation".

Alternately, what is the minimum you would reasonably expect to see in order to accept it as direct evidence of atmospheric circulation?