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Chad
In: Ramsbottom
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Anyway... as Mick requested I've been looking at the deserts of North America... and I need help.
Can anybody explain what is going on up there in Washington and Oregon? We have an area of torrential precipitation directly to the west of desert... Isn't that rather 'off plan'..?
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Ishmael
In: Toronto
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| Grant wrote: | | Or to put it another way, if South America can make its own sand, why can't India? |
Because it is a factor of relative windspeed and the width of the relative oceans. Oceans strip the air of moisture but also they strip it of sand. If the water does less harm than good, you get India.
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Ishmael
In: Toronto
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| Chad wrote: | | But as I said before, in the case of the Sahara the Eastern Effect simply isn't doing its job... the Indian Ocean coast of Arabia and the Horn of Africa have not been shielded from sand... the Sahara stretches from coast to coast. |
My friend, there is a reason why the fertility of the Horn of Africa has long been recognized. And the fact of the matter is the east coast of Africa is far less deserty than is the interior.
What reduces the desertiness?
Ok. That's my last post on the subject. None of you are worthy of my brilliance.
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Ishmael
In: Toronto
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| Chad wrote: | | The thing that most struck me when I first saw this, was the way the rain bands move up and down in line with the Earth's seasonal inclination through its solar orbit... (Something none of us have so far mentioned.) |
Because atmospheric moisture follows the sun.
Not the wind.
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Chad
In: Ramsbottom
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| Ish wrote: | | My friend, there is a reason why the fertility of the Horn of Africa has long been recognized. And the fact of the matter is the east coast of Africa is far less deserty than is the interior. |
OK. Let's look at the Horn of Africa in a little more detail:
The fertility you refer to is "recognized" only for the Ethiopian Highlands... not the arid coastal portion, which is anomalous not only in its ability to shrug off the Eastern Effect, but also in the fact that it lies directly east of some of the wettest, most lush, parts of the African continent... It's actually far more deserty than the interior.
What increases the desertiness?
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Mallas
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I think evaporation occurs due to the wind as Chad has explained, but I am going to have to check out that HEXOS project you mentioned. I will try and google it today between my breaks.
The way I see it is, I imagine that gas molecules moving rapidly (wind) that skim across a body of water. The gas molecules collide with top layers of the water molecules and they carry them away.
I had to think of why the heavier water molecules would not just fall back to the surface straight away due to gravity. Well then I thought that solar radiation, which is the main cause of wind - Solar radiation heats the earths crust, warm air rises, cool air descends. The rising of this warm air may be enough to lift the water molecules higher, where they cool.
So then how does it rain?
Well when the water molecules get caught in the cool air descending.
Anyway this is far from a perfect explanation, but I am working on it.
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Chad
In: Ramsbottom
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| Mallas wrote: | | I had to think of why the heavier water molecules would not just fall back to the surface straight away due to gravity. |
Individual water molecules (1 oxygen atom + 2 hydrogen atoms) are lighter than most atmospheric oxygen molecules (normally 2 oxygen atoms) so wouldn't be especially affected by gravity.
Even solid ice can give up gaseous water molecules directly from its surface (without an intervening liquid stage) at temperatures below freezing.
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Mallas
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I overlooked that H2O was lighter than some of the other gases.
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Ishmael
In: Toronto
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| Chad wrote: | | What increases the desertiness? |
Relative windspeed.
I keep answering your question.
Why am I still here?
This is the problem to solve: "Where the eastern effect exists, what is its cause?"
I told you already.
The only principle you need is the one Mick started with: "Plants grow wherever they can get a toe-hold." Where they don't grow, they can't.
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Chad
In: Ramsbottom
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| Ishmael wrote: | | This is the problem to solve: "Where the eastern effect exists, what is its cause?". |
You have probably noticed that I have been trying to avoid answering this... and basically what you are suggesting (I think) is not too dissimilar to what I was saying about a balance between forces from west and east.
If I'm right (didn't Mick mention this?) what you are saying is that the east coast forms a barrier to the desert advancing from the west and any sand coming from that direction is blown into the eastern ocean. But when the wind reverses direction (as it does on occasions) the sand lost to the ocean can't be blown back, so there is less sand on the eastern margin and plants are better able to take root.
I'm rather hoping this is not what your idea is based on... because if it is, it suffers from the same pitfall that awaits mine.
There is essentially no difference between easterly and westerly winds... they should both be equally capable of building deserts.
Take the case of South Africa: how is the Kalahari formed?
Westerly winds arrive from a dryish part of South America, becoming even drier as the pass over the South Atlantic. They then reach the west coast of South Africa and blow the sands (produced on that coast) inland forming the desert.
Now let's reverse the wind direction for a while... where has that wind come from?... It's come from the dry Australian Desert via the Indian Ocean.
We know that the Indian Ocean is quite capable of drying out the air when it travels west to east (to form the Australian Desert) so it should produce bone dry air also from the opposite direction.
The east coast of South Africa is just as capable of producing sand as the west coast, so the easterly winds arriving from the Indian Ocean should be just as capable of blowing these east coast sands inland to form a reverse Kalahari.
Why doesn't this happen?
I think it's because of the longitudinal shift in the rain bands (due to the Earths axial tilt)... occurring in synch with a reversal in wind direction.
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Grant
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| The only principle you need is the one Mick started with: "Plants grow wherever they can get a toe-hold." Where they don't grow, they can't. |
Remember those natural history films about deserts. At some stage in the film it rains and within days the desert blooms. It isn't the drifting sand which stops plant life; it's the lack of water. And don't forget that plants help to fix the soil.
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Ishmael
In: Toronto
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| Chad wrote: | | ...the east coast forms a barrier to the desert advancing from the west and any sand coming from that direction is blown into the eastern ocean....when the wind reverses direction (as it does on occasions) the sand lost to the ocean can't be blown back, so there is less sand on the eastern margin and plants are better able to take root. |
That's it. That's all there is to it. And I am sorry to report to you that there are no problems with your idea. I am afraid you will have to come to grips with the fact that it works everywhere, every time.
Let's go through it slowly so Grant will be able to keep up.
Look at the middle of the Sahara. What is the "desert to the west" when you stand in the middle of the Sahara?
It is the Sahara.
On the interior of a continent, the desert to the west is the desert to the east.
But on the eastern edge, the desert to the west does not continue to the east.
This is significant because, though winds do generally blow from west to to east, they sometimes blow from east to west.
Thus, in the middle of the Sahara, a single grain of sand might fly 5 miles east one day, four miles west the next, 5 miles east the following day, and four miles west the day following. And so on. Thus the sand trends eastward but, almost as much, moves westward.
So, on the inside of the continent, plants are harassed by sand from two directions (actually all directions but we will keep it 2 dimensional for simplicity). But on the east coast, they are harassed from only one direction. They are shielded from more than half of the sand.
Five miles inland from the east coast, in this hypothetical scenario, the grain of sand flies up into the air, blows 5 miles, and then lands in the ocean. Once it is in the ocean, it is never heard from again. It will not blow back westward again, to fly eastward once more the next day, neither will it do this on any subsequent day. It is fixed and immobile. Removed from the system.
Now the reason that the east coast of Northern Africa is more deserty than many other east coasts -- though still less deserty than the interior of the Sahara, is because this phenomenon of westward winds is strongest at the equator. This means that in Northern Africa, sand is blown inland from both the west and eastern coasts.
However, sand blown inland from the east coast is short-lived. It is quickly removed from the system again by the world-wide eastward trend of the winds (it's actually shifted north and south to be ejected from the system). This is why the most fertile areas are found some distance inland from the shore.
But this is true of all shorelines!
A beach is, by definition, an area where plants are unable to get a toe-hold (and this principle of separation, I believe, is highly significant to understanding all life processes and forms the cornerstone of my new theory regarding speciation). Despite Mick's mixup, his theory has never required that plants dip their roots in the eastern waves.
It just so happens then that, due to the strength of the westward winds in the area, the Somalian beach extends much further inland than it does on other eastern shores. But the Eastern effect is still functioning -- for the reasons described above -- as evidenced by the general fertility of the area when compared to the land to the immediate west.
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Ishmael
In: Toronto
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| Chad wrote: | | Now let's reverse the wind direction for a while... where has that wind come from?... It's come from the dry Australian Desert via the Indian Ocean. |
No.
Air from the Australian Desert will not reach Africa.
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Ishmael
In: Toronto
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| Grant wrote: | | At some stage in the film it rains and within days the desert blooms. It isn't the drifting sand which stops plant life; it's the lack of water. |
Water makes muck.
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Mick Harper
Site Admin
In: London
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Let us return (in my case with some relief) to North American deserts. The general picture is rather easy to sketch out:
1. Beginning at the Panama isthmus, draw a line northwards parallel to but 500 kms west of the east coast. This gives you the eastern line of a triangle and means, for instance, that the whole of Central America is prevented from desert by the Eastern Effect all the way up to Northern Mexico where the continent widens out to more than 500 kms in width. But north of this point, anywhere in North America can be desert (save for the far eastern margin)
2. Look at the Pacific Ocean between North America and Asia. It gets progressively narrower the further north you go -- the continents are half-a-world apart at the bottom and practically touching at the top. Using your skill, judgement and (by now hopefully) experience, choose a point where you think the Pacific is narrow enough to permit Asian plants to provide North American rain in goodish quantities. Draw that line across North America. No deserts will be possible north of this line.
3. Line 1 and Line 2 gives you the Basic North American Desert Triangle from whom all else flows.
PS If somebody could illustrate this pictorially I would be, to use the local desert patois, mucho grateful.
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